Network Working Group U. Blumenthal
Request for Comments: 2274 IBM T. J. Watson Research
Obsoletes: 2264 B. Wijnen
Category: Standards Track IBM T. J. Watson Research
January 1998
User-based Security Model (USM) for version 3 of the
Simple Network Management Protocol (SNMPv3)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
IANA Note
Due to a clerical error in the assignment of the snmpModules in this
memo, this RFC provides the corrected number assignment for this
protocol. This memo obsoletes RFC 2264.
Abstract
This document describes the User-based Security Model (USM) for SNMP
version 3 for use in the SNMP architecture [RFC2271]. It defines the
Elements of Procedure for providing SNMP message level security.
This document also includes a MIB for remotely monitoring/managing
the configuration parameters for this Security Model.
Table of Contents
1. Introduction 3
1.1. Threats 4
1.2. Goals and Constraints 5
1.3. Security Services 6
1.4. Module Organization 7
1.4.1. Timeliness Module 7
1.4.2. Authentication Protocol 8
1.4.3. Privacy Protocol 8
1.5. Protection against Message Replay, Delay and Redirection 8
1.5.1. Authoritative SNMP engine 8
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1.5.2. Mechanisms 9
1.6. Abstract Service Interfaces. 10
1.6.1. User-based Security Model Primitives for Authentication 11
1.6.2. User-based Security Model Primitives for Privacy 11
2. Elements of the Model 12
2.1. User-based Security Model Users 12
2.2. Replay Protection 13
2.2.1. msgAuthoritativeEngineID 13
2.2.2. msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime 14
2.2.3. Time Window 15
2.3. Time Synchronization 15
2.4. SNMP Messages Using this Security Model 16
2.5. Services provided by the User-based Security Model 17
2.5.1. Services for Generating an Outgoing SNMP Message 17
2.5.2. Services for Processing an Incoming SNMP Message 19
2.6. Key Localization Algorithm. 21
3. Elements of Procedure 21
3.1. Generating an Outgoing SNMP Message 22
3.2. Processing an Incoming SNMP Message 25
4. Discovery 30
5. Definitions 31
6. HMAC-MD5-96 Authentication Protocol 45
6.1. Mechanisms 45
6.1.1. Digest Authentication Mechanism 46
6.2. Elements of the Digest Authentication Protocol 46
6.2.1. Users 46
6.2.2. msgAuthoritativeEngineID 47
6.2.3. SNMP Messages Using this Authentication Protocol 47
6.2.4. Services provided by the HMAC-MD5-96 Authentication Module 47
6.2.4.1. Services for Generating an Outgoing SNMP Message 47
6.2.4.2. Services for Processing an Incoming SNMP Message 48
6.3. Elements of Procedure 49
6.3.1. Processing an Outgoing Message 49
6.3.2. Processing an Incoming Message 50
7. HMAC-SHA-96 Authentication Protocol 51
7.1. Mechanisms 51
7.1.1. Digest Authentication Mechanism 51
7.2. Elements of the HMAC-SHA-96 Authentication Protocol 52
7.2.1. Users 52
7.2.2. msgAuthoritativeEngineID 52
7.2.3. SNMP Messages Using this Authentication Protocol 53
7.2.4. Services provided by the HMAC-SHA-96 Authentication Module 53
7.2.4.1. Services for Generating an Outgoing SNMP Message 53
7.2.4.2. Services for Processing an Incoming SNMP Message 54
7.3. Elements of Procedure 54
7.3.1. Processing an Outgoing Message 55
7.3.2. Processing an Incoming Message 55
8. CBC-DES Symmetric Encryption Protocol 56
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8.1. Mechanisms 56
8.1.1. Symmetric Encryption Protocol 57
8.1.1.1. DES key and Initialization Vector. 57
8.1.1.2. Data Encryption. 58
8.1.1.3. Data Decryption 59
8.2. Elements of the DES Privacy Protocol 59
8.2.1. Users 59
8.2.2. msgAuthoritativeEngineID 59
8.2.3. SNMP Messages Using this Privacy Protocol 60
8.2.4. Services provided by the DES Privacy Module 60
8.2.4.1. Services for Encrypting Outgoing Data 60
8.2.4.2. Services for Decrypting Incoming Data 61
8.3. Elements of Procedure. 61
8.3.1. Processing an Outgoing Message 61
8.3.2. Processing an Incoming Message 62
9. Intellectual Property 62
10. Acknowledgements 63
11. Security Considerations 64
11.1. Recommended Practices 64
11.2. Defining Users 66
11.3. Conformance 67
12. References 67
13. Editors' Addresses 69
A.1. SNMP engine Installation Parameters 70
A.2. Password to Key Algorithm 71
A.2.1. Password to Key Sample Code for MD5 71
A.2.2. Password to Key Sample Code for SHA 72
A.3. Password to Key Sample Results 73
A.3.1. Password to Key Sample Results using MD5 73
A.3.2. Password to Key Sample Results using SHA 74
A.4. Sample encoding of msgSecurityParameters 74
B. Full Copyright Statement 76
1. Introduction
The Architecture for describing Internet Management Frameworks
[RFC2271] describes that an SNMP engine is composed of:
1) a Dispatcher
2) a Message Processing Subsystem,
3) a Security Subsystem, and
4) an Access Control Subsystem.
Applications make use of the services of these subsystems.
It is important to understand the SNMP architecture and the
terminology of the architecture to understand where the Security
Model described in this document fits into the architecture and
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interacts with other subsystems within the architecture. The reader
is expected to have read and understood the description of the SNMP
architecture, as defined in [RFC2271].
This memo [RFC2274] describes the User-based Security Model as it is
used within the SNMP Architecture. The main idea is that we use the
traditional concept of a user (identified by a userName) with which
to associate security information.
This memo describes the use of HMAC-MD5-96 and HMAC-SHA-96 as the
authentication protocols and the use of CBC-DES as the privacy
protocol. The User-based Security Model however allows for other such
protocols to be used instead of or concurrent with these protocols.
Therefore, the description of HMAC-MD5-96, HMAC-SHA-96 and CBC-DES
are in separate sections to reflect their self-contained nature and
to indicate that they can be replaced or supplemented in the future.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
1.1. Threats
Several of the classical threats to network protocols are applicable
to the network management problem and therefore would be applicable
to any SNMP Security Model. Other threats are not applicable to the
network management problem. This section discusses principal
threats, secondary threats, and threats which are of lesser
importance.
The principal threats against which this SNMP Security Model should
provide protection are:
- Modification of Information
The modification threat is the danger that some unauthorized entity
may alter in-transit SNMP messages generated on behalf of an
authorized user in such a way as to effect unauthorized management
operations, including falsifying the value of an object.
- Masquerade
The masquerade threat is the danger that management operations not
authorized for some user may be attempted by assuming the identity
of another user that has the appropriate authorizations.
Two secondary threats are also identified. The Security Model
defined in this memo provides limited protection against:
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- Disclosure
The disclosure threat is the danger of eavesdropping on the
exchanges between managed agents and a management station.
Protecting against this threat may be required as a matter of local
policy.
- Message Stream Modification
The SNMP protocol is typically based upon a connection-less
transport service which may operate over any sub-network service.
The re-ordering, delay or replay of messages can and does occur
through the natural operation of many such sub-network services.
The message stream modification threat is the danger that messages
may be maliciously re-ordered, delayed or replayed to an extent
which is greater than can occur through the natural operation of a
sub-network service, in order to effect unauthorized management
operations.
There are at least two threats that an SNMP Security Model need not
protect against. The security protocols defined in this memo do not
provide protection against:
- Denial of Service
This SNMP Security Model does not attempt to address the broad
range of attacks by which service on behalf of authorized users is
denied. Indeed, such denial-of-service attacks are in many cases
indistinguishable from the type of network failures with which any
viable network management protocol must cope as a matter of course.
- Traffic Analysis
This SNMP Security Model does not attempt to address traffic
analysis attacks. Indeed, many traffic patterns are predictable -
devices may be managed on a regular basis by a relatively small
number of management applications - and therefore there is no
significant advantage afforded by protecting against traffic
analysis.
1.2. Goals and Constraints
Based on the foregoing account of threats in the SNMP network
management environment, the goals of this SNMP Security Model are as
follows.
1) Provide for verification that each received SNMP message has
not been modified during its transmission through the network.
2) Provide for verification of the identity of the user on whose
behalf a received SNMP message claims to have been generated.
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3) Provide for detection of received SNMP messages, which request
or contain management information, whose time of generation was
not recent.
4) Provide, when necessary, that the contents of each received
SNMP message are protected from disclosure.
In addition to the principal goal of supporting secure network
management, the design of this SNMP Security Model is also influenced
by the following constraints:
1) When the requirements of effective management in times of
network stress are inconsistent with those of security, the design
should prefer the former.
2) Neither the security protocol nor its underlying security
mechanisms should depend upon the ready availability of other
network services (e.g., Network Time Protocol (NTP) or key
management protocols).
3) A security mechanism should entail no changes to the basic
SNMP network management philosophy.
1.3. Security Services
The security services necessary to support the goals of this SNMP
Security Model are as follows:
- Data Integrity
is the provision of the property that data has not been altered or
destroyed in an unauthorized manner, nor have data sequences been
altered to an extent greater than can occur non-maliciously.
- Data Origin Authentication
is the provision of the property that the claimed identity of the
user on whose behalf received data was originated is corroborated.
- Data Confidentiality
is the provision of the property that information is not made
available or disclosed to unauthorized individuals, entities, or
processes.
- Message timeliness and limited replay protection
is the provision of the property that a message whose generation
time is outside of a specified time window is not accepted. Note
that message reordering is not dealt with and can occur in normal
conditions too.
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For the protocols specified in this memo, it is not possible to
assure the specific originator of a received SNMP message; rather, it
is the user on whose behalf the message was originated that is
authenticated.
For these protocols, it not possible to obtain data integrity without
data origin authentication, nor is it possible to obtain data origin
authentication without data integrity. Further, there is no
provision for data confidentiality without both data integrity and
data origin authentication.
The security protocols used in this memo are considered acceptably
secure at the time of writing. However, the procedures allow for new
authentication and privacy methods to be specified at a future time
if the need arises.
1.4. Module Organization
The security protocols defined in this memo are split in three
different modules and each has its specific responsibilities such
that together they realize the goals and security services described
above:
- The authentication module MUST provide for:
- Data Integrity,
- Data Origin Authentication
- The timeliness module MUST provide for:
- Protection against message delay or replay (to an extent
greater than can occur through normal operation)
The privacy module MUST provide for
- Protection against disclosure of the message payload.
The timeliness module is fixed for the User-based Security Model
while there is provision for multiple authentication and/or privacy
modules, each of which implements a specific authentication or
privacy protocol respectively.
1.4.1. Timeliness Module
Section 3 (Elements of Procedure) uses the timeliness values in an
SNMP message to do timeliness checking. The timeliness check is only
performed if authentication is applied to the message. Since the
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complete message is checked for integrity, we can assume that the
timeliness values in a message that passes the authentication module
are trustworthy.
1.4.2. Authentication Protocol
Section 6 describes the HMAC-MD5-96 authentication protocol which is
the first authentication protocol that MUST be supported with the
User-based Security Model. Section 7 describes the HMAC-SHA-96
authentication protocol which is another authentication protocol that
SHOULD be supported with the User-based Security Model. In the
future additional or replacement authentication protocols may be
defined as new needs arise.
The User-based Security Model prescribes that, if authentication is
used, then the complete message is checked for integrity in the
authentication module.
For a message to be authenticated, it needs to pass authentication
check by the authentication module and the timeliness check which is
a fixed part of this User-based Security model.
1.4.3. Privacy Protocol
Section 8 describes the CBC-DES Symmetric Encryption Protocol which
is the first privacy protocol to be used with the User-based Security
Model. In the future additional or replacement privacy protocols may
be defined as new needs arise.
The User-based Security Model prescribes that the scopedPDU is
protected from disclosure when a message is sent with privacy.
The User-based Security Model also prescribes that a message needs to
be authenticated if privacy is in use.
1.5. Protection against Message Replay, Delay and Redirection
1.5.1. Authoritative SNMP engine
In order to protect against message replay, delay and redirection,
one of the SNMP engines involved in each communication is designated
to be the authoritative SNMP engine. When an SNMP message contains a
payload which expects a response (for example a Get, GetNext,
GetBulk, Set or Inform PDU), then the receiver of such messages is
authoritative. When an SNMP message contains a payload which does
not expect a response (for example an SNMPv2-Trap, Response or Report
PDU), then the sender of such a message is authoritative.
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1.5.2. Mechanisms
The following mechanisms are used:
1) To protect against the threat of message delay or replay (to an
extent greater than can occur through normal operation), a set of
timeliness indicators (for the authoritative SNMP engine) are
included in each message generated. An SNMP engine evaluates the
timeliness indicators to determine if a received message is
recent. An SNMP engine may evaluate the timeliness indicators to
ensure that a received message is at least as recent as the last
message it received from the same source. A non-authoritative
SNMP engine uses received authentic messages to advance its notion
of the timeliness indicators at the remote authoritative source.
An SNMP engine MUST also use a mechanism to match incoming
Responses to outstanding Requests and it MUST drop any Responses
that do not match an outstanding request. For example, a msgID can
be inserted in every message to cater for this functionality.
These mechanisms provide for the detection of authenticated
messages whose time of generation was not recent.
This protection against the threat of message delay or replay does
not imply nor provide any protection against unauthorized deletion
or suppression of messages. Also, an SNMP engine may not be able
to detect message reordering if all the messages involved are sent
within the Time Window interval. Other mechanisms defined
independently of the security protocol can also be used to detect
the re-ordering replay, deletion, or suppression of messages
containing Set operations (e.g., the MIB variable snmpSetSerialNo
[RFC1907]).
2) Verification that a message sent to/from one authoritative SNMP
engine cannot be replayed to/as-if-from another authoritative SNMP
engine.
Included in each message is an identifier unique to the
authoritative SNMP engine associated with the sender or intended
recipient of the message.
A Report, Response or Trap message sent by an authoritative SNMP
engine to one non-authoritative SNMP engine can potentially be
replayed to another non-authoritative SNMP engine. The latter
non-authoritative SNMP engine might (if it knows about the same
userName with the same secrets at the authoritative SNMP engine)
as a result update its notion of timeliness indicators of the
authoritative SNMP engine, but that is not considered a threat.
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In this case, A Report or Response message will be discarded by
the Message Processing Model, because there should not be an
outstanding Request message. A Trap will possibly be accepted.
Again, that is not considered a threat, because the communication
was authenticated and timely. It is as if the authoritative SNMP
engine was configured to start sending Traps to the second SNMP
engine, which theoretically can happen without the knowledge of
the second SNMP engine anyway. Anyway, the second SNMP engine may
not expect to receive this Trap, but is allowed to see the
management information contained in it.
3) Detection of messages which were not recently generated.
A set of time indicators are included in the message, indicating
the time of generation. Messages without recent time indicators
are not considered authentic. In addition, an SNMP engine MUST
drop any Responses that do not match an outstanding request. This
however is the responsibility of the Message Processing Model.
