1. RFC 4902
Network Working Group                                         M. Stecher
Request for Comments: 4902                              Secure Computing
Category: Informational                                         May 2007

                   Integrity, Privacy, and Security
            in Open Pluggable Edge Services (OPES) for SMTP

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).


   The Open Pluggable Edge Services (OPES) framework is application
   agnostic.  Application-specific adaptations extend that framework.
   Previous work has focused on HTTP and work for SMTP is in progress.
   These protocols differ fundamentally in the way data flows, and it
   turns out that existing OPES requirements and IAB considerations for
   OPES need to be reviewed with regards to how well they fit for SMTP
   adaptation.  This document analyzes aspects about the integrity of
   SMTP and mail message adaptation by OPES systems and about privacy
   and security issues when the OPES framework is adapted to SMTP.  It
   also lists requirements that must be considered when creating the
   "SMTP adaptation with OPES" document.

   The intent of this document is to capture this information before the
   current OPES working group shuts down.  This is to provide input for
   subsequent working groups or individual contributors that may pick up
   the OPES/SMTP work at a later date.

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Table of Contents

   1. Introduction ....................................................3
      1.1. Differences between Unidirectional and
           Bidirectional Application Protocols ........................3
      1.2. Non-Standardized SMTP Adaptations at SMTP Gateways .........3
      1.3. Non-OPES Issues of SMTP ....................................4
      1.4. Opportunities of OPES/SMTP to Address Some Issues ..........4
      1.5. Limitations of OPES in Regards to Fixing SMTP Issues .......4
   2. Terminology .....................................................5
   3. Integrity, Privacy, and Security Considerations .................5
      3.1. Tracing Information in OPES/SMTP ...........................5
      3.2. Bypass in OPES/SMTP ........................................6
      3.3. Compatibility with Cryptographic Protection Mechanisms .....7
   4. Protocol Requirements for OPES/SMTP .............................8
   5. IAB Considerations for OPES/SMTP ................................9
      5.1. IAB Consideration (2.1) One-Party Consent ..................9
      5.2. IAB Consideration (2.2) IP-Layer Communications ............9
      5.3. IAB Consideration (3.1) Notification .......................9
      5.4. IAB Consideration (3.2) Notification ......................10
      5.5. IAB Consideration (3.3) Non-Blocking ......................10
      5.6. IAB Consideration Application Layer Addresses (4.x) .......10
      5.7. IAB Consideration (5.1) Privacy ...........................10
      5.8. IAB Consideration Encryption ..............................11
   6. Security Considerations ........................................11
   7. References .....................................................11
      7.1. Normative References ......................................11
      7.2. Informative References ....................................11
   Appendix A. Acknowledgements ......................................13

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1.  Introduction

   Because OPES is a protocol that is built over application layer
   transports, its security may depend on the specifics of the
   transport.  OPES designs are guided by the IAB considerations for
   OPES document [2], and those considerations are revisited here in the
   context of the SMTP protocol.

   Section 3 of the OPES SMTP use cases document [6] maps some email and
   SMTP elements to OPES names that are used in this document.

1.1.  Differences between Unidirectional and Bidirectional Application

   The IAB listed considerations for Open Pluggable Edge Services (OPES)
   in [2] and OPES treatment of those considerations has been discussed
   in [3].  Both documents make use of HTTP as an example for the
   underlying protocol in OPES flows, and focus on web protocols that
   have requests and responses in the classic form (client sends a
   request to a server that replies with a response of the same protocol
   within a single protocol transaction).

   RFC 3914 [3] already indicates that other protocols may not fit in
   this context, for example in Section 5.3, "Moreover, some application
   protocols may not have explicit responses...".

   When using SMTP there are still client and server applications, and
   requests and responses handled within SMTP, but email messages are
   sent by the data provider to the recipients (data consumers) without
   a previous request.  At that abstraction layer, email delivery via
   SMTP is a unidirectional process and different from the previously
   handled web protocols such as HTTP.  For example, bypass has been
   defined for OPES, so far, by the data consumer requesting an OPES
   bypass by adding information to the application protocol request; the
   OPES system can then react on the bypass request in both the
   application request and response.  For SMTP, the data consumer (email
   recipient) cannot request in-band that the OPES bypass handling of
   his/her messages.

