Open Pluggable Edge Services                                  M. Stecher
Internet-Draft                                          Secure Computing
Expires: November 23, 2006                                  May 22, 2006

            Integrity, privacy and security in OPES for SMTP

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   Copyright (C) The Internet Society (2006).


   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 . . . . . . . . .  7
   4.  Requirements for OPES/SMTP . . . . . . . . . . . . . . . . . .  8
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     6.1   Normative References . . . . . . . . . . . . . . . . . . . 10
     6.2   Informative References . . . . . . . . . . . . . . . . . . 10
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 10
       Intellectual Property and Copyright Statements . . . . . . . . 11

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1.  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.

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2.  Introduction

2.1  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 request
   to 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 the OPES bypass of his

   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 is 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 completly but
   at least might be able to improve the situaton to some extend.

2.3  Non-OPES issues of SMTP

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

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   are sent to the originator of an undeliverable mail that has been
   accepted by an SMTP server.  But it became common practice that some
   sort of mail (spam, worms) is silently dropped without sending an NDR
   in violation of that 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 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 OPES systems may change messages in a way that the
   consumer cannot longer read them or that messages are not longer
   delivered at all.

   Defining a standard way to mark mails that are handled by OPES
   systems is fairly simple and does not require new techniques by SMTP
   gateways that already today MUST leave tracing information by adding
   "Received" headers to mails.  Therefore, recipients receiving broken
   mail have a fair chance to find the compromised OPES system by using
   the trace information.  There is still no guarantee as the email may
   be 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 instrument to show
   the HTTP headers that may 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 no reliable medium of 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, following a similar syntax and structure
   as 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 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 to 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 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 sent to itself which
   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 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

   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

   End-to-end email encryption is a proven technology although still the
   majority of mails are sent unencrypted.  Encrpyting and signing email
   is done on the content of mails and transparent for SMTP.  Encrypted
   mails can either be used to prevent OPES systems to inspect or modify
   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
   which must be addressed by implementors.  Beause this is not a simple
   task, it is only suggested as an option, not as a requirement for

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4.  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 that has been accepted by an OPES system cannot be
      delivered, the non delivery report MUST include trace information
      of the OPES system.

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

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

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5.  Security Considerations

   The document itself discusses security considerations of OPES/SMTP.

   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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6.  References

6.1  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.

6.2  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.

Author's Address

   Martin Stecher
   Secure Computing Corporation
   Vattmannstr. 3
   33100 Paderborn


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