This memo allows the same user to be defined on multiple SNMP
engines. Each SNMP engine maintains a value, snmpEngineID, which
uniquely identifies the SNMP engine. This value is included in each
message sent to/from the SNMP engine that is authoritative (see
section 1.5.1). On receipt of a message, an authoritative SNMP
engine checks the value to ensure that it is the intended recipient,
and a non-authoritative SNMP engine uses the value to ensure that the
message is processed using the correct state information.
Each SNMP engine maintains two values, snmpEngineBoots and
snmpEngineTime, which taken together provide an indication of time at
that SNMP engine. Both of these values are included in an
authenticated message sent to/received from that SNMP engine. On
receipt, the values are checked to ensure that the indicated
timeliness value is within a Time Window of the current time. The
Time Window represents an administrative upper bound on acceptable
delivery delay for protocol messages.
For an SNMP engine to generate a message which an authoritative SNMP
engine will accept as authentic, and to verify that a message
received from that authoritative SNMP engine is authentic, such an
SNMP engine must first achieve timeliness synchronization with the
authoritative SNMP engine. See section 2.3.
1.6. Abstract Service Interfaces.
Abstract service interfaces have been defined to describe the
conceptual interfaces between the various subsystems within an SNMP
entity. Similarly a set of abstract service interfaces have been
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defined within the User-based Security Model (USM) to describe the
conceptual interfaces between the generic USM services and the self-
contained authentication and privacy services.
These abstract service interfaces are defined by a set of primitives
that define the services provided and the abstract data elements that
must be passed when the services are invoked. This section lists the
primitives that have been defined for the User-based Security Model.
1.6.1. User-based Security Model Primitives for Authentication
The User-based Security Model provides the following internal
primitives to pass data back and forth between the Security Model
itself and the authentication service:
statusInformation =
authenticateOutgoingMsg(
IN authKey -- secret key for authentication
IN wholeMsg -- unauthenticated complete message
OUT authenticatedWholeMsg -- complete authenticated message
)
statusInformation =
authenticateIncomingMsg(
IN authKey -- secret key for authentication
IN authParameters -- as received on the wire
IN wholeMsg -- as received on the wire
OUT authenticatedWholeMsg -- complete authenticated message
)
1.6.2. User-based Security Model Primitives for Privacy
The User-based Security Model provides the following internal
primitives to pass data back and forth between the Security Model
itself and the privacy service:
statusInformation =
encryptData(
IN encryptKey -- secret key for encryption
IN dataToEncrypt -- data to encrypt (scopedPDU)
OUT encryptedData -- encrypted data (encryptedPDU)
OUT privParameters -- filled in by service provider
)
statusInformation =
decryptData(
IN decryptKey -- secret key for decrypting
IN privParameters -- as received on the wire
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IN encryptedData -- encrypted data (encryptedPDU)
OUT decryptedData -- decrypted data (scopedPDU)
)
2. Elements of the Model
This section contains definitions required to realize the security
model defined by this memo.
2.1. User-based Security Model Users
Management operations using this Security Model make use of a defined
set of user identities. For any user on whose behalf management
operations are authorized at a particular SNMP engine, that SNMP
engine must have knowledge of that user. An SNMP engine that wishes
to communicate with another SNMP engine must also have knowledge of a
user known to that engine, including knowledge of the applicable
attributes of that user.
A user and its attributes are defined as follows:
userName
A string representing the name of the user.
securityName
A human-readable string representing the user in a format that is
Security Model independent.
authProtocol
An indication of whether messages sent on behalf of this user can
be authenticated, and if so, the type of authentication protocol
which is used. Two such protocols are defined in this memo:
- the HMAC-MD5-96 authentication protocol.
- the HMAC-SHA-96 authentication protocol.
authKey
If messages sent on behalf of this user can be authenticated,
the (private) authentication key for use with the authentication
protocol. Note that a user's authentication key will normally
be different at different authoritative SNMP engines. The authKey
is not accessible via SNMP. The length requirements of the authKey
are defined by the authProtocol in use.
authKeyChange and authOwnKeyChange
The only way to remotely update the authentication key. Does
that in a secure manner, so that the update can be completed
without the need to employ privacy protection.
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privProtocol
An indication of whether messages sent on behalf of this user
can be protected from disclosure, and if so, the type of privacy
protocol which is used. One such protocol is defined in this
memo: the CBC-DES Symmetric Encryption Protocol.
privKey
If messages sent on behalf of this user can be en/decrypted,
the (private) privacy key for use with the privacy protocol.
Note that a user's privacy key will normally be different at
different authoritative SNMP engines. The privKey is not
accessible via SNMP. The length requirements of the privKey are
defined by the privProtocol in use.
privKeyChange and privOwnKeyChange
The only way to remotely update the encryption key. Does that
in a secure manner, so that the update can be completed without
the need to employ privacy protection.
2.2. Replay Protection
Each SNMP engine maintains three objects:
- snmpEngineID, which (at least within an administrative domain)
uniquely and unambiguously identifies an SNMP engine.
- snmpEngineBoots, which is a count of the number of times the
SNMP engine has re-booted/re-initialized since snmpEngineID
was last configured; and,
- snmpEngineTime, which is the number of seconds since the
snmpEngineBoots counter was last incremented.
Each SNMP engine is always authoritative with respect to these
objects in its own SNMP entity. It is the responsibility of a
non-authoritative SNMP engine to synchronize with the
authoritative SNMP engine, as appropriate.
An authoritative SNMP engine is required to maintain the values of
its snmpEngineID and snmpEngineBoots in non-volatile storage.
2.2.1. msgAuthoritativeEngineID
The msgAuthoritativeEngineID value contained in an authenticated
message is used to defeat attacks in which messages from one SNMP
engine to another SNMP engine are replayed to a different SNMP
engine. It represents the snmpEngineID at the authoritative SNMP
engine involved in the exchange of the message.
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When an authoritative SNMP engine is first installed, it sets its
local value of snmpEngineID according to a enterprise-specific
algorithm (see the definition of the Textual Convention for
SnmpEngineID in the SNMP Architecture document [RFC2271]).
2.2.2. msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime
The msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime
values contained in an authenticated message are used to defeat
attacks in which messages are replayed when they are no longer
valid. They represent the snmpEngineBoots and snmpEngineTime
values at the authoritative SNMP engine involved in the exchange
of the message.
Through use of snmpEngineBoots and snmpEngineTime, there is no
requirement for an SNMP engine to have a non-volatile clock which
ticks (i.e., increases with the passage of time) even when the
SNMP engine is powered off. Rather, each time an SNMP engine
re-boots, it retrieves, increments, and then stores snmpEngineBoots
in non-volatile storage, and resets snmpEngineTime to zero.
When an SNMP engine is first installed, it sets its local values
of snmpEngineBoots and snmpEngineTime to zero. If snmpEngineTime
ever reaches its maximum value (2147483647), then snmpEngineBoots
is incremented as if the SNMP engine has re-booted and
snmpEngineTime is reset to zero and starts incrementing again.
Each time an authoritative SNMP engine re-boots, any SNMP engines
holding that authoritative SNMP engine's values of snmpEngineBoots
and snmpEngineTime need to re-synchronize prior to sending
correctly authenticated messages to that authoritative SNMP engine
(see Section 2.3 for (re-)synchronization procedures). Note,
however, that the procedures do provide for a notification to be
accepted as authentic by a receiving SNMP engine, when sent by an
authoritative SNMP engine which has re-booted since the receiving
SNMP engine last (re-)synchronized.
If an authoritative SNMP engine is ever unable to determine its
latest snmpEngineBoots value, then it must set its snmpEngineBoots
value to 2147483647.
Whenever the local value of snmpEngineBoots has the value 2147483647
it latches at that value and an authenticated message always causes
an notInTimeWindow authentication failure.
In order to reset an SNMP engine whose snmpEngineBoots value has
reached the value 2147483647, manual intervention is required.
The engine must be physically visited and re-configured, either
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with a new snmpEngineID value, or with new secret values for the
authentication and privacy protocols of all users known to that
SNMP engine. Note that even if an SNMP engine re-boots once a second
that it would still take approximately 68 years before the max value
of 2147483647 would be reached.
2.2.3. Time Window
The Time Window is a value that specifies the window of time in
which a message generated on behalf of any user is valid. This
memo specifies that the same value of the Time Window, 150 seconds,
is used for all users.
2.3. Time Synchronization
Time synchronization, required by a non-authoritative SNMP engine
in order to proceed with authentic communications, has occurred
when the non-authoritative SNMP engine has obtained a local notion
of the authoritative SNMP engine's values of snmpEngineBoots and
snmpEngineTime from the authoritative SNMP engine. These values
must be (and remain) within the authoritative SNMP engine's Time
Window. So the local notion of the authoritative SNMP engine's
values must be kept loosely synchronized with the values stored
at the authoritative SNMP engine. In addition to keeping a local
copy of snmpEngineBoots and snmpEngineTime from the authoritative
SNMP engine, a non-authoritative SNMP engine must also keep one
local variable, latestReceivedEngineTime. This value records the
highest value of snmpEngineTime that was received by the
non-authoritative SNMP engine from the authoritative SNMP engine
and is used to eliminate the possibility of replaying messages
that would prevent the non-authoritative SNMP engine's notion of
the snmpEngineTime from advancing.
A non-authoritative SNMP engine must keep local notions of these
values for each authoritative SNMP engine with which it wishes to
communicate. Since each authoritative SNMP engine is uniquely
and unambiguously identified by its value of snmpEngineID, the
non-authoritative SNMP engine may use this value as a key in
order to cache its local notions of these values.
Time synchronization occurs as part of the procedures of receiving
an SNMP message (Section 3.2, step 7b). As such, no explicit time
synchronization procedure is required by a non-authoritative SNMP
engine. Note, that whenever the local value of snmpEngineID is
changed (e.g., through discovery) or when secure communications
are first established with an authoritative SNMP engine, the local
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values of snmpEngineBoots and latestReceivedEngineTime should be
set to zero. This will cause the time synchronization to occur
when the next authentic message is received.
2.4. SNMP Messages Using this Security Model
The syntax of an SNMP message using this Security Model adheres
to the message format defined in the version-specific Message
Processing Model document (for example [RFC2272]).
The field msgSecurityParameters in SNMPv3 messages has a data type
of OCTET STRING. Its value is the BER serialization of the
following ASN.1 sequence:
USMSecurityParametersSyntax DEFINITIONS IMPLICIT TAGS ::= BEGIN
UsmSecurityParameters ::=
SEQUENCE {
-- global User-based security parameters
msgAuthoritativeEngineID OCTET STRING,
msgAuthoritativeEngineBoots INTEGER (0..2147483647),
msgAuthoritativeEngineTime INTEGER (0..2147483647),
msgUserName OCTET STRING (SIZE(1..32)),
-- authentication protocol specific parameters
msgAuthenticationParameters OCTET STRING,
-- privacy protocol specific parameters
msgPrivacyParameters OCTET STRING
}
END
The fields of this sequence are:
- The msgAuthoritativeEngineID specifies the snmpEngineID of the
authoritative SNMP engine involved in the exchange of the message.
- The msgAuthoritativeEngineBoots specifies the snmpEngineBoots
value at the authoritative SNMP engine involved in the exchange of
the message.
- The msgAuthoritativeEngineTime specifies the snmpEngineTime value
at the authoritative SNMP engine involved in the exchange of the
message.
- The msgUserName specifies the user (principal) on whose behalf
the message is being exchanged.
Blumenthal & Wijnen Standards Track [Page 16]RFC 2274 USM for SNMPv3 January 1998
- The msgAuthenticationParameters are defined by the authentication
protocol in use for the message, as defined by the
usmUserAuthProtocol column in the user's entry in the usmUserTable.
- The msgPrivacyParameters are defined by the privacy protocol in
use for the message, as defined by the usmUserPrivProtocol column
in the user's entry in the usmUserTable).
See appendix A.4 for an example of the BER encoding of field
msgSecurityParameters.
2.5. Services provided by the User-based Security Model
This section describes the services provided by the User-based
Security Model with their inputs and outputs.
The services are described as primitives of an abstract service
interface and the inputs and outputs are described as abstract data
elements as they are passed in these abstract service primitives.
2.5.1. Services for Generating an Outgoing SNMP Message
When the Message Processing (MP) Subsystem invokes the User-based
Security module to secure an outgoing SNMP message, it must use the
appropriate service as provided by the Security module. These two
services are provided:
1) A service to generate a Request message. The abstract service
primitive is:
statusInformation = -- success or errorIndication
generateRequestMsg(
IN messageProcessingModel -- typically, SNMP version
IN globalData -- message header, admin data
IN maxMessageSize -- of the sending SNMP entity
IN securityModel -- for the outgoing message
IN securityEngineID -- authoritative SNMP entity
IN securityName -- on behalf of this principal
IN securityLevel -- Level of Security requested
IN scopedPDU -- message (plaintext) payload
OUT securityParameters -- filled in by Security Module
OUT wholeMsg -- complete generated message
OUT wholeMsgLength -- length of generated message
)
2) A service to generate a Response message. The abstract service
primitive is:
Blumenthal & Wijnen Standards Track [Page 17]RFC 2274 USM for SNMPv3 January 1998
statusInformation = -- success or errorIndication
generateResponseMsg(
IN messageProcessingModel -- typically, SNMP version
IN globalData -- message header, admin data
IN maxMessageSize -- of the sending SNMP entity
IN securityModel -- for the outgoing message
IN securityEngineID -- authoritative SNMP entity
IN securityName -- on behalf of this principal
IN securityLevel -- Level of Security requested
IN scopedPDU -- message (plaintext) payload
IN securityStateReference -- reference to security state
-- information from original
-- request
OUT securityParameters -- filled in by Security Module
OUT wholeMsg -- complete generated message
OUT wholeMsgLength -- length of generated message
)
The abstract data elements passed as parameters in the abstract
service primitives are as follows:
statusInformation
An indication of whether the encoding and securing of the message
was successful. If not it is an indication of the problem.
essageProcessingModel
The SNMP version number for the message to be generated. This
data is not used by the User-based Security module.
globalData
The message header (i.e., its administrative information). This
data is not used by the User-based Security module.
maxMessageSize
The maximum message size as included in the message. This data is
not used by the User-based Security module.
securityParameters
These are the security parameters. They will be filled in by the
User-based Security module.
securityModel
The securityModel in use. Should be User-based Security Model.
This data is not used by the User-based Security module.
securityName
Together with the snmpEngineID it identifies a row in the
usmUserTable that is to be used for securing the message. The
securityName has a format that is independent of the Security
Model. In case of a response this parameter is ignored and the
value from the cache is used.
securityLevel
The Level of Security from which the User-based Security module
determines if the message needs to be protected from disclosure
Blumenthal & Wijnen Standards Track [Page 18]RFC 2274 USM for SNMPv3 January 1998
and if the message needs to be authenticated. In case of a
response this parameter is ignored and the value from the cache is
used.
securityEngineID
The snmpEngineID of the authoritative SNMP engine to which a
Request message is to be sent. In case of a response it is implied
to be the processing SNMP engine's snmpEngineID and so if it is
specified, then it is ignored.
scopedPDU
The message payload. The data is opaque as far as the User-based
Security Model is concerned.
securityStateReference
A handle/reference to cachedSecurityData to be used when securing
an outgoing Response message. This is the exact same
handle/reference as it was generated by the User-based Security
module when processing the incoming Request message to which this
is the Response message.
wholeMsg
The fully encoded and secured message ready for sending on the
wire.
wholeMsgLength
The length of the encoded and secured message (wholeMsg).