   The IAB considerations need to be revisited and special requirements
   may be needed for OPES handling of SMTP.

1.2.  Non-Standardized SMTP Adaptations at SMTP Gateways

   A large number of email filters are deployed at SMTP gateways today.
   In fact, all use cases listed in "OPES SMTP Use Cases" [6] are
   already deployed, often in non-standardized ways.  This opens a
   number of integrity, privacy, and security concerns that are not

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   addressed, and SMTP itself does not provide effective measures to
   detect and defend against compromised implementations.

   OPES will most likely not be able to solve these issues completely,
   but at least should be able to improve the situation to some extent.

1.3.  Non-OPES Issues of SMTP

   The SMTP specifications [4] require that NDRs (Non-Delivery Reports)
   be sent to the originator of an undeliverable mail that has been
   accepted by an SMTP server.  But it has become common practice for
   some sorts of mail (spam, worms) to be silently dropped without
   sending an NDR, a violation of the MUST statement of SMTP (see
   Section 3.7 of [4]).  While the user of a web protocol notices if a
   resource cannot be fetched, neither the email sender nor email
   recipient may notice that an email was not delivered.  These kind of
   issues already exist and are not introduced by OPES.

1.4.  Opportunities of OPES/SMTP to Address Some Issues

   Adding SMTP adaptations with OPES allows us to define a standardized
   way for SMTP gateway filtering, to offload filtering services to
   callout servers and address a number of the integrity, privacy, and
   security issues.  OPES offers methods to add OPES tracing information
   and to request a bypass of filtering, and by that can make email
   gateway filtering a more reliable and standardized function.  But
   OPES won't make email delivery via SMTP a reliable communication.

1.5.  Limitations of OPES in Regards to Fixing SMTP Issues

   The biggest concerns when adding OPES services to a network flow are
   that compromised, misconfigured, or faulty OPES systems may change
   messages in a way that the consumer can no longer read them or that
   messages are no longer delivered at all.

   Defining a standard way to mark mails that have been handled by OPES
   systems is fairly simple and does not require new techniques by SMTP
   gateways; they already today MUST leave tracing information by adding
   "Received" headers to mails.  Therefore, recipients receiving broken
   mail have a fair chance of finding the compromised OPES system by
   using the trace information.  There is still no guarantee, as the
   email may have been broken in a way that makes even the tracing
   information unreadable.  But the chance will be even better than with
   other protocols such as HTTP, because most email clients allow the
   user to display mail headers, while many browsers have no mechanism
   to show the HTTP headers that might include tracing info.

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   Email that cannot be delivered, because a compromised OPES system
   prevented the delivery of legitimate mail, MUST result in a an NDR to
   be sent to the originator of the mail according to the SMTP
   specifications [4].  OPES should not be forced to fix the issue that
   NDRs are not reliable over SMTP.

2.  Terminology

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [1].  When used with
   the normative meanings, these keywords will be all uppercase.
   Occurrences of these words in lowercase comprise normal prose usage,
   with no normative implications.

3.  Integrity, Privacy, and Security Considerations

3.1.  Tracing Information in OPES/SMTP

   Tracing OPES operations is an important requirement for OPES systems.
   Tracing information added to email should follow a similar syntax and
   structure to that defined for OPES/HTTP in HTTP Adaptation with Open
   Pluggable Edge Services [5], and with the same guidelines as the SMTP
   specifications [4] defined for the "Received" headers.  (We do not
   specify here whether "Received" headers would be used to carry the
   OPES information, or new trace headers should be defined, such as
   OPES-System and OPES-Via.)

   OPES/SMTP specifications defining tracing requirements MUST be
   compliant with the general OPES tracing requirements defined in OPES
   Entities & End Points Communication [12], but MAY further restrict
   those.  For example, they might require the following: that the OPES
   processor must add tracing information for the OPES system before
   calling the first callout server; that it has to augment the tracing
   information with additional data if necessary after the message
   returns from each service it is calling; and that it must ensure that
   the tracing information has not been deleted by an OPES service
   before it forwards the SMTP message.

   Trace information can then be seen by mail recipients when the mail
   message reaches the recipient.

