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Versions: 00 01 02 03 RFC 4902

Open Pluggable Edge Services                                  M. Stecher
Internet-Draft                                          Secure Computing
Expires: March 26, 2007                               September 22, 2006


            Integrity, privacy and security in OPES for SMTP
                    draft-ietf-opes-smtp-security-01

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Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   The Open Pluggable Edge Services (OPES) framework is application
   agnostic.  Application specific adaptations extend that framework.
   Previous work has focussed 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 analysis aspects about the integrity of
   SMTP and mail message adaptation by OPES systems and privacy and
   security issues when the OPES framework is adapted to SMTP and lists



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   requirements that must be considered when creating the "SMTP
   adaptation with OPES" document.

Table of Contents

   1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1   Differences between unidirectional and bidirectional
           application protocols  . . . . . . . . . . . . . . . . . .  4
     2.2   Non-standardized SMTP adaptations at SMTP gateways . . . .  4
     2.3   Non-OPES issues of SMTP  . . . . . . . . . . . . . . . . .  4
     2.4   Opportunities of OPES/SMTP to address some issues  . . . .  5
     2.5   Limitations of OPES in regards to fixing SMTP issues . . .  5
   3.  Integrity, privacy and security considerations . . . . . . . .  6
     3.1   Tracing info in OPES/SMTP  . . . . . . . . . . . . . . . .  6
     3.2   Bypass in OPES/SMTP  . . . . . . . . . . . . . . . . . . .  6
     3.3   Compatibility with end-to-end encryption and signatures  .  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 . . . . . . . . . . .  9
     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  . . . . . . . . . . . . . . . . . . . 12
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     7.1   Normative References . . . . . . . . . . . . . . . . . . . 13
     7.2   Informative References . . . . . . . . . . . . . . . . . . 13
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 13
       Intellectual Property and Copyright Statements . . . . . . . . 14


















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1.  Terminology

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   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.











































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2.  Introduction

2.1  Differences between unidirectional and bidirectional application
     protocols

   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; on 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 allowing the data consumer to request 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.

2.2  Non-standardized SMTP adaptations at SMTP gateways

   A large number of email filters are deployed at SMTP gateways today;
   in fact all usecases 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 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 might be able to improve the situaton to some extent.

2.3  Non-OPES issues of SMTP

   The SMTP specifications [4] require that NDRs (Non Delivery Reports)



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   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.

2.4  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 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.

2.5  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 not 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 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.

   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.








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3.  Integrity, privacy and security considerations

3.1  Tracing info in OPES/SMTP

   Tracing is an important requirement for OPES systems.  Tracing
   information added to mails 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] define for the "Received" headers.

   Trace information is then seen by mail recipients when the mails
   reach 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 a NDR MUST be sent
   to the originator; OPES requires that if a NDR is sent that the
   report MUST also contain information about the OPES system so that
   the sender gets informed.  If an email is rejected, 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 a 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.

   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 it
   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 quarantaine 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 quarantaine.

   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 by bypassed (such as virus scanners for
   example).  In general, the receiver should be able to configure a



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   client centric OPES system, i.e. the receiver should be able to
   indicate if he/she wants to receive a non-OPES version if the OPES
   service would result in rejection of the email.

   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
   commands).

   Bypass request data sent at the beginning of a SMTP dialog may not
   reach the OPES system if intermediate SMTP relays do not support
   those bypass request commands and don't forward that information.

3.3  Compatibility with end-to-end encryption and signatures

   End-to-end email encryption is a proven technology, although the
   majority of mails are still sent unencrypted.  Encrypting and signing
   email messages is done on the content of the mail and would be
   transparent to SMTP.  Encrypted mail messages can either be used to
   prevent OPES systems from inspecting or modifying the content, or it
   can be used as an explicit approval to filter the mail by the OPES
   system, if keys for decryption of the message are made available to
   the OPES system.  Signing of mails can be used to trace whether
   content has been changed by intermediates.

   There are security risks associated with storing cryptographic keys
   that must be addressed by implementors.  Because this is not a simple
   task, it is only suggested as an option, not as a requirement for
   OPES/SMTP.





















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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
   recipient.

   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 incompliant 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"
   [2].

   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 2.3 OPES/SMTP
   cannot prevent that NDRs are not sent or get blocked before reaching
   the sender of the original message.

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



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   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 is 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 apply to some degree only.

   The SMTP use cases document [6] includes a use case for Mail
   Rerouting and Address Rewriting.  Alias and email list address
   resolution are standard function of an email gateway described in
   [4].

   Translating the reference validity consideration regarding inter- and
   intra-document reference validity to SMTP, OPES services mapping
   internal to external email addresses MUST ensure to properly map
   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 of OPES intermediaries.

   The application agnostic part for OPES and 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 additional privacy issues that MUST be added to the privacy
   policy description of the OPES system.





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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.










































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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 (about compatibility with end-to-end encryption) mentions
   that an OPES system could be approved to inspect encrypted mails by
   making keys available for decryption.  It must be noted that an
   implementation of the decryption key handling raises security issues
   (such as availability and storage of cryptographic keys) that must be
   addressed by the implementer.







































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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 2004.

7.2  Informative References

   [4]  Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
        April 2001.

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

   [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.


Author's Address

   Martin Stecher
   Secure Computing Corporation
   Vattmannstr. 3
   33100 Paderborn
   Germany

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







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