Upon completion of the process, the User-based Security module
returns statusInformation. If the process was successful, the
completed message with privacy and authentication applied if such was
requested by the specified securityLevel is returned. If the process
was not successful, then an errorIndication is returned.
2.5.2. Services for Processing an Incoming SNMP Message
When the Message Processing (MP) Subsystem invokes the User-based
Security module to verify proper security of an incoming message, it
must use the service provided for an incoming message. The abstract
service primitive is:
statusInformation = -- errorIndication or success
-- error counter OID/value if error
processIncomingMsg(
IN messageProcessingModel -- typically, SNMP version
IN maxMessageSize -- of the sending SNMP entity
IN securityParameters -- for the received message
IN securityModel -- for the received message
IN securityLevel -- Level of Security
IN wholeMsg -- as received on the wire
IN wholeMsgLength -- length as received on the wire
OUT securityEngineID -- authoritative SNMP entity
OUT securityName -- identification of the principal
Blumenthal & Wijnen Standards Track [Page 19]RFC 2274 USM for SNMPv3 January 1998
OUT scopedPDU, -- message (plaintext) payload
OUT maxSizeResponseScopedPDU -- maximum size of the Response PDU
OUT securityStateReference -- reference to security state
) -- information, needed for response
The abstract data elements passed as parameters in the abstract
service primitives are as follows:
statusInformation
An indication of whether the process was successful or not. If
not, then the statusInformation includes the OID and the value of
the error counter that was incremented.
messageProcessingModel
The SNMP version number as received in the message. This data is
not used by the User-based Security module.
maxMessageSize
The maximum message size as included in the message. The User-
based Security module uses this value to calculate the
maxSizeResponseScopedPDU.
securityParameters
These are the security parameters as received in the message.
securityModel
The securityModel in use. Should be the User-based Security
Model. This data is not used by the User-based Security module.
securityLevel
The Level of Security from which the User-based Security module
determines if the message needs to be protected from disclosure
and if the message needs to be authenticated.
wholeMsg
The whole message as it was received.
wholeMsgLength
The length of the message as it was received (wholeMsg).
securityEngineID
The snmpEngineID that was extracted from the field
msgAuthoritativeEngineID and that was used to lookup the secrets
in the usmUserTable.
securityName
The security name representing the user on whose behalf the
message was received. The securityName has a format that is
independent of the Security Model.
scopedPDU
The message payload. The data is opaque as far as the User-based
Security Model is concerned.
maxSizeResponseScopedPDU
The maximum size of a scopedPDU to be included in a possible
Response message. The User-base Security module calculates
Blumenthal & Wijnen Standards Track [Page 20]RFC 2274 USM for SNMPv3 January 1998
this size based on the mms (as received in the message) and the
space required for the message header (including the
securityParameters) for such a Response message.
securityStateReference
A handle/reference to cachedSecurityData to be used when securing
an outgoing Response message. When the Message Processing
Subsystem calls the User-based Security module to generate a
response to this incoming message it must pass this
handle/reference.
Upon completion of the process, the User-based Security module
returns statusInformation and, if the process was successful, the
additional data elements for further processing of the message. If
the process was not successful, then an errorIndication, possibly
with a OID and value pair of an error counter that was incremented.
2.6. Key Localization Algorithm.
A localized key is a secret key shared between a user U and one
authoritative SNMP engine E. Even though a user may have only one
password and therefore one key for the whole network, the actual
secrets shared between the user and each authoritative SNMP engine
will be different. This is achieved by key localization [Localized-
key].
First, if a user uses a password, then the user's password is
converted into a key Ku using one of the two algorithms described in
Appendices A.2.1 and A.2.2.
To convert key Ku into a localized key Kul of user U at the
authoritative SNMP engine E, one appends the snmpEngineID of the
authoritative SNMP engine to the key Ku and then appends the key Ku
to the result, thus enveloping the snmpEngineID within the two copies
of user's key Ku. Then one runs a secure hash function (which one
depends on the authentication protocol defined for this user U at
authoritative SNMP engine E; this document defines two authentication
protocols with their associated algorithms based on MD5 and SHA). The
output of the hash-function is the localized key Kul for user U at
the authoritative SNMP engine E.
3. Elements of Procedure
This section describes the security related procedures followed by an
SNMP engine when processing SNMP messages according to the User-based
Security Model.
Blumenthal & Wijnen Standards Track [Page 21]RFC 2274 USM for SNMPv3 January 1998
3.1. Generating an Outgoing SNMP Message
This section describes the procedure followed by an SNMP engine
whenever it generates a message containing a management operation
(like a request, a response, a notification, or a report) on behalf
of a user, with a particular securityLevel.
1) a) If any securityStateReference is passed (Response message),
then information concerning the user is extracted from the
cachedSecurityData. The securityEngineID and the
securityLevel are extracted from the cachedSecurityData. The
cachedSecurityData can now be discarded.
Otherwise,
b) based on the securityName, information concerning the
user at the destination snmpEngineID, specified by the
securityEngineID, is extracted from the Local Configuration
Datastore (LCD, usmUserTable). If information about the user
is absent from the LCD, then an error indication
(unknownSecurityName) is returned to the calling module.
2) If the securityLevel specifies that the message is to be
protected from disclosure, but the user does not support both an
authentication and a privacy protocol then the message cannot be
sent. An error indication (unsupportedSecurityLevel) is returned
to the calling module.
3) If the securityLevel specifies that the message is to be
authenticated, but the user does not support an authentication
protocol, then the message cannot be sent. An error indication
(unsupportedSecurityLevel) is returned to the calling module.
4) a) If the securityLevel specifies that the message is to be
protected from disclosure, then the octet sequence
representing the serialized scopedPDU is encrypted according
to the user's privacy protocol. To do so a call is made to the
privacy module that implements the user's privacy protocol
according to the abstract primitive:
statusInformation = -- success or failure
encryptData(
IN encryptKey -- user's localized privKey
IN dataToEncrypt -- serialized scopedPDU
OUT encryptedData -- serialized encryptedPDU
OUT privParameters -- serialized privacy parameters
)
Blumenthal & Wijnen Standards Track [Page 22]RFC 2274 USM for SNMPv3 January 1998
statusInformation
indicates if the encryption process was successful or not.
encryptKey
the user's localized private privKey is the secret key that
can be used by the encryption algorithm.
dataToEncrypt
the serialized scopedPDU is the data that to be encrypted.
encryptedData
the encryptedPDU represents the encrypted scopedPDU,
encoded as an OCTET STRING.
privParameters
the privacy parameters, encoded as an OCTET STRING.
If the privacy module returns failure, then the message cannot
be sent and an error indication (encryptionError) is returned
to the calling module.
If the privacy module returns success, then the returned
privParameters are put into the msgPrivacyParameters field of
the securityParameters and the encryptedPDU serves as the
payload of the message being prepared.
Otherwise,
b) If the securityLevel specifies that the message is not to be
protected from disclosure, then the NULL string is encoded as
an OCTET STRING and put into the msgPrivacyParameters field of
the securityParameters and the plaintext scopedPDU serves as
the payload of the message being prepared.
5) The snmpEngineID is encoded as an OCTET STRING into the
msgAuthoritativeEngineID field of the securityParameters. Note
that an empty (zero length) snmpEngineID is OK for a Request
message, because that will cause the remote (authoritative) SNMP
engine to return a Report PDU with the proper snmpEngineID
included in the msgAuthoritativeEngineID in the
securityParameters of that returned Report PDU.
6) a) If the securityLevel specifies that the message is to be
authenticated, then the current values of snmpEngineBoots and
snmpEngineTime corresponding to the snmpEngineID from the LCD
are used.
Otherwise,
b) If this is a Response message, then the current value of
snmpEngineBoots and snmpEngineTime corresponding to the local
snmpEngineID from the LCD are used.
Blumenthal & Wijnen Standards Track [Page 23]RFC 2274 USM for SNMPv3 January 1998
Otherwise,
c) If this is a Request message, then a zero value is used
for both snmpEngineBoots and snmpEngineTime. This zero value
gets used if snmpEngineID is empty.
The values are encoded as INTEGER respectively into the
msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime fields
of the securityParameters.
7) The userName is encoded as an OCTET STRING into the msgUserName
field of the securityParameters.
8) a) If the securityLevel specifies that the message is to be
authenticated, the message is authenticated according to the
user's authentication protocol. To do so a call is made to the
authentication module that implements the user's
authentication protocol according to the abstract service
primitive:
statusInformation =
authenticateOutgoingMsg(
IN authKey -- the user's localized authKey
IN wholeMsg -- unauthenticated message
OUT authenticatedWholeMsg -- authenticated complete message
)
statusInformation
indicates if authentication was successful or not.
authKey
the user's localized private authKey is the secret key that
can be used by the authentication algorithm.
wholeMsg
the complete serialized message to be authenticated.
authenticatedWholeMsg
the same as the input given to the authenticateOutgoingMsg
service, but with msgAuthenticationParameters properly
filled in.
If the authentication module returns failure, then the message
cannot be sent and an error indication (authenticationFailure)
is returned to the calling module.
If the authentication module returns success, then the
msgAuthenticationParameters field is put into the
securityParameters and the authenticatedWholeMsg represents
the serialization of the authenticated message being prepared.
Blumenthal & Wijnen Standards Track [Page 24]RFC 2274 USM for SNMPv3 January 1998
Otherwise,
b) If the securityLevel specifies that the message is not to
be authenticated then the NULL string is encoded as an OCTET
STRING into the msgAuthenticationParameters field of the
securityParameters. The wholeMsg is now serialized and then
represents the unauthenticated message being prepared.
9) The completed message with its length is returned to the
calling module with the statusInformation set to success.
3.2. Processing an Incoming SNMP Message
This section describes the procedure followed by an SNMP engine
whenever it receives a message containing a management operation on
behalf of a user, with a particular securityLevel.
To simplify the elements of procedure, the release of state
information is not always explicitly specified. As a general rule, if
state information is available when a message gets discarded, the
state information should also be released. Also, when an error
indication with an OID and value for an incremented counter is
returned, then the available information (like
securityStateReference) must be passed back to the caller so it can
generate a Report PDU.
1) If the received securityParameters is not the serialization
(according to the conventions of [RFC1906]) of an OCTET STRING
formatted according to the UsmSecurityParameters defined in
section 2.4, then the snmpInASNParseErrs counter [RFC1907] is
incremented, and an error indication (parseError) is returned to
the calling module. Note that we return without the OID and
value of the incremented counter, because in this case there is
not enough information to generate a Report PDU.
2) The values of the security parameter fields are extracted from
the securityParameters. The securityEngineID to be returned to
the caller is the value of the msgAuthoritativeEngineID field.
The cachedSecurityData is prepared and a securityStateReference
is prepared to reference this data. Values to be cached are:
msgUserName
securityEngineID
securityLevel
3) If the value of the msgAuthoritativeEngineID field in the
securityParameters is unknown then:
Blumenthal & Wijnen Standards Track [Page 25]RFC 2274 USM for SNMPv3 January 1998
a) a non-authoritative SNMP engine that performs discovery may
optionally create a new entry in its Local Configuration
Datastore (LCD) and continue processing;
or
b) the usmStatsUnknownEngineIDs counter is incremented, and
an error indication (unknownEngineID) together with the
OID and value of the incremented counter is returned to
the calling module.
4) Information about the value of the msgUserName and
msgAuthoritativeEngineID fields is extracted from the Local
Configuration Datastore (LCD, usmUserTable). If no information
is available for the user, then the usmStatsUnknownUserNames
counter is incremented and an error indication
(unknownSecurityName) together with the OID and value of the
incremented counter is returned to the calling module.
5) If the information about the user indicates that it does not
support the securityLevel requested by the caller, then the
usmStatsUnsupportedSecLevels counter is incremented and an
error indication (unsupportedSecurityLevel) together with the
OID and value of the incremented counter is returned to the
calling module.
6) If the securityLevel specifies that the message is to be
authenticated, then the message is authenticated according to
the user's authentication protocol. To do so a call is made
to the authentication module that implements the user's
authentication protocol according to the abstract service
primitive:
statusInformation = -- success or failure
authenticateIncomingMsg(
IN authKey -- the user's localized authKey
IN authParameters -- as received on the wire
IN wholeMsg -- as received on the wire
OUT authenticatedWholeMsg -- checked for authentication
)
statusInformation
indicates if authentication was successful or not.
authKey
the user's localized private authKey is the secret key that
can be used by the authentication algorithm.
wholeMsg
the complete serialized message to be authenticated.
Blumenthal & Wijnen Standards Track [Page 26]RFC 2274 USM for SNMPv3 January 1998
authenticatedWholeMsg
the same as the input given to the authenticateIncomingMsg
service, but after authentication has been checked.
If the authentication module returns failure, then the message
cannot be trusted, so the usmStatsWrongDigests counter is
incremented and an error indication (authenticationFailure)
together with the OID and value of the incremented counter is
returned to the calling module.
If the authentication module returns success, then the message
is authentic and can be trusted so processing continues.
7) If the securityLevel indicates an authenticated message, then
the local values of snmpEngineBoots and snmpEngineTime
corresponding to the value of the msgAuthoritativeEngineID
field are extracted from the Local Configuration Datastore.
a) If the extracted value of msgAuthoritativeEngineID is the
same as the value of snmpEngineID of the processing SNMP
engine (meaning this is the authoritative SNMP engine),
then if any of the following conditions is true, then the
message is considered to be outside of the Time Window:
- the local value of snmpEngineBoots is 2147483647;
- the value of the msgAuthoritativeEngineBoots field differs
from the local value of snmpEngineBoots; or,
- the value of the msgAuthoritativeEngineTime field differs
from the local notion of snmpEngineTime by more than
+/- 150 seconds.
If the message is considered to be outside of the Time Window
then the usmStatsNotInTimeWindows counter is incremented and
an error indication (notInTimeWindow) together with the OID
and value of the incremented counter is returned to the
calling module.
b) If the extracted value of msgAuthoritativeEngineID is not the
same as the value snmpEngineID of the processing SNMP engine
(meaning this is not the authoritative SNMP engine), then:
1) if at least one of the following conditions is true:
- the extracted value of the msgAuthoritativeEngineBoots
field is greater than the local notion of the value of
snmpEngineBoots; or,
Blumenthal & Wijnen Standards Track [Page 27]RFC 2274 USM for SNMPv3 January 1998
- the extracted value of the msgAuthoritativeEngineBoots
field is equal to the local notion of the value of
snmpEngineBoots, the extracted value of
msgAuthoritativeEngineTime field is greater than the
value of latestReceivedEngineTime,
then the LCD entry corresponding to the extracted value
of the msgAuthoritativeEngineID field is updated, by
setting:
- the local notion of the value of snmpEngineBoots to
the value of the msgAuthoritativeEngineBoots field,
- the local notion of the value of snmpEngineTime to
the value of the msgAuthoritativeEngineTime field,
and
- the latestReceivedEngineTime to the value of the
value of the msgAuthoritativeEngineTime field.