   Mail that cannot be delivered or that is blocked by the OPES service
   will either be rejected or cannot be delivered after it has been
   accepted by an SMTP server.  In the latter case, SMTP specifications
   [4] require that an NDR MUST be sent to the originator; OPES further
   requires that an NDR generated due to OPES processing MUST also
   contain information about the OPES system so that the sender gets

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   informed.  If an email is rejected at the SMTP protocol level due to
   OPES processing, an OPES system MUST also include trace data in the
   SMTP response so that the originator can find out why and where the
   mail was rejected.

3.2.  Bypass in OPES/SMTP

   If a mail message was rejected or could not be delivered (and an NDR
   was sent), the originator of the message may want to bypass the OPES
   system that blocked the message.

   If the recipient of a message receives a mail with OPES trace
   information, he may want to receive a non-OPES version of the
   message.  Although there is no direct in-band request from the
   recipient back to the OPES system, the recipient can contact the
   sender and ask her to send the message again and to add a bypass
   request for the OPES system.  Not all OPES systems will be allowed to
   fulfill a bypass request according to their policy.  For example,
   malware scanners should not be bypassed.  But other OPES services are
   good candidates for bypass requests, such as language translation of
   the email message.  Translation could be bypassed after the recipient
   has noticed that the translated result does not meet his/her
   expectations and that the original message would be preferred.

   An OPES system MAY also define out-of-band methods to request a
   bypass, for example, a web interface or an email message sent to the
   server that results in the creation of a white list entry for the
   sender/recipient pair.  Examples for these out-of-band methods are
   email systems that keep a copy of the original email in a quarantine
   queue and only send the recipient a block notification, plus either a
   direct link or a digest notification, with the ability to retrieve
   the original message from quarantine.  These out-of-band methods are
   typically offered by spam filters today.

   OPES MUST implement methods to request a bypass, but there cannot be
   a guarantee that the bypass request will be approved.  The security
   needs of the receiver or the receiver's network may demand that
   certain filters must not be bypassed (such as virus scanners).  In
   general, the receiver should be able to configure a client centric
   OPES system, i.e. the receiver should be able to indicate if he/she
   wants to receive a non-OPES version if it is available.

   Bypass requests could be added to the mail message or within the SMTP
   dialog.  Bypass request data added to the mail message cannot bypass
   OPES services that operate on other SMTP dialog commands, which are
   sent before the mail message has been received (such as RCPT

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   Bypass request data sent using an ESMTP extension as part of the SMTP
   dialog may not reach the OPES system if intermediate SMTP relays do
   not support those bypass request commands and don't forward that

3.3.  Compatibility with Cryptographic Protection Mechanisms

   Cryptography can be used to assure message privacy, to authenticate
   the originator of messages, and to detect message modification.
   There are standard methods for achieving some or all these
   protections for generic messages ([9], [10], [11]), and these can be
   used to protect SMTP data without changing the SMTP protocol.

   The content of encrypted mail messages cannot be inspected by OPES
   systems because only the intended recipient has the information
   necessary for decryption.  The IAB and others have suggested that
   users might want to share that information with OPES systems, thus
   permitting decryption by intermediates.  For most cryptographic
   systems that are compatible with email, this would require end users
   to share their most valuable keys, in essence their "identities",
   with OPES machines.  Some key management systems, particularly those
   which have centralized administrative control of keys, might have
   trust models in which such sharing would be sensible and secure.

   After decrypting the message, an OPES box that modified the content
   would be faced with the task of re-encrypting it in order to maintain
   some semblance of "end-to-end" privacy.

   If OPES/SMTP had a way to interact with end users on a per-message
   basis, it might be possible to communicate cryptographic key
   information from individual messages to end users, have them compute
   the message encrypting key for particular message, and to send that
   back to the OPES box.  This would perhaps ameliorate the need to
   share a user's "master" message decrypting key with the OPES box.
   This kind of communication has not been defined for OPES.

   Message protection systems generally include some message integrity
   mechanisms by which a recipient can check for a message modification
   that may have occurred after the sender released the message.  This
   protection can be applied to encrypted or plaintext messages and can
   be accomplished through either symmetric or asymmetric cryptography.
   In the case of symmetric cryptography, the key sharing problem is
   exactly similar to the encryption case discussed previously.  If the
   OPES box modified the content, then the message integrity (or
   authentication) code would have to be recalculated and included with
   the modified message.