2) if any of the following conditions is true, then the
message is considered to be outside of the Time Window:
- the local notion of the value of snmpEngineBoots is
2147483647;
- the value of the msgAuthoritativeEngineBoots field is
less than the local notion of the value of
snmpEngineBoots; or,
- the value of the msgAuthoritativeEngineBoots field is
equal to the local notion of the value of
snmpEngineBoots and the value of the
msgAuthoritativeEngineTime field is more than 150
seconds less than the local notion of of the value of
snmpEngineTime.
If the message is considered to be outside of the Time
Window then an error indication (notInTimeWindow) is
returned to the calling module;
Note that this means that a too old (possibly replayed)
message has been detected and is deemed unauthentic.
Note that this procedure allows for the value of
msgAuthoritativeEngineBoots in the message to be greater
than the local notion of the value of snmpEngineBoots to
allow for received messages to be accepted as authentic
Blumenthal & Wijnen Standards Track [Page 28]RFC 2274 USM for SNMPv3 January 1998
when received from an authoritative SNMP engine that has
re-booted since the receiving SNMP engine last
(re-)synchronized.
Note that this procedure does not allow for automatic
time synchronization if the non-authoritative SNMP engine
has a real out-of-sync situation whereby the authoritative
SNMP engine is more than 150 seconds behind the
non-authoritative SNMP engine.
8) a) If the securityLevel indicates that the message was protected
from disclosure, then the OCTET STRING representing the
encryptedPDU is decrypted according to the user's privacy
protocol to obtain an unencrypted serialized scopedPDU value.
To do so a call is made to the privacy module that implements
the user's privacy protocol according to the abstract
primitive:
statusInformation = -- success or failure
decryptData(
IN decryptKey -- the user's localized privKey
IN privParameters -- as received on the wire
IN encryptedData -- encryptedPDU as received
OUT decryptedData -- serialized decrypted scopedPDU
)
statusInformation
indicates if the decryption process was successful or not.
decryptKey
the user's localized private privKey is the secret key that
can be used by the decryption algorithm.
privParameters
the msgPrivacyParameters, encoded as an OCTET STRING.
encryptedData
the encryptedPDU represents the encrypted scopedPDU, encoded
as an OCTET STRING.
decryptedData
the serialized scopedPDU if decryption is successful.
If the privacy module returns failure, then the message can
not be processed, so the usmStatsDecryptionErrors counter is
incremented and an error indication (decryptionError) together
with the OID and value of the incremented counter is returned
to the calling module.
If the privacy module returns success, then the decrypted
scopedPDU is the message payload to be returned to the calling
module.
Blumenthal & Wijnen Standards Track [Page 29]RFC 2274 USM for SNMPv3 January 1998
Otherwise,
b) The scopedPDU component is assumed to be in plain text
and is the message payload to be returned to the calling
module.
9) The maxSizeResponseScopedPDU is calculated. This is the
maximum size allowed for a scopedPDU for a possible Response
message. Provision is made for a message header that allows the
same securityLevel as the received Request.
10) The securityName for the user is retrieved from the
usmUserTable.
11) The security data is cached as cachedSecurityData, so that a
possible response to this message can and will use the same
authentication and privacy secrets, the same securityLevel and
the same value for msgAuthoritativeEngineID. Information to be
saved/cached is as follows:
msgUserName,
usmUserAuthProtocol, usmUserAuthKey
usmUserPrivProtocol, usmUserPrivKey
securityEngineID, securityLevel
12) The statusInformation is set to success and a return is made to
the calling module passing back the OUT parameters as specified
in the processIncomingMsg primitive.
4. Discovery
The User-based Security Model requires that a discovery process
obtains sufficient information about other SNMP engines in order to
communicate with them. Discovery requires an non-authoritative SNMP
engine to learn the authoritative SNMP engine's snmpEngineID value
before communication may proceed. This may be accomplished by
generating a Request message with a securityLevel of noAuthNoPriv, a
msgUserName of "initial", a msgAuthoritativeEngineID value of zero
length, and the varBindList left empty. The response to this message
will be a Report message containing the snmpEngineID of the
authoritative SNMP engine as the value of the
msgAuthoritativeEngineID field within the msgSecurityParameters
field. It contains a Report PDU with the usmStatsUnknownEngineIDs
counter in the varBindList.
If authenticated communication is required, then the discovery
process should also establish time synchronization with the
authoritative SNMP engine. This may be accomplished by sending an
Blumenthal & Wijnen Standards Track [Page 30]RFC 2274 USM for SNMPv3 January 1998
authenticated Request message with the value of
msgAuthoritativeEngineID set to the newly learned snmpEngineID and
with the values of msgAuthoritativeEngineBoots and
msgAuthoritativeEngineTime set to zero. The response to this
authenticated message will be a Report message containing the up to
date values of the authoritative SNMP engine's snmpEngineBoots and
snmpEngineTime as the value of the msgAuthoritativeEngineBoots and
msgAuthoritativeEngineTime fields respectively. It also contains the
usmStatsNotInTimeWindows counter in the varBindList of the Report
PDU. The time synchronization then happens automatically as part of
the procedures in section 3.2 step 7b. See also section 2.3.
5. Definitions
SNMP-USER-BASED-SM-MIB DEFINITIONS ::= BEGIN
IMPORTS
MODULE-IDENTITY, OBJECT-TYPE,
OBJECT-IDENTITY,
snmpModules, Counter32 FROM SNMPv2-SMI
TEXTUAL-CONVENTION, TestAndIncr,
RowStatus, RowPointer,
StorageType, AutonomousType FROM SNMPv2-TC
MODULE-COMPLIANCE, OBJECT-GROUP FROM SNMPv2-CONF
SnmpAdminString, SnmpEngineID,
snmpAuthProtocols, snmpPrivProtocols FROM SNMP-FRAMEWORK-MIB;
snmpUsmMIB MODULE-IDENTITY
LAST-UPDATED "9711200000Z" -- 20 Nov 1997, midnight
ORGANIZATION "SNMPv3 Working Group"
CONTACT-INFO "WG-email: snmpv3@tis.com
Subscribe: majordomo@tis.com
In msg body: subscribe snmpv3
Chair: Russ Mundy
Trusted Information Systems
postal: 3060 Washington Rd
Glenwood MD 21738
USA
email: mundy@tis.com
phone: +1-301-854-6889
Co-editor Uri Blumenthal
IBM T. J. Watson Research
postal: 30 Saw Mill River Pkwy,
Hawthorne, NY 10532
USA
email: uri@watson.ibm.com
Blumenthal & Wijnen Standards Track [Page 31]RFC 2274 USM for SNMPv3 January 1998
phone: +1-914-784-7964
Co-editor: Bert Wijnen
IBM T. J. Watson Research
postal: Schagen 33
3461 GL Linschoten
Netherlands
email: wijnen@vnet.ibm.com
phone: +31-348-432-794
"
DESCRIPTION "The management information definitions for the
SNMP User-based Security Model.
"
::= { snmpModules 15 }
-- Administrative assignments ****************************************
usmMIBObjects OBJECT IDENTIFIER ::= { snmpUsmMIB 1 }
usmMIBConformance OBJECT IDENTIFIER ::= { snmpUsmMIB 2 }
-- Identification of Authentication and Privacy Protocols ************
usmNoAuthProtocol OBJECT-IDENTITY
STATUS current
DESCRIPTION "No Authentication Protocol."
::= { snmpAuthProtocols 1 }
usmHMACMD5AuthProtocol OBJECT-IDENTITY
STATUS current
DESCRIPTION "The HMAC-MD5-96 Digest Authentication Protocol."
REFERENCE "- H. Krawczyk, M. Bellare, R. Canetti HMAC:
Keyed-Hashing for Message Authentication,
RFC2104, Feb 1997.
- Rivest, R., Message Digest Algorithm MD5, RFC1321.
"
::= { snmpAuthProtocols 2 }
usmHMACSHAAuthProtocol OBJECT-IDENTITY
STATUS current
DESCRIPTION "The HMAC-SHA-96 Digest Authentication Protocol."
REFERENCE "- H. Krawczyk, M. Bellare, R. Canetti, HMAC:
Keyed-Hashing for Message Authentication,
RFC2104, Feb 1997.
- Secure Hash Algorithm. NIST FIPS 180-1.
"
::= { snmpAuthProtocols 3 }
Blumenthal & Wijnen Standards Track [Page 32]RFC 2274 USM for SNMPv3 January 1998
usmNoPrivProtocol OBJECT-IDENTITY
STATUS current
DESCRIPTION "No Privacy Protocol."
::= { snmpPrivProtocols 1 }
usmDESPrivProtocol OBJECT-IDENTITY
STATUS current
DESCRIPTION "The CBC-DES Symmetric Encryption Protocol."
REFERENCE "- Data Encryption Standard, National Institute of
Standards and Technology. Federal Information
Processing Standard (FIPS) Publication 46-1.
Supersedes FIPS Publication 46,
(January, 1977; reaffirmed January, 1988).
- Data Encryption Algorithm, American National
Standards Institute. ANSI X3.92-1981,
(December, 1980).
- DES Modes of Operation, National Institute of
Standards and Technology. Federal Information
Processing Standard (FIPS) Publication 81,
(December, 1980).
- Data Encryption Algorithm - Modes of Operation,
American National Standards Institute.
ANSI X3.106-1983, (May 1983).
"
::= { snmpPrivProtocols 2 }
-- Textual Conventions ***********************************************
KeyChange ::= TEXTUAL-CONVENTION
STATUS current
DESCRIPTION
"Every definition of an object with this syntax must identify
a protocol P, a secret key K, and a hash algorithm H
that produces output of L octets.
The object's value is a manager-generated, partially-random
value which, when modified, causes the value of the secret
key K, to be modified via a one-way function.
The value of an instance of this object is the concatenation
of two components: first a 'random' component and then a
'delta' component.
Blumenthal & Wijnen Standards Track [Page 33]RFC 2274 USM for SNMPv3 January 1998
The lengths of the random and delta components
are given by the corresponding value of the protocol P;
if P requires K to be a fixed length, the length of both the
random and delta components is that fixed length; if P
allows the length of K to be variable up to a particular
maximum length, the length of the random component is that
maximum length and the length of the delta component is any
length less than or equal to that maximum length.
For example, usmHMACMD5AuthProtocol requires K to be a fixed
length of 16 octets and L - of 16 octets.
usmHMACSHAAuthProtocol requires K to be a fixed length of
20 octets and L - of 20 octets. Other protocols may define
other sizes, as deemed appropriate.
When a requestor wants to change the old key K to a new
key keyNew on a remote entity, the 'random' component is
obtained from either a true random generator, or from a
pseudorandom generator, and the 'delta' component is
computed as follows:
- a temporary variable is initialized to the existing value
of K;
- if the length of the keyNew is greater than L octets,
then:
- the random component is appended to the value of the
temporary variable, and the result is input to the
the hash algorithm H to produce a digest value, and
the temporary variable is set to this digest value;
- the value of the temporary variable is XOR-ed with
the first (next) L-octets (16 octets in case of MD5)
of the keyNew to produce the first (next) L-octets
(16 octets in case of MD5) of the 'delta' component.
- the above two steps are repeated until the unused
portion of the delta component is L octets or less,
- the random component is appended to the value of the
temporary variable, and the result is input to the
hash algorithm H to produce a digest value;
- this digest value, truncated if necessary to be the same
length as the unused portion of the keyNew, is XOR-ed
with the unused portion of the keyNew to produce the
(final portion of the) 'delta' component.
For example, using MD5 as the hash algorithm H:
iterations = (lenOfDelta - 1)/16; /* integer division */
temp = keyOld;
for (i = 0; i < iterations; i++) {
temp = MD5 (temp || random);
Blumenthal & Wijnen Standards Track [Page 34]RFC 2274 USM for SNMPv3 January 1998
delta[i*16 .. (i*16)+15] =
temp XOR keyNew[i*16 .. (i*16)+15];
}
temp = MD5 (temp || random);
delta[i*16 .. lenOfDelta-1] =
temp XOR keyNew[i*16 .. lenOfDelta-1];
The 'random' and 'delta' components are then concatenated as
described above, and the resulting octet string is sent to
the receipient as the new value of an instance of this
object.
At the receiver side, when an instance of this object is set
to a new value, then a new value of K is computed as follows:
- a temporary variable is initialized to the existing value
of K;
- if the length of the delta component is greater than L
octets, then:
- the random component is appended to the value of the
temporary variable, and the result is input to the
the hash algorithm H to produce a digest value, and
the temporary variable is set to this digest value;
- the value of the temporary variable is XOR-ed with
the first (next) L-octets (16 octets in case of MD5)
of the delta component to produce the first (next)
L-octets (16 octets in case of MD5) of the new value
of K.
- the above two steps are repeated until the unused
portion of the delta component is L octets or less,
- the random component is appended to the value of the
temporary variable, and the result is input to the
hash algorithm H to produce a digest value;
- this digest value, truncated if necessary to be the same
length as the unused portion of the delta component, is
XOR-ed with the unused portion of the delta component to
produce the (final portion of the) new value of K.
For example, using MD5 as the hash algorithm H:
iterations = (lenOfDelta - 1)/16; /* integer division */
temp = keyOld;
for (i = 0; i < iterations; i++) {
temp = MD5 (temp || random);
keyNew[i*16 .. (i*16)+15] =
temp XOR delta[i*16 .. (i*16)+15];
}
temp = MD5 (temp || random);
Blumenthal & Wijnen Standards Track [Page 35]RFC 2274 USM for SNMPv3 January 1998
keyNew[i*16 .. lenOfDelta-1] =
temp XOR delta[i*16 .. lenOfDelta-1];
The value of an object with this syntax, whenever it is
retrieved by the management protocol, is always the zero
length string.
"
SYNTAX OCTET STRING
-- Statistics for the User-based Security Model **********************
usmStats OBJECT IDENTIFIER ::= { usmMIBObjects 1 }
usmStatsUnsupportedSecLevels OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The total number of packets received by the SNMP
engine which were dropped because they requested a
securityLevel that was unknown to the SNMP engine
or otherwise unavailable.
"
::= { usmStats 1 }
usmStatsNotInTimeWindows OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The total number of packets received by the SNMP
engine which were dropped because they appeared
outside of the authoritative SNMP engine's window.
"
::= { usmStats 2 }
usmStatsUnknownUserNames OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The total number of packets received by the SNMP
engine which were dropped because they referenced a
user that was not known to the SNMP engine.
"
::= { usmStats 3 }
usmStatsUnknownEngineIDs OBJECT-TYPE
Blumenthal & Wijnen Standards Track [Page 36]RFC 2274 USM for SNMPv3 January 1998
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The total number of packets received by the SNMP
engine which were dropped because they referenced an
snmpEngineID that was not known to the SNMP engine.
"
::= { usmStats 4 }
usmStatsWrongDigests OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The total number of packets received by the SNMP
engine which were dropped because they didn't
contain the expected digest value.
"
::= { usmStats 5 }
usmStatsDecryptionErrors OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The total number of packets received by the SNMP
engine which were dropped because they could not be
decrypted.
"
::= { usmStats 6 }
-- The usmUser Group ************************************************
usmUser OBJECT IDENTIFIER ::= { usmMIBObjects 2 }
usmUserSpinLock OBJECT-TYPE
SYNTAX TestAndIncr
MAX-ACCESS read-write
STATUS current
DESCRIPTION "An advisory lock used to allow several cooperating
Command Generator Applications to coordinate their
use of facilities to alter secrets in the
usmUserTable.
"
::= { usmUser 1 }
-- The table of valid users for the User-based Security Model ********
usmUserTable OBJECT-TYPE
SYNTAX SEQUENCE OF UsmUserEntry
Blumenthal & Wijnen Standards Track [Page 37]RFC 2274 USM for SNMPv3 January 1998
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION "The table of users configured in the SNMP engine's
Local Configuration Datastore (LCD)."
::= { usmUser 2 }
usmUserEntry OBJECT-TYPE
SYNTAX UsmUserEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION "A user configured in the SNMP engine's Local
Configuration Datastore (LCD) for the User-based
Security Model.
"
INDEX { usmUserEngineID,
usmUserName
}
::= { usmUserTable 1 }
UsmUserEntry ::= SEQUENCE
{
usmUserEngineID SnmpEngineID,
usmUserName SnmpAdminString,
usmUserSecurityName SnmpAdminString,
usmUserCloneFrom RowPointer,
usmUserAuthProtocol AutonomousType,
usmUserAuthKeyChange KeyChange,
usmUserOwnAuthKeyChange KeyChange,
usmUserPrivProtocol AutonomousType,
usmUserPrivKeyChange KeyChange,
usmUserOwnPrivKeyChange KeyChange,
usmUserPublic OCTET STRING,
usmUserStorageType StorageType,
usmUserStatus RowStatus
}
usmUserEngineID OBJECT-TYPE
SYNTAX SnmpEngineID
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION "An SNMP engine's administratively-unique identifier.
In a simple agent, this value is always that agent's
own snmpEngineID value.
The value can also take the value of the snmpEngineID
of a remote SNMP engine with which this user can
communicate.
Blumenthal & Wijnen Standards Track [Page 38]RFC 2274 USM for SNMPv3 January 1998
"
::= { usmUserEntry 1 }
usmUserName OBJECT-TYPE
SYNTAX SnmpAdminString (SIZE(1..32))
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION "A human readable string representing the name of
the user.
This is the (User-based Security) Model dependent
security ID.
"
::= { usmUserEntry 2 }
usmUserSecurityName OBJECT-TYPE
SYNTAX SnmpAdminString
MAX-ACCESS read-only
STATUS current
DESCRIPTION "A human readable string representing the user in
Security Model independent format.
The default transformation of the User-based Security
Model dependent security ID to the securityName and
vice versa is the identity function so that the
securityName is the same as the userName.
"
::= { usmUserEntry 3 }
usmUserCloneFrom OBJECT-TYPE
SYNTAX RowPointer
MAX-ACCESS read-create
STATUS current
DESCRIPTION "A pointer to another conceptual row in this
usmUserTable. The user in this other conceptual
row is called the clone-from user.
When a new user is created (i.e., a new conceptual
row is instantiated in this table), the privacy and
authentication parameters of the new user are cloned
from its clone-from user.
The first time an instance of this object is set by
a management operation (either at or after its
instantiation), the cloning process is invoked.
Subsequent writes are successful but invoke no
action to be taken by the receiver.
The cloning process fails with an 'inconsistentName'
Blumenthal & Wijnen Standards Track [Page 39]RFC 2274 USM for SNMPv3 January 1998
error if the conceptual row representing the
clone-from user is not in an active state when the
cloning process is invoked.
Cloning also causes the initial values of the secret
authentication key and the secret encryption key of
the new user to be set to the same value as the
corresponding secret of the clone-from user.
When this object is read, the ZeroDotZero OID
is returned.
"
::= { usmUserEntry 4 }
usmUserAuthProtocol OBJECT-TYPE
SYNTAX AutonomousType
MAX-ACCESS read-create
STATUS current
DESCRIPTION "An indication of whether messages sent on behalf of
this user to/from the SNMP engine identified by
usmUserEngineID, can be authenticated, and if so,
the type of authentication protocol which is used.
An instance of this object is created concurrently
with the creation of any other object instance for
the same user (i.e., as part of the processing of
the set operation which creates the first object
instance in the same conceptual row). Once created,
the value of an instance of this object can not be
changed.
If a set operation tries to set a value for an unknown
or unsupported protocol, then a wrongValue error must
be returned.
"
DEFVAL { usmHMACMD5AuthProtocol }
::= { usmUserEntry 5 }
usmUserAuthKeyChange OBJECT-TYPE
SYNTAX KeyChange -- typically (SIZE (0..32))
MAX-ACCESS read-create
STATUS current
DESCRIPTION "An object, which when modified, causes the secret
authentication key used for messages sent on behalf
of this user to/from the SNMP engine identified by
usmUserEngineID, to be modified via a one-way
function.
Blumenthal & Wijnen Standards Track [Page 40]RFC 2274 USM for SNMPv3 January 1998
The associated protocol is the usmUserAuthProtocol.
The associated secret key is the user's secret
authentication key (authKey). The associated hash
algorithm is the algorithm used by the user's
usmUserAuthProtocol.
When creating a new user, it is an 'inconsistentName'
error for a Set operation to refer to this object
unless it is previously or concurrently initialized
through a set operation on the corresponding value
of usmUserCloneFrom.
"
DEFVAL { ''H } -- the empty string
::= { usmUserEntry 6 }
usmUserOwnAuthKeyChange OBJECT-TYPE
SYNTAX KeyChange -- typically (SIZE (0..32))
MAX-ACCESS read-create
STATUS current
DESCRIPTION "Behaves exactly as usmUserAuthKeyChange, with one
notable difference: in order for the Set operation
to succeed, the usmUserName of the operation
requester must match the usmUserName that
indexes the row which is targeted by this
operation.
The idea here is that access to this column can be
public, since it will only allow a user to change
his own secret authentication key (authKey).
"
DEFVAL { ''H } -- the empty string
::= { usmUserEntry 7 }
usmUserPrivProtocol OBJECT-TYPE
SYNTAX AutonomousType
MAX-ACCESS read-create
STATUS current
DESCRIPTION "An indication of whether messages sent on behalf of
this user to/from the SNMP engine identified by
usmUserEngineID, can be protected from disclosure,
and if so, the type of privacy protocol which is used.
An instance of this object is created concurrently
with the creation of any other object instance for
the same user (i.e., as part of the processing of
the set operation which creates the first object
instance in the same conceptual row). Once created,
the value of an instance of this object can not be
Blumenthal & Wijnen Standards Track [Page 41]RFC 2274 USM for SNMPv3 January 1998
changed.
If a set operation tries to set a value for an unknown
or unsupported protocol, then a wrongValue error must
be returned.
"
DEFVAL { usmNoPrivProtocol }
::= { usmUserEntry 8 }
usmUserPrivKeyChange OBJECT-TYPE
SYNTAX KeyChange -- typically (SIZE (0..32))
MAX-ACCESS read-create
STATUS current
DESCRIPTION "An object, which when modified, causes the secret
encryption key used for messages sent on behalf
of this user to/from the SNMP engine identified by
usmUserEngineID, to be modified via a one-way
function.
The associated protocol is the usmUserPrivProtocol.
The associated secret key is the user's secret
privacy key (privKey). The associated hash
algorithm is the algorithm used by the user's
usmUserAuthProtocol.
When creating a new user, it is an 'inconsistentName'
error for a set operation to refer to this object
unless it is previously or concurrently initialized
through a set operation on the corresponding value
of usmUserCloneFrom.
"
DEFVAL { ''H } -- the empty string
::= { usmUserEntry 9 }
usmUserOwnPrivKeyChange OBJECT-TYPE
SYNTAX KeyChange -- typically (SIZE (0..32))
MAX-ACCESS read-create
STATUS current
DESCRIPTION "Behaves exactly as usmUserPrivKeyChange, with one
notable difference: in order for the Set operation
to succeed, the usmUserName of the operation
requester must match the usmUserName that indexes
the row which is targeted by this operation.
The idea here is that access to this column can be
public, since it will only allow a user to change
his own secret privacy key (privKey).
"
Blumenthal & Wijnen Standards Track [Page 42]RFC 2274 USM for SNMPv3 January 1998
DEFVAL { ''H } -- the empty string
::= { usmUserEntry 10 }
usmUserPublic OBJECT-TYPE
SYNTAX OCTET STRING (SIZE(0..32))
MAX-ACCESS read-create
STATUS current
DESCRIPTION "A publicly-readable value which is written as part
of the procedure for changing a user's secret
authentication and/or privacy key, and later read to
determine whether the change of the secret was
effected.
"
DEFVAL { ''H } -- the empty string
::= { usmUserEntry 11 }
usmUserStorageType OBJECT-TYPE
SYNTAX StorageType
MAX-ACCESS read-create
STATUS current
DESCRIPTION "The storage type for this conceptual row.
Conceptual rows having the value 'permanent'
must allow write-access at a minimum to:
- usmUserAuthKeyChange, usmUserOwnAuthKeyChange
and usmUserPublic for a user who employs
authentication, and
- usmUserPrivKeyChange, usmUserOwnPrivKeyChange
and usmUserPublic for a user who employs
privacy.
Note that any user who employs authentication or
privacy must allow its secret(s) to be updated and
thus cannot be 'readOnly'.
"
DEFVAL { nonVolatile }
::= { usmUserEntry 12 }
usmUserStatus OBJECT-TYPE
SYNTAX RowStatus
MAX-ACCESS read-create
STATUS current
DESCRIPTION "The status of this conceptual row.
Until instances of all corresponding columns are
appropriately configured, the value of the
corresponding instance of the usmUserStatus column
Blumenthal & Wijnen Standards Track [Page 43]RFC 2274 USM for SNMPv3 January 1998
is 'notReady'.
In particular, a newly created row cannot be made
active until the corresponding usmUserCloneFrom,
usmUserAuthKeyChange, usmUserOwnAuthKeyChange,
usmUserPrivKeyChange and usmUserOwnPrivKeyChange
have all been set.
The RowStatus TC [RFC1903] requires that this
DESCRIPTION clause states under which circumstances
other objects in this row can be modified:
The value of this object has no effect on whether
other objects in this conceptual row can be modified.
"
::= { usmUserEntry 13 }
-- Conformance Information *******************************************
usmMIBCompliances OBJECT IDENTIFIER ::= { usmMIBConformance 1 }
usmMIBGroups OBJECT IDENTIFIER ::= { usmMIBConformance 2 }
-- Compliance statements
usmMIBCompliance MODULE-COMPLIANCE
STATUS current
DESCRIPTION "The compliance statement for SNMP engines which
implement the SNMP-USER-BASED-SM-MIB.
"
MODULE -- this module
MANDATORY-GROUPS { usmMIBBasicGroup }
OBJECT usmUserAuthProtocol
MIN-ACCESS read-only
DESCRIPTION "Write access is not required."
OBJECT usmUserPrivProtocol
MIN-ACCESS read-only
DESCRIPTION "Write access is not required."
::= { usmMIBCompliances 1 }
-- Units of compliance
usmMIBBasicGroup OBJECT-GROUP
OBJECTS {
usmStatsUnsupportedSecLevels,
usmStatsNotInTimeWindows,
Blumenthal & Wijnen Standards Track [Page 44]RFC 2274 USM for SNMPv3 January 1998
usmStatsUnknownUserNames,
usmStatsUnknownEngineIDs,
usmStatsWrongDigests,
usmStatsDecryptionErrors,
usmUserSpinLock,
usmUserSecurityName,
usmUserCloneFrom,
usmUserAuthProtocol,
usmUserAuthKeyChange,
usmUserOwnAuthKeyChange,
usmUserPrivProtocol,
usmUserPrivKeyChange,
usmUserOwnPrivKeyChange,
usmUserPublic,
usmUserStorageType,
usmUserStatus
}
STATUS current
DESCRIPTION "A collection of objects providing for configuration
of an SNMP engine which implements the SNMP
User-based Security Model.
"
::= { usmMIBGroups 1 }
END
6. HMAC-MD5-96 Authentication Protocol
This section describes the HMAC-MD5-96 authentication protocol. This
authentication protocol is the first defined for the User-based
Security Model. It uses MD5 hash-function which is described in
[MD5], in HMAC mode described in [RFC2104], truncating the output to
96 bits.
This protocol is identified by usmHMACMD5AuthProtocol.
Over time, other authentication protocols may be defined either as a
replacement of this protocol or in addition to this protocol.
6.1. Mechanisms
- In support of data integrity, a message digest algorithm is
required. A digest is calculated over an appropriate portion of an
SNMP message and included as part of the message sent to the
recipient.
Blumenthal & Wijnen Standards Track [Page 45]RFC 2274 USM for SNMPv3 January 1998
- In support of data origin authentication and data integrity,
a secret value is prepended to SNMP message prior to computing the
digest; the calculated digest is partially inserted into the SNMP
message prior to transmission, and the prepended value is not
transmitted. The secret value is shared by all SNMP engines
authorized to originate messages on behalf of the appropriate user.
6.1.1. Digest Authentication Mechanism
The Digest Authentication Mechanism defined in this memo provides
for:
- verification of the integrity of a received message, i.e., the
message received is the message sent.
The integrity of the message is protected by computing a digest
over an appropriate portion of the message. The digest is computed
by the originator of the message, transmitted with the message, and
verified by the recipient of the message.
- verification of the user on whose behalf the message was generated.
A secret value known only to SNMP engines authorized to generate
messages on behalf of a user is used in HMAC mode (see [RFC2104]).
It also recommends the hash-function output used as Message
Authentication Code, to be truncated.
This protocol uses the MD5 [MD5] message digest algorithm. A 128-bit
MD5 digest is calculated in a special (HMAC) way over the designated
portion of an SNMP message and the first 96 bits of this digest is
included as part of the message sent to the recipient. The size of
the digest carried in a message is 12 octets. The size of the private
authentication key (the secret) is 16 octets. For the details see
section 6.3.
6.2. Elements of the Digest Authentication Protocol
This section contains definitions required to realize the
authentication module defined in this section of this memo.
6.2.1. Users
Authentication using this authentication protocol makes use of a
defined set of userNames. For any user on whose behalf a message must
be authenticated at a particular SNMP engine, that SNMP engine must
have knowledge of that user. An SNMP engine that wishes to
Blumenthal & Wijnen Standards Track [Page 46]RFC 2274 USM for SNMPv3 January 1998
communicate with another SNMP engine must also have knowledge of a
user known to that engine, including knowledge of the applicable
attributes of that user.
A user and its attributes are defined as follows:
<userName>
A string representing the name of the user.
<authKey>
A user's secret key to be used when calculating a digest.
It MUST be 16 octets long for MD5.