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   For asymmetric cryptography the situation is more complicated.  The
   message integrity is tied to the sender's public key, and although
   anyone who can get the sender's public key can also check for a
   message modification, no one but the sender can compute the sender's
   signature on a modified message.  Thus, an OPES system could not
   modify messages and have them appear to come from the purported
   sender.  The notion of sharing the sender's signing key with the OPES
   system is unpalatable because few trust models would be compatible
   with sharing digital identities across organization boundaries.
   However, if the OPES system doing the modification were under the
   control of the sender's local administration, the sharing might be
   sensible (as discussed for decryption, above).

   OPES/SMTP systems could present modified content showing the modified
   regions in a form that permits the authentication of the original
   message and authentication of the OPES modifications (assuming the
   OPES box had a digital signature identity and key).  One method for
   doing this is outlined in [13], but to our knowledge this method is
   not in any standard.

   There are security risks associated with sharing cryptographic keys
   that must be addressed by implementers.  Because this is not a simple
   task, it is not a requirement for OPES/SMTP.

4.  Protocol Requirements for OPES/SMTP

   In addition to other documents listing requirements for OPES, the
   discussion in this document implies specific requirements for
   designing and implementing SMTP adaptations with OPES:

   o  OPES Systems MUST add tracing headers to mail messages

   o  If an email message that has been accepted by an OPES system
      cannot be delivered, the non-delivery report MUST include trace
      information of the OPES system.

   o  The OPES/SMTP specifications MUST define a bypass request option
      that can be included in mail messages.

   o  The OPES/SMTP specifications MUST define a bypass request option
      as an extension for SMTP dialogs.

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5.  IAB Considerations for OPES/SMTP

   This section lists the IAB considerations for OPES [2] and summarizes
   how OPES/SMTP addresses them.

5.1.  IAB Consideration (2.1) One-Party Consent

   The IAB recommends that all OPES services be explicitly authorized by
   one of the application-layer end-hosts (that is, either the data
   consumer application or the data provider application).  For OPES/
   SMTP, this means consent of either the email message sender or the

   The application agnostic architecture of OPES [7] requires that "OPES
   processors MUST be consented to by either the data consumer or data
   provider application" (OPES processor is the email gateway for OPES/
   SMTP).  This cannot prevent the consent-less introduction of OPES
   processors by noncompliant OPES entities.

5.2.  IAB Consideration (2.2) IP-Layer Communications

   The IAB recommends that OPES processors must be explicitly addressed
   at the IP layer by the end user (data consumer application).

   This requirement has been addressed by the architecture requirements
   in Section 2.1 of [7] and has been further clarified in Section 2.2
   of [3].

5.3.  IAB Consideration (3.1) Notification

   "The overall OPES framework needs to assist content providers in
   detecting and responding to client-centric actions by OPES
   intermediaries that are deemed inappropriate by the content provider"

   For OPES/SMTP this translates into assistance for the email message
   sender to detect and respond to recipient-centric actions that are
   deemed inappropriate by the sender.

   This has been addressed in Section 3.1 and by the second tracing
   requirements in Section 4.  As discussed in Section 1.3, OPES/SMTP
   cannot fix cases where NDRs are not sent or get blocked before
   reaching the sender of the original message.

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5.4.  IAB Consideration (3.2) Notification

   "The overall OPES framework should assist end users in detecting the
   behavior of OPES intermediaries, potentially allowing them to
   identify imperfect or compromised intermediaries" [2].

   This is addressed in Section 3.1 and by the first tracing requirement
   in Section 4.  It must be noted that some email systems do not make
   the email headers available to the end user, although the headers
   belong to the payload that is transferred via SMTP.  Building an OPES
   architecture with those email systems should be avoided or requires
   that the tracing information be made available to the end users in a
   different way.

5.5.  IAB Consideration (3.3) Non-Blocking

   "If there exists a "non-OPES" version of content available from the
   content provider, the OPES architecture must not prevent users from
   retrieving this "non-OPES" version from the content provider" [2].