6.2.2. msgAuthoritativeEngineID
The msgAuthoritativeEngineID value contained in an authenticated
message specifies the authoritative SNMP engine for that particular
message (see the definition of SnmpEngineID in the SNMP Architecture
document [RFC2271]).
The user's (private) authentication key is normally different at each
authoritative SNMP engine and so the snmpEngineID is used to select
the proper key for the authentication process.
6.2.3. SNMP Messages Using this Authentication Protocol
Messages using this authentication protocol carry a
msgAuthenticationParameters field as part of the
msgSecurityParameters. For this protocol, the
msgAuthenticationParameters field is the serialized OCTET STRING
representing the first 12 octets of the HMAC-MD5-96 output done over
the wholeMsg.
The digest is calculated over the wholeMsg so if a message is
authenticated, that also means that all the fields in the message are
intact and have not been tampered with.
6.2.4. Services provided by the HMAC-MD5-96 Authentication Module
This section describes the inputs and outputs that the HMAC-MD5-96
Authentication module expects and produces when the User-based
Security module calls the HMAC-MD5-96 Authentication module for
services.
6.2.4.1. Services for Generating an Outgoing SNMP Message
The HMAC-MD5-96 authentication protocol assumes that the selection of
the authKey is done by the caller and that the caller passes the
secret key to be used.
Blumenthal & Wijnen Standards Track [Page 47]RFC 2274 USM for SNMPv3 January 1998
Upon completion the authentication module returns statusInformation
and, if the message digest was correctly calculated, the wholeMsg
with the digest inserted at the proper place. The abstract service
primitive is:
statusInformation = -- success or failure
authenticateOutgoingMsg(
IN authKey -- secret key for authentication
IN wholeMsg -- unauthenticated complete message
OUT authenticatedWholeMsg -- complete authenticated message
)
The abstract data elements are:
statusInformation
An indication of whether the authentication process was
successful. If not it is an indication of the problem.
authKey
The secret key to be used by the authentication algorithm.
The length of this key MUST be 16 octets.
wholeMsg
The message to be authenticated.
authenticatedWholeMsg
The authenticated message (including inserted digest) on output.
Note, that authParameters field is filled by the authentication
module and this field should be already present in the wholeMsg
before the Message Authentication Code (MAC) is generated.
6.2.4.2. Services for Processing an Incoming SNMP Message
The HMAC-MD5-96 authentication protocol assumes that the selection of
the authKey is done by the caller and that the caller passes the
secret key to be used.
Upon completion the authentication module returns statusInformation
and, if the message digest was correctly calculated, the wholeMsg as
it was processed. The abstract service primitive is:
statusInformation = -- success or failure
authenticateIncomingMsg(
IN authKey -- secret key for authentication
IN authParameters -- as received on the wire
IN wholeMsg -- as received on the wire
OUT authenticatedWholeMsg -- complete authenticated message
)
The abstract data elements are:
Blumenthal & Wijnen Standards Track [Page 48]RFC 2274 USM for SNMPv3 January 1998
statusInformation
An indication of whether the authentication process was
successful. If not it is an indication of the problem.
authKey
The secret key to be used by the authentication algorithm.
The length of this key MUST be 16 octets.
authParameters
The authParameters from the incoming message.
wholeMsg
The message to be authenticated on input and the authenticated
message on output.
authenticatedWholeMsg
The whole message after the authentication check is complete.
6.3. Elements of Procedure
This section describes the procedures for the HMAC-MD5-96
authentication protocol.
6.3.1. Processing an Outgoing Message
This section describes the procedure followed by an SNMP engine
whenever it must authenticate an outgoing message using the
usmHMACMD5AuthProtocol.
1) The msgAuthenticationParameters field is set to the
serialization, according to the rules in [RFC1906], of an OCTET
STRING containing 12 zero octets.
2) From the secret authKey, two keys K1 and K2 are derived:
a) extend the authKey to 64 octets by appending 48 zero
octets; save it as extendedAuthKey
b) obtain IPAD by replicating the octet 0x36 64 times;
c) obtain K1 by XORing extendedAuthKey with IPAD;
d) obtain OPAD by replicating the octet 0x5C 64 times;
e) obtain K2 by XORing extendedAuthKey with OPAD.
4) Prepend K1 to the wholeMsg and calculate MD5 digest over it
according to [MD5].
5) Prepend K2 to the result of the step 4 and calculate MD5 digest
over it according to [MD5]. Take the first 12 octets of the final
digest - this is Message Authentication Code (MAC).
6) Replace the msgAuthenticationParameters field with MAC obtained
in the step 5.
Blumenthal & Wijnen Standards Track [Page 49]RFC 2274 USM for SNMPv3 January 1998
7) The authenticatedWholeMsg is then returned to the caller
together with statusInformation indicating success.
6.3.2. Processing an Incoming Message
This section describes the procedure followed by an SNMP engine
whenever it must authenticate an incoming message using the
usmHMACMD5AuthProtocol.
1) If the digest received in the msgAuthenticationParameters field
is not 12 octets long, then an failure and an errorIndication
(authenticationError) is returned to the calling module.
2) The MAC received in the msgAuthenticationParameters field
is saved.
3) The digest in the msgAuthenticationParameters field is replaced
by the 12 zero octets.
4) From the secret authKey, two keys K1 and K2 are derived:
a) extend the authKey to 64 octets by appending 48 zero
octets; save it as extendedAuthKey
b) obtain IPAD by replicating the octet 0x36 64 times;
c) obtain K1 by XORing extendedAuthKey with IPAD;
d) obtain OPAD by replicating the octet 0x5C 64 times;
e) obtain K2 by XORing extendedAuthKey with OPAD.
5) The MAC is calculated over the wholeMsg:
a) prepend K1 to the wholeMsg and calculate the MD5 digest
over it;
b) prepend K2 to the result of step 5.a and calculate the
MD5 digest over it;
c) first 12 octets of the result of step 5.b is the MAC.
The msgAuthenticationParameters field is replaced with the MAC
value that was saved in step 2.
6) Then the newly calculated MAC is compared with the MAC
saved in step 2. If they do not match, then an failure and an
errorIndication (authenticationFailure) is returned to the
calling module.
7) The authenticatedWholeMsg and statusInformation indicating
success are then returned to the caller.
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7. HMAC-SHA-96 Authentication Protocol
This section describes the HMAC-SHA-96 authentication protocol. This
protocol uses the SHA hash-function which is described in [SHA-NIST],
in HMAC mode described in [RFC2104], truncating the output to 96
bits.
This protocol is identified by usmHMACSHAAuthProtocol.
Over time, other authentication protocols may be defined either as a
replacement of this protocol or in addition to this protocol.
7.1. Mechanisms
- In support of data integrity, a message digest algorithm is
required. A digest is calculated over an appropriate portion of an
SNMP message and included as part of the message sent to the
recipient.
- In support of data origin authentication and data integrity,
a secret value is prepended to the SNMP message prior to computing
the digest; the calculated digest is then partially inserted into
the message prior to transmission. The prepended secret is not
transmitted. The secret value is shared by all SNMP engines
authorized to originate messages on behalf of the appropriate user.
7.1.1. Digest Authentication Mechanism
The Digest Authentication Mechanism defined in this memo provides
for:
- verification of the integrity of a received message, i.e., the
the message received is the message sent.
The integrity of the message is protected by computing a digest
over an appropriate portion of the message. The digest is computed
by the originator of the message, transmitted with the message, and
verified by the recipient of the message.
- verification of the user on whose behalf the message was generated.
A secret value known only to SNMP engines authorized to generate
messages on behalf of a user is used in HMAC mode (see [RFC2104]).
It also recommends the hash-function output used as Message
Authentication Code, to be truncated.
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This mechanism uses the SHA [SHA-NIST] message digest algorithm. A
160-bit SHA digest is calculated in a special (HMAC) way over the
designated portion of an SNMP message and the first 96 bits of this
digest is included as part of the message sent to the recipient. The
size of the digest carried in a message is 12 octets. The size of the
private authentication key (the secret) is 20 octets. For the details
see section 7.3.
7.2. Elements of the HMAC-SHA-96 Authentication Protocol
This section contains definitions required to realize the
authentication module defined in this section of this memo.
7.2.1. Users
Authentication using this authentication protocol makes use of a
defined set of userNames. For any user on whose behalf a message
must be authenticated at a particular SNMP engine, that SNMP engine
must have knowledge of that user. An SNMP engine that wishes to
communicate with another SNMP engine must also have knowledge of a
user known to that engine, including knowledge of the applicable
attributes of that user.
A user and its attributes are defined as follows:
<userName>
A string representing the name of the user.
<authKey>
A user's secret key to be used when calculating a digest.
It MUST be 20 octets long for SHA.
7.2.2. msgAuthoritativeEngineID
The msgAuthoritativeEngineID value contained in an authenticated
message specifies the authoritative SNMP engine for that particular
message (see the definition of SnmpEngineID in the SNMP Architecture
document [RFC2271]).
The user's (private) authentication key is normally different at each
authoritative SNMP engine and so the snmpEngineID is used to select
the proper key for the authentication process.
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7.2.3. SNMP Messages Using this Authentication Protocol
Messages using this authentication protocol carry a
msgAuthenticationParameters field as part of the
msgSecurityParameters. For this protocol, the
msgAuthenticationParameters field is the serialized OCTET STRING
representing the first 12 octets of HMAC-SHA-96 output done over the
wholeMsg.
The digest is calculated over the wholeMsg so if a message is
authenticated, that also means that all the fields in the message are
intact and have not been tampered with.
7.2.4. Services provided by the HMAC-SHA-96 Authentication Module
This section describes the inputs and outputs that the HMAC-SHA-96
Authentication module expects and produces when the User-based
Security module calls the HMAC-SHA-96 Authentication module for
services.
7.2.4.1. Services for Generating an Outgoing SNMP Message
HMAC-SHA-96 authentication protocol assumes that the selection of the
authKey is done by the caller and that the caller passes the secret
key to be used.
Upon completion the authentication module returns statusInformation
and, if the message digest was correctly calculated, the wholeMsg
with the digest inserted at the proper place. The abstract service
primitive is:
statusInformation = -- success or failure
authenticateOutgoingMsg(
IN authKey -- secret key for authentication
IN wholeMsg -- unauthenticated complete message
OUT authenticatedWholeMsg -- complete authenticated message
)
The abstract data elements are:
statusInformation
An indication of whether the authentication process was
successful. If not it is an indication of the problem.
authKey
The secret key to be used by the authentication algorithm.
The length of this key MUST be 20 octets.
wholeMsg
The message to be authenticated.
Blumenthal & Wijnen Standards Track [Page 53]RFC 2274 USM for SNMPv3 January 1998
authenticatedWholeMsg
The authenticated message (including inserted digest) on output.
Note, that authParameters field is filled by the authentication
module and this field should be already present in the wholeMsg
before the Message Authentication Code (MAC) is generated.
7.2.4.2. Services for Processing an Incoming SNMP Message
HMAC-SHA-96 authentication protocol assumes that the selection of the
authKey is done by the caller and that the caller passes the secret
key to be used.
Upon completion the authentication module returns statusInformation
and, if the message digest was correctly calculated, the wholeMsg as
it was processed. The abstract service primitive is:
statusInformation = -- success or failure
authenticateIncomingMsg(
IN authKey -- secret key for authentication
IN authParameters -- as received on the wire
IN wholeMsg -- as received on the wire
OUT authenticatedWholeMsg -- complete authenticated message
)
The abstract data elements are:
statusInformation
An indication of whether the authentication process was
successful. If not it is an indication of the problem.
authKey
The secret key to be used by the authentication algorithm.
The length of this key MUST be 20 octets.
authParameters
The authParameters from the incoming message.
wholeMsg
The message to be authenticated on input and the authenticated
message on output.
authenticatedWholeMsg
The whole message after the authentication check is complete.
7.3. Elements of Procedure
This section describes the procedures for the HMAC-SHA-96
authentication protocol.
Blumenthal & Wijnen Standards Track [Page 54]RFC 2274 USM for SNMPv3 January 1998
7.3.1. Processing an Outgoing Message
This section describes the procedure followed by an SNMP engine
whenever it must authenticate an outgoing message using the
usmHMACSHAAuthProtocol.
1) The msgAuthenticationParameters field is set to the
serialization, according to the rules in [RFC1906], of an OCTET
STRING containing 12 zero octets.
2) From the secret authKey, two keys K1 and K2 are derived:
a) extend the authKey to 64 octets by appending 44 zero
octets; save it as extendedAuthKey
b) obtain IPAD by replicating the octet 0x36 64 times;
c) obtain K1 by XORing extendedAuthKey with IPAD;
d) obtain OPAD by replicating the octet 0x5C 64 times;
e) obtain K2 by XORing extendedAuthKey with OPAD.
3) Prepend K1 to the wholeMsg and calculate the SHA digest over it
according to [SHA-NIST].
4) Prepend K2 to the result of the step 4 and calculate SHA digest
over it according to [SHA-NIST]. Take the first 12 octets of the
final digest - this is Message Authentication Code (MAC).
5) Replace the msgAuthenticationParameters field with MAC obtained
in the step 5.
6) The authenticatedWholeMsg is then returned to the caller
together with statusInformation indicating success.
7.3.2. Processing an Incoming Message
This section describes the procedure followed by an SNMP engine
whenever it must authenticate an incoming message using the
usmHMACSHAAuthProtocol.
1) If the digest received in the msgAuthenticationParameters field
is not 12 octets long, then an failure and an errorIndication
(authenticationError) is returned to the calling module.
2) The MAC received in the msgAuthenticationParameters field
is saved.
3) The digest in the msgAuthenticationParameters field is
replaced by the 12 zero octets.
Blumenthal & Wijnen Standards Track [Page 55]RFC 2274 USM for SNMPv3 January 1998
4) From the secret authKey, two keys K1 and K2 are derived:
a) extend the authKey to 64 octets by appending 44 zero
octets; save it as extendedAuthKey
b) obtain IPAD by replicating the octet 0x36 64 times;
c) obtain K1 by XORing extendedAuthKey with IPAD;
d) obtain OPAD by replicating the octet 0x5C 64 times;
e) obtain K2 by XORing extendedAuthKey with OPAD.
5) The MAC is calculated over the wholeMsg:
a) prepend K1 to the wholeMsg and calculate the SHA digest
over it;
b) prepend K2 to the result of step 5.a and calculate the
SHA digest over it;
c) first 12 octets of the result of step 5.b is the MAC.
The msgAuthenticationParameters field is replaced with the MAC
value that was saved in step 2.
6) The the newly calculated MAC is compared with the MAC saved in
step 2. If they do not match, then a failure and an
errorIndication (authenticationFailure) are returned to the
calling module.
7) The authenticatedWholeMsg and statusInformation indicating
success are then returned to the caller.
8. CBC-DES Symmetric Encryption Protocol
This section describes the CBC-DES Symmetric Encryption Protocol.
This protocol is the first privacy protocol defined for the User-
based Security Model.
This protocol is identified by usmDESPrivProtocol.
Over time, other privacy protocols may be defined either as a
replacement of this protocol or in addition to this protocol.
8.1. Mechanisms
- In support of data confidentiality, an encryption algorithm is
required. An appropriate portion of the message is encrypted prior
to being transmitted. The User-based Security Model specifies that
the scopedPDU is the portion of the message that needs to be
encrypted.
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- A secret value in combination with a timeliness value is used
to create the en/decryption key and the initialization vector. The
secret value is shared by all SNMP engines authorized to originate
messages on behalf of the appropriate user.
8.1.1. Symmetric Encryption Protocol
The Symmetric Encryption Protocol defined in this memo provides
support for data confidentiality. The designated portion of an SNMP
message is encrypted and included as part of the message sent to the
recipient.
Two organizations have published specifications defining the DES: the
National Institute of Standards and Technology (NIST) [DES-NIST] and
the American National Standards Institute [DES-ANSI]. There is a
companion Modes of Operation specification for each definition
([DESO-NIST] and [DESO-ANSI], respectively).
The NIST has published three additional documents that implementors
may find useful.
- There is a document with guidelines for implementing and using
the DES, including functional specifications for the DES and its
modes of operation [DESG-NIST].
- There is a specification of a validation test suite for the DES
[DEST-NIST]. The suite is designed to test all aspects of the DES
and is useful for pinpointing specific problems.
- There is a specification of a maintenance test for the DES
[DESM-NIST]. The test utilizes a minimal amount of data and
processing to test all components of the DES. It provides a simple
yes-or-no indication of correct operation and is useful to run as
part of an initialization step, e.g., when a computer re-boots.
8.1.1.1. DES key and Initialization Vector.
The first 8 octets of the 16-octet secret (private privacy key) are
used as a DES key. Since DES uses only 56 bits, the Least
Significant Bit in each octet is disregarded.
The Initialization Vector for encryption is obtained using the
following procedure.
The last 8 octets of the 16-octet secret (private privacy key) are
used as pre-IV.
Blumenthal & Wijnen Standards Track [Page 57]RFC 2274 USM for SNMPv3 January 1998
In order to ensure that the IV for two different packets encrypted by
the same key, are not the same (i.e., the IV does not repeat) we need
to "salt" the pre-IV with something unique per packet. An 8-octet
string is used as the "salt". The concatenation of the generating
SNMP engine's 32-bit snmpEngineBoots and a local 32-bit integer, that
the encryption engine maintains, is input to the "salt". The 32-bit
integer is initialized to an arbitrary value at boot time.
The 32-bit snmpEngineBoots is converted to the first 4 octets (Most
Significant Byte first) of our "salt". The 32-bit integer is then
converted to the last 4 octet (Most Significant Byte first) of our
"salt". The resulting "salt" is then XOR-ed with the pre-IV. The 8-
octet "salt" is then put into the privParameters field encoded as an
OCTET STRING. The "salt" integer is then modified. We recommend
that it be incremented by one and wrap when it reaches the maximum
value.
How exactly the value of the "salt" (and thus of the IV) varies, is
an implementation issue, as long as the measures are taken to avoid
producing a duplicate IV.
The "salt" must be placed in the privParameters field to enable the
receiving entity to compute the correct IV and to decrypt the
message.
8.1.1.2. Data Encryption.
The data to be encrypted is treated as sequence of octets. Its length
should be an integral multiple of 8 - and if it is not, the data is
padded at the end as necessary. The actual pad value is irrelevant.
The data is encrypted in Cipher Block Chaining mode.
The plaintext is divided into 64-bit blocks.
The plaintext for each block is XOR-ed with the ciphertext of the
previous block, the result is encrypted and the output of the
encryption is the ciphertext for the block. This procedure is
repeated until there are no more plaintext blocks.
For the very first block, the Initialization Vector is used instead
of the ciphertext of the previous block.
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RFC 2274 USM for SNMPv3 January 1998
8.1.1.3. Data Decryption
Before decryption, the encrypted data length is verified. If the
length of the OCTET STRING to be decrypted is not an integral
multiple of 8 octets, the decryption process is halted and an
appropriate exception noted. When decrypting, the padding is
ignored.
The first ciphertext block is decrypted, the decryption output is
XOR-ed with the Initialization Vector, and the result is the first
plaintext block.
For each subsequent block, the ciphertext block is decrypted, the
decryption output is XOR-ed with the previous ciphertext block and
the result is the plaintext block.
8.2. Elements of the DES Privacy Protocol
This section contains definitions required to realize the privacy
module defined by this memo.
8.2.1. Users
Data en/decryption using this Symmetric Encryption Protocol makes use
of a defined set of userNames. For any user on whose behalf a
message must be en/decrypted at a particular SNMP engine, that SNMP
engine must have knowledge of that user. An SNMP engine that wishes
to communicate with another SNMP engine must also have knowledge of a
user known to that SNMP engine, including knowledge of the applicable
attributes of that user.
A user and its attributes are defined as follows:
<userName>
An octet string representing the name of the user.
<privKey>
A user's secret key to be used as input for the DES key and IV.
The length of this key MUST be 16 octets.
8.2.2. msgAuthoritativeEngineID
The msgAuthoritativeEngineID value contained in an authenticated
message specifies the authoritative SNMP engine for that particular
message (see the definition of SnmpEngineID in the SNMP Architecture
document [RFC2271]).
Blumenthal & Wijnen Standards Track [Page 59]RFC 2274 USM for SNMPv3 January 1998
The user's (private) privacy key is normally different at each
authoritative SNMP engine and so the snmpEngineID is used to select
the proper key for the en/decryption process.
8.2.3. SNMP Messages Using this Privacy Protocol
Messages using this privacy protocol carry a msgPrivacyParameters
field as part of the msgSecurityParameters. For this protocol, the
msgPrivacyParameters field is the serialized OCTET STRING
representing the "salt" that was used to create the IV.
8.2.4. Services provided by the DES Privacy Module
This section describes the inputs and outputs that the DES Privacy
module expects and produces when the User-based Security module
invokes the DES Privacy module for services.
8.2.4.1. Services for Encrypting Outgoing Data
This DES privacy protocol assumes that the selection of the privKey
is done by the caller and that the caller passes the secret key to be
used.
Upon completion the privacy module returns statusInformation and, if
the encryption process was successful, the encryptedPDU and the
msgPrivacyParameters encoded as an OCTET STRING. The abstract
service primitive is:
statusInformation = -- success of failure
encryptData(
IN encryptKey -- secret key for encryption
IN dataToEncrypt -- data to encrypt (scopedPDU)
OUT encryptedData -- encrypted data (encryptedPDU)
OUT privParameters -- filled in by service provider
)
The abstract data elements are:
statusInformation
An indication of the success or failure of the encryption
process. In case of failure, it is an indication of the error.
encryptKey
The secret key to be used by the encryption algorithm.
The length of this key MUST be 16 octets.
dataToEncrypt
The data that must be encrypted.
encryptedData
The encrypted data upon successful completion.
Blumenthal & Wijnen Standards Track [Page 60]RFC 2274 USM for SNMPv3 January 1998
privParameters
The privParameters encoded as an OCTET STRING.
8.2.4.2. Services for Decrypting Incoming Data
This DES privacy protocol assumes that the selection of the privKey
is done by the caller and that the caller passes the secret key to be
used.
Upon completion the privacy module returns statusInformation and, if
the decryption process was successful, the scopedPDU in plain text.
The abstract service primitive is:
statusInformation =
decryptData(
IN decryptKey -- secret key for decryption
IN privParameters -- as received on the wire
IN encryptedData -- encrypted data (encryptedPDU)
OUT decryptedData -- decrypted data (scopedPDU)
)
The abstract data elements are:
statusInformation
An indication whether the data was successfully decrypted
and if not an indication of the error.
decryptKey
The secret key to be used by the decryption algorithm.
The length of this key MUST be 16 octets.
privParameters
The "salt" to be used to calculate the IV.
encryptedData
The data to be decrypted.
decryptedData
The decrypted data.
8.3. Elements of Procedure.
This section describes the procedures for the DES privacy protocol.
8.3.1. Processing an Outgoing Message
This section describes the procedure followed by an SNMP engine
whenever it must encrypt part of an outgoing message using the
usmDESPrivProtocol.
1) The secret cryptKey is used to construct the DES encryption key,
the "salt" and the DES pre-IV (as described in section 8.1.1.1).
Blumenthal & Wijnen Standards Track [Page 61]RFC 2274 USM for SNMPv3 January 1998
2) The privParameters field is set to the serialization according
to the rules in [RFC1906] of an OCTET STRING representing the the
"salt" string.
3) The scopedPDU is encrypted (as described in section 8.1.1.2)
and the encrypted data is serialized according to the rules in
[RFC1906] as an OCTET STRING.
4) The serialized OCTET STRING representing the encrypted
scopedPDU together with the privParameters and statusInformation
indicating success is returned to the calling module.
8.3.2. Processing an Incoming Message
This section describes the procedure followed by an SNMP engine
whenever it must decrypt part of an incoming message using the
usmDESPrivProtocol.
1) If the privParameters field is not an 8-octet OCTET STRING,
then an error indication (decryptionError) is returned to the
calling module.
2) The "salt" is extracted from the privParameters field.
3) The secret cryptKey and the "salt" are then used to construct the
DES decryption key and pre-IV (as described in section 8.1.1.1).
4) The encryptedPDU is then decrypted (as described in
section 8.1.1.3).
5) If the encryptedPDU cannot be decrypted, then an error
indication (decryptionError) is returned to the calling module.
6) The decrypted scopedPDU and statusInformation indicating
success are returned to the calling module.
9. Intellectual Property
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
Blumenthal & Wijnen Standards Track [Page 62]RFC 2274 USM for SNMPv3 January 1998
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
10. Acknowledgements
This document is the result of the efforts of the SNMPv3 Working
Group. Some special thanks are in order to the following SNMPv3 WG
members:
Dave Battle (SNMP Research, Inc.)
Uri Blumenthal (IBM T.J. Watson Research Center)
Jeff Case (SNMP Research, Inc.)
John Curran (BBN)
T. Max Devlin (Hi-TECH Connections)
John Flick (Hewlett Packard)
David Harrington (Cabletron Systems Inc.)
N.C. Hien (IBM T.J. Watson Research Center)
Dave Levi (SNMP Research, Inc.)
Louis A Mamakos (UUNET Technologies Inc.)
Paul Meyer (Secure Computing Corporation)
Keith McCloghrie (Cisco Systems)
Russ Mundy (Trusted Information Systems, Inc.)
Bob Natale (ACE*COMM Corporation)
Mike O'Dell (UUNET Technologies Inc.)
Dave Perkins (DeskTalk)
Peter Polkinghorne (Brunel University)
Randy Presuhn (BMC Software, Inc.)
David Reid (SNMP Research, Inc.)
Shawn Routhier (Epilogue)
Juergen Schoenwaelder (TU Braunschweig)
Bob Stewart (Cisco Systems)
Bert Wijnen (IBM T.J. Watson Research Center)
The document is based on recommendations of the IETF Security and
Administrative Framework Evolution for SNMP Advisory Team. Members
of that Advisory Team were:
David Harrington (Cabletron Systems Inc.)
Jeff Johnson (Cisco Systems)
David Levi (SNMP Research Inc.)
John Linn (Openvision)
Blumenthal & Wijnen Standards Track [Page 63]RFC 2274 USM for SNMPv3 January 1998
Russ Mundy (Trusted Information Systems) chair
Shawn Routhier (Epilogue)
Glenn Waters (Nortel)
Bert Wijnen (IBM T. J. Watson Research Center)
As recommended by the Advisory Team and the SNMPv3 Working Group
Charter, the design incorporates as much as practical from previous
RFCs and drafts. As a result, special thanks are due to the authors
of previous designs known as SNMPv2u and SNMPv2*:
Jeff Case (SNMP Research, Inc.)
David Harrington (Cabletron Systems Inc.)
David Levi (SNMP Research, Inc.)
Keith McCloghrie (Cisco Systems)
Brian O'Keefe (Hewlett Packard)
Marshall T. Rose (Dover Beach Consulting)
Jon Saperia (BGS Systems Inc.)
Steve Waldbusser (International Network Services)
Glenn W. Waters (Bell-Northern Research Ltd.)
11. Security Considerations
11.1. Recommended Practices
This section describes practices that contribute to the secure,
effective operation of the mechanisms defined in this memo.
- An SNMP engine must discard SNMP Response messages that do not
correspond to any currently outstanding Request message. It is the
responsibility of the Message Processing module to take care of
this. For example it can use a msgID for that.
An SNMP Command Generator Application must discard any Response PDU
for which there is no currently outstanding Request PDU; for
example for SNMPv2 [RFC1905] PDUs, the request-id component in the
PDU can be used to correlate Responses to outstanding Requests.
Although it would be typical for an SNMP engine and an SNMP Command
Generator Application to do this as a matter of course, when using
these security protocols it is significant due to the possibility
of message duplication (malicious or otherwise).
- If an SNMP engine uses a msgID for correlating Response messages
to outstanding Request messages, then it MUST use different msgIDs
in all such Request messages that it sends out during a Time Window
(150 seconds) period.
Blumenthal & Wijnen Standards Track [Page 64]RFC 2274 USM for SNMPv3 January 1998
A Command Generator or Notification Originator Application MUST use
different request-ids in all Request PDUs that it sends out during
a TimeWindow (150 seconds) period.
This must be done to protect against the possibility of message
duplication (malicious or otherwise).
For example, starting operations with a msgID and/or request-id
value of zero is not a good idea. Initializing them with an
unpredictable number (so they do not start out the same after each
reboot) and then incrementing by one would be acceptable.
- An SNMP engine should perform time synchronization using
authenticated messages in order to protect against the possibility
of message duplication (malicious or otherwise).
- When sending state altering messages to a managed authoritative
SNMP engine, a Command Generator Application should delay sending
successive messages to that managed SNMP engine until a positive
acknowledgement is received for the previous message or until the
previous message expires.
No message ordering is imposed by the SNMP. Messages may be
received in any order relative to their time of generation and each
will be processed in the ordered received. Note that when an
authenticated message is sent to a managed SNMP engine, it will be
valid for a period of time of approximately 150 seconds under
normal circumstances, and is subject to replay during this period.
Indeed, an SNMP engine and SNMP Command Generator Applications must
cope with the loss and re-ordering of messages resulting from
anomalies in the network as a matter of course.
However, a managed object, snmpSetSerialNo [RFC1907], is
specifically defined for use with SNMP Set operations in order to
provide a mechanism to ensure that the processing of SNMP messages
occurs in a specific order.
- The frequency with which the secrets of a User-based Security
Model user should be changed is indirectly related to the frequency
of their use.
Protecting the secrets from disclosure is critical to the overall
security of the protocols. Frequent use of a secret provides a
continued source of data that may be useful to a cryptanalyst in
exploiting known or perceived weaknesses in an algorithm. Frequent
changes to the secret avoid this vulnerability.
Blumenthal & Wijnen Standards Track [Page 65]RFC 2274 USM for SNMPv3 January 1998
Changing a secret after each use is generally regarded as the most
secure practice, but a significant amount of overhead may be
associated with that approach.
Note, too, in a local environment the threat of disclosure may be
less significant, and as such the changing of secrets may be less
frequent. However, when public data networks are used as the
communication paths, more caution is prudent.