   For OPES/SMTP, this has been discussed in Section 3.2 and is
   addressed by the two bypass requirements of Section 4.

5.6.  IAB Consideration Application Layer Addresses (4.x)

   While "most application layer addressing revolves around URIs"
   (section 8 of [2]), SMTP uses email addresses, for which the
   considerations only apply to some degree.

   The SMTP use cases document [6] includes a use case for Mail
   Rerouting and Address Rewriting.  Alias and email list address
   resolution are standard functions of an email gateway described in

   Translating the reference validity consideration regarding inter- and
   intra-document reference validity to SMTP, OPES services mapping
   internal to external email addresses MUST ensure the proper mapping
   of addresses in all affected email headers.

5.7.  IAB Consideration (5.1) Privacy

   This consideration recommends that the overall OPES framework must
   provide for mechanisms for end users to determine the privacy
   policies that were used by OPES intermediaries.

   The application agnostic part for OPES has been discussed in Section
   10 of [3].  Email-specific trace information that will be added to
   OPES/SMTP according to the requirements in Section 4 may raise

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   additional privacy issues that MUST be added to the privacy policy
   description of the OPES system.

5.8.  IAB Consideration Encryption

   "If OPES was compatible with end-to-end encryption, this would
   effectively ensure that OPES boxes would be restricted to ones that
   are known, trusted, explicitly addressed at the IP layer, and
   authorized (by the provision of decryption keys) by at least one of
   the ends" [2].

   This has been discussed in Section 3.3.

6.  Security Considerations

   The document itself discusses security considerations of OPES/SMTP.
   General security threats of OPES are described in Security Threats
   for OPES [8]

   Section 3.3 ("Compatibility with Cryptographic Protection
   Mechanisms") mentions that an OPES system could eventually deal with
   cryptographic keys.  This raises security issues (such as
   availability and storage of cryptographic keys) that must be
   addressed by the implementer.

7.  References

7.1.  Normative References

   [1]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

   [2]   Floyd, S. and L. Daigle, "IAB Architectural and Policy
         Considerations for Open Pluggable Edge Services", RFC 3238,
         January 2002.

   [3]   Barbir, A. and A. Rousskov, "Open Pluggable Edge Services
         (OPES) Treatment of IAB Considerations", RFC 3914, October

7.2.  Informative References

   [4]   Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April

   [5]   Rousskov, A. and M. Stecher, "HTTP Adaptation with Open
         Pluggable Edge Services (OPES)", RFC 4236, November 2005.

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   [6]   Stecher, M. and A. Barbir, "Open Pluggable Edge Services (OPES)
         SMTP Use Cases", RFC 4496, May 2006.

   [7]   Barbir, A., Penno, R., Chen, R., Hofmann, M., and H. Orman, "An
         Architecture for Open Pluggable Edge Services (OPES)", RFC
         3835, August 2004.

   [8]   Barbir, A., Batuner, O., Srinivas, B., Hofmann, M., and H.
         Orman, "Security Threats and Risks for Open Pluggable Edge
         Services (OPES)", RFC 3837, August 2004.

   [9]   Elkins, M., Del Torto, D., Levien, R., and T. Roessler, "MIME
         Security with OpenPGP", RFC 3156, August 2001.

   [10]  Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3852,
         July 2004.

   [11]  Eastlake, D., Reagle, J., and D. Solo, "(Extensible Markup
         Language) XML-Signature Syntax and Processing", RFC 3275, March

   [12]  Barbir, A., "Open Pluggable Edge Services (OPES) Entities and
         End Points Communication", RFC 3897, September 2004.

   [13]  Orman, H., "Data Integrity for Mildly Active Content",
         Proceedings of the Third Annual International Workshop on
         Active Middleware Services, p.73, August 2001.

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Appendix A.  Acknowledgements

   Many thanks to everybody who provided input and feedback for this
   document.  Very special thanks to Hilarie Orman for her input and
   suggestions, especially for the content of Section 3.3
   ("Compatibility with Cryptographic Protection Mechanisms").

Author's Address

   Martin Stecher
   Secure Computing Corporation
   Vattmannstr. 3
   33100 Paderborn

   EMail: martin.stecher@webwasher.com
   URI:   http://www.securecomputing.com/

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