11.2 Defining Users
The mechanisms defined in this document employ the notion of users on
whose behalf messages are sent. How "users" are defined is subject
to the security policy of the network administration. For example,
users could be individuals (e.g., "joe" or "jane"), or a particular
role (e.g., "operator" or "administrator"), or a combination (e.g.,
"joe-operator", "jane-operator" or "joe-admin"). Furthermore, a user
may be a logical entity, such as an SNMP Application or a set of SNMP
Applications, acting on behalf of an individual or role, or set of
individuals, or set of roles, including combinations.
Appendix A describes an algorithm for mapping a user "password" to a
16 octet value for use as either a user's authentication key or
privacy key (or both). Note however, that using the same password
(and therefore the same key) for both authentication and privacy is
very poor security practice and should be strongly discouraged.
Passwords are often generated, remembered, and input by a human.
Human-generated passwords may be less than the 16 octets required by
the authentication and privacy protocols, and brute force attacks can
be quite easy on a relatively short ASCII character set. Therefore,
the algorithm is Appendix A performs a transformation on the
password. If the Appendix A algorithm is used, SNMP implementations
(and SNMP configuration applications) must ensure that passwords are
at least 8 characters in length.
Because the Appendix A algorithm uses such passwords (nearly)
directly, it is very important that they not be easily guessed. It
is suggested that they be composed of mixed-case alphanumeric and
punctuation characters that don't form words or phrases that might be
found in a dictionary. Longer passwords improve the security of the
system. Users may wish to input multiword phrases to make their
password string longer while ensuring that it is memorable.
Since it is infeasible for human users to maintain different
passwords for every SNMP engine, but security requirements strongly
discourage having the same key for more than one SNMP engine, the
User-based Security Model employs a compromise proposed in
[Localized-key]. It derives the user keys for the SNMP engines from
Blumenthal & Wijnen Standards Track [Page 66]RFC 2274 USM for SNMPv3 January 1998
user's password in such a way that it is practically impossible to
either determine the user's password, or user's key for another SNMP
engine from any combination of user's keys on SNMP engines.
Note however, that if user's password is disclosed, then key
localization will not help and network security may be compromised in
this case. Therefore a user's password or non-localized key MUST NOT
be stored on a managed device/node. Instead the localized key SHALL
be stored (if at all) , so that, in case a device does get
compromised, no other managed or managing devices get compromised.
11.3. Conformance
To be termed a "Secure SNMP implementation" based on the User-based
Security Model, an SNMP implementation MUST:
- implement one or more Authentication Protocol(s). The HMAC-MD5-96
and HMAC-SHA-96 Authentication Protocols defined in this memo are
examples of such protocols.
- to the maximum extent possible, prohibit access to the secret(s)
of each user about which it maintains information in a Local
Configuration Datastore (LCD) under all circumstances except as
required to generate and/or validate SNMP messages with respect to
that user.
- implement the key-localization mechanism.
- implement the SNMP-USER-BASED-SM-MIB.
In addition, an authoritative SNMP engine SHOULD provide initial
configuration in accordance with Appendix A.1.
Implementation of a Privacy Protocol (the DES Symmetric Encryption
Protocol defined in this memo is one such protocol) is optional.
12. References
[RFC1321] Rivest, R., "Message Digest Algorithm MD5",
RFC 1321, April 1992.
[RFC1903] Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
"Textual Conventions for Version 2 of the Simple Network
Management Protocol (SNMPv2)", RFC 1903, January 1996.
[RFC1905] Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
"Protocol Operations for Version 2 of the Simple Network
Management Protocol (SNMPv2)", RFC 1905, January 1996.
Blumenthal & Wijnen Standards Track [Page 67]RFC 2274 USM for SNMPv3 January 1998
[RFC1906] Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
"Transport Mappings for Version 2 of the Simple Network Management
Protocol (SNMPv2)", RFC 1906, January 1996.
[RFC1907] Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
"Management Information Base for Version 2 of the Simple Network
Management Protocol (SNMPv2)", RFC 1907 January 1996.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
Keyed-Hashing for Message Authentication", RFC 2104, February
1997.
[RFC2028] Hovey, R., and S. Bradner, "The Organizations Involved in
the IETF Standards Process", BCP 11, RFC 2028, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2271] Harrington, D., Presuhn, R., and B. Wijnen, "An
Architecture for describing SNMP Management Frameworks", RFC 2271,
January 1998.
[RFC2272] Case, J., Harrington, D., Presuhn, R., and B. Wijnen,
"Message Processing and Dispatching for the Simple Network
Management Protocol (SNMP)", RFC 2272, January 1998.
[Localized-Key] U. Blumenthal, N. C. Hien, B. Wijnen
"Key Derivation for Network Management Applications" IEEE Network
Magazine, April/May issue, 1997.
[DES-NIST] Data Encryption Standard, National Institute of Standards
and Technology. Federal Information Processing Standard (FIPS)
Publication 46-1. Supersedes FIPS Publication 46, (January, 1977;
reaffirmed January, 1988).
[DES-ANSI] Data Encryption Algorithm, American National Standards
Institute. ANSI X3.92-1981, (December, 1980).
[DESO-NIST] DES Modes of Operation, National Institute of Standards
and Technology. Federal Information Processing Standard (FIPS)
Publication 81, (December, 1980).
[DESO-ANSI] Data Encryption Algorithm - Modes of Operation, American
National Standards Institute. ANSI X3.106-1983, (May 1983).
[DESG-NIST] Guidelines for Implementing and Using the NBS Data
Encryption Standard, National Institute of Standards and
Technology. Federal Information Processing Standard (FIPS)
Blumenthal & Wijnen Standards Track [Page 68]RFC 2274 USM for SNMPv3 January 1998
Publication 74, (April, 1981).
[DEST-NIST] Validating the Correctness of Hardware Implementations of
the NBS Data Encryption Standard, National Institute of Standards
and Technology. Special Publication 500-20.
[DESM-NIST] Maintenance Testing for the Data Encryption Standard,
National Institute of Standards and Technology. Special
Publication 500-61, (August, 1980).
[SHA-NIST] Secure Hash Algorithm. NIST FIPS 180-1, (April, 1995)
http://csrc.nist.gov/fips/fip180-1.txt (ASCII)
http://csrc.nist.gov/fips/fip180-1.ps (Postscript)
13. Editors' Addresses
Uri Blumenthal
IBM T. J. Watson Research
30 Saw Mill River Pkwy,
Hawthorne, NY 10532
USA
EMail: uri@watson.ibm.com
Phone: +1-914-784-7064
Bert Wijnen
IBM T. J. Watson Research
Schagen 33
3461 GL Linschoten
Netherlands
EMail: wijnen@vnet.ibm.com
Phone: +31-348-432-794
Blumenthal & Wijnen Standards Track [Page 69]RFC 2274 USM for SNMPv3 January 1998
APPENDIX A - Installation
A.1. SNMP engine Installation Parameters
During installation, an authoritative SNMP engine SHOULD (in the
meaning as defined in [RFC2119]) be configured with several initial
parameters. These include:
1) A security posture
The choice of security posture determines if initial configuration
is implemented and if so how. One of three possible choices is
selected:
minimum-secure,
semi-secure,
very-secure (i.e., no-initial-configuration)
In the case of a very-secure posture, there is no initial
configuration, and so the following steps are irrelevant.
2) one or more secrets
These are the authentication/privacy secrets for the first user to be
configured.
One way to accomplish this is to have the installer enter a
"password" for each required secret. The password is then
algorithmically converted into the required secret by:
- forming a string of length 1,048,576 octets by repeating the
value of the password as often as necessary, truncating
accordingly, and using the resulting string as the input to the MD5
algorithm [MD5]. The resulting digest, termed "digest1", is used
in the next step.
- a second string is formed by concatenating digest1, the SNMP
engine's snmpEngineID value, and digest1. This string is used as
input to the MD5 algorithm [MD5].
The resulting digest is the required secret (see Appendix A.2).
With these configured parameters, the SNMP engine instantiates the
following usmUserEntry in the usmUserTable:
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no privacy support privacy support
------------------ ---------------
usmUserEngineID localEngineID localEngineID
usmUserName "initial" "initial"
usmUserSecurityName "initial" "initial"
usmUserCloneFrom ZeroDotZero ZeroDotZero
usmUserAuthProtocol usmHMACMD5AuthProtocol usmHMACMD5AuthProtocol
usmUserAuthKeyChange "" ""
usmUserOwnAuthKeyChange "" ""
usmUserPrivProtocol none usmDESPrivProtocol
usmUserPrivKeyChange "" ""
usmUserOwnPrivKeyChange "" ""
usmUserPublic "" ""
usmUserStorageType anyValidStorageType anyValidStorageType
usmUserStatus active active
A.2. Password to Key Algorithm
A sample code fragment (section A.2.1) demonstrates the password to
key algorithm which can be used when mapping a password to an
authentication or privacy key using MD5. The reference source code of
MD5 is available in [RFC1321].
Another sample code fragment (section A.2.2) demonstrates the
password to key algorithm which can be used when mapping a password
to an authentication or privacy key using SHA (documented in SHA-
NIST).
An example of the results of a correct implementation is provided
(section A.3) which an implementor can use to check if his
implementation produces the same result.
A.2.1. Password to Key Sample Code for MD5
void password_to_key_md5(
u_char *password, /* IN */
u_int passwordlen, /* IN */
u_char *engineID, /* IN - pointer to snmpEngineID */
u_int engineLength /* IN - length of snmpEngineID */
u_char *key) /* OUT - pointer to caller 16-octet buffer */
{
MD5_CTX MD;
u_char *cp, password_buf[64];
u_long password_index = 0;
u_long count = 0, i;
MD5Init (&MD); /* initialize MD5 */
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/**********************************************/
/* Use while loop until we've done 1 Megabyte */
/**********************************************/
while (count < 1048576) {
cp = password_buf;
for (i = 0; i < 64; i++) {
/*************************************************/
/* Take the next octet of the password, wrapping */
/* to the beginning of the password as necessary.*/
/*************************************************/
*cp++ = password[password_index++ % passwordlen];
}
MD5Update (&MD, password_buf, 64);
count += 64;
}
MD5Final (key, &MD); /* tell MD5 we're done */
/*****************************************************/
/* Now localize the key with the engineID and pass */
/* through MD5 to produce final key */
/* May want to ensure that engineLength <= 32, */
/* otherwise need to use a buffer larger than 64 */
/*****************************************************/
memcpy(password_buf, key, 16);
memcpy(password_buf+16, engineID, engineLength);
memcpy(password_buf+engineLength, key, 16);
MD5Init(&MD);
MD5Update(&MD, password_buf, 32+engineLength);
MD5Final(key, &MD);
return;
}
A.2.2. Password to Key Sample Code for SHA
void password_to_key_sha(
u_char *password, /* IN */
u_int passwordlen, /* IN */
u_char *engineID, /* IN - pointer to snmpEngineID */
u_int engineLength /* IN - length of snmpEngineID */
u_char *key) /* OUT - pointer to caller 20-octet buffer */
{
SHA_CTX SH;
u_char *cp, password_buf[72];
u_long password_index = 0;
u_long count = 0, i;
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SHAInit (&SH); /* initialize SHA */
/**********************************************/
/* Use while loop until we've done 1 Megabyte */
/**********************************************/
while (count < 1048576) {
cp = password_buf;
for (i = 0; i < 64; i++) {
/*************************************************/
/* Take the next octet of the password, wrapping */
/* to the beginning of the password as necessary.*/
/*************************************************/
*cp++ = password[password_index++ % passwordlen];
}
SHAUpdate (&SH, password_buf, 64);
count += 64;
}
SHAFinal (key, &SH); /* tell SHA we're done */
/*****************************************************/
/* Now localize the key with the engineID and pass */
/* through SHA to produce final key */
/* May want to ensure that engineLength <= 32, */
/* otherwise need to use a buffer larger than 72 */
/*****************************************************/
memcpy(password_buf, key, 20);
memcpy(password_buf+20, engineID, engineLength);
memcpy(password_buf+engineLength, key, 20);
SHAInit(&SH);
SHAUpdate(&SH, password_buf, 40+engineLength);
SHAFinal(key, &SH);
return;
}
A.3. Password to Key Sample Results
A.3.1. Password to Key Sample Results using MD5
The following shows a sample output of the password to key algorithm
for a 16-octet key using MD5.
With a password of "maplesyrup" the output of the password to key
algorithm before the key is localized with the SNMP engine's
snmpEngineID is:
'9f af 32 83 88 4e 92 83 4e bc 98 47 d8 ed d9 63'H
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After the intermediate key (shown above) is localized with the
snmpEngineID value of:
'00 00 00 00 00 00 00 00 00 00 00 02'H
the final output of the password to key algorithm is:
'52 6f 5e ed 9f cc e2 6f 89 64 c2 93 07 87 d8 2b'H
A.3.2. Password to Key Sample Results using SHA
The following shows a sample output of the password to key
algorithm for a 20-octet key using SHA.
With a password of "maplesyrup" the output of the password to key
algorithm before the key is localized with the SNMP engine's
snmpEngineID is:
'f1 be a9 ae 66 7f 4f b6 34 1e 51 af 06 80 7e 91 e4 3b 01 ac'H
After the intermediate key (shown above) is localized with the
snmpEngineID value of:
'00 00 00 00 00 00 00 00 00 00 00 02'H
the final output of the password to key algorithm is:
'8a a3 d9 9e 3e 30 56 f2 bf e3 a9 ee f3 45 d5 39 54 91 12 be'H
A.4. Sample encoding of msgSecurityParameters
The msgSecurityParameters in an SNMP message are represented as an
OCTET STRING. This OCTET STRING should be considered opaque outside a
specific Security Model.
The User-based Security Model defines the contents of the OCTET
STRING as a SEQUENCE (see section 2.4).
Given these two properties, the following is an example of the
msgSecurityParameters for the User-based Security Model, encoded as
an OCTET STRING:
04 <length>
30 <length>
04 <length> <msgAuthoritativeEngineID>
02 <length> <msgAuthoritativeEngineBoots>
02 <length> <msgAuthoritativeEngineTime>
04 <length> <msgUserName>
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04 0c <HMAC-MD5-96-digest>
04 08 <salt>
Here is the example once more, but now with real values (except for
the digest in msgAuthenticationParameters and the salt in
msgPrivacyParameters, which depend on variable data that we have not
defined here):
Hex Data Description
-------------- -----------------------------------------------
04 39 OCTET STRING, length 57
30 37 SEQUENCE, length 55
04 0c 80000002 msgAuthoritativeEngineID: IBM
01 IPv4 address
09840301 9.132.3.1
02 01 01 msgAuthoritativeEngineBoots: 1
02 02 0101 msgAuthoritativeEngineTime: 257
04 04 62657274 msgUserName: bert
04 0c 01234567 msgAuthenticationParameters: sample value
89abcdef
fedcba98
04 08 01234567 msgPrivacyParameters: sample value
89abcdef
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B. Full Copyright Statement
Copyright (C) The Internet Society (1998). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Blumenthal & Wijnen Standards Track [Page 76]
