Redaction of Potentially Sensitive Data from Mail Abuse Reports
draft-ietf-marf-redaction-08

quot;, frustrated any kind
   of prioritizing or grouping of reports.  This memo presents a
   practice for conducting redaction in a manner that allows a report
   receiver to detect that two reports were caused by the same end user
   without revealing the identify of that user.  That is, the report
   receiver can use the redacted string, such as an obscured email
   address, to determine that two such unredacted strings were
   identical; the reports originally contained the same address.

   Generally, it is assumed that the recipient-identifying fields of a
   message, when copied into a report, are to be obscured to protect the
   identity of the end user who submitted the complaint about the
   message.  However, it is also presumed that other data will be left
   intact, and those data could be correlated against log files or other
   resources to determine the intended recipient of the original
   message.

2.  Keywords

   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 [KEYWORDS].

3.  Recommended Practice

   When redacting of reports is desired, in order to enable a report
   receiver to correlate reports that might refer to a common but
   anonymous source, the report generator SHOULD use the following
   practice:

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   1.  Select a transformation mechanism (see Section 4) that is
       consistent (i.e., the same input string produces the same output
       each time) and reasonably collision-resistant (i.e., two
       different inputs are unlikely to produce the same output).
   2.  Identify string(s) (such as local-parts of email addresses) in a
       message that need to be redacted.  Call these strings the
       "private data".
   3.  For each piece of private data, apply the selected transformation
       mechanism.
   4.  If the output of the transformation can contain bytes that are
       not printable ASCII, or if the output can include characters not
       appropriate to replace the private data directly, encode the
       output with the base64 algorithm as defined in Section 4 of
       [BASE64], or some similar translation to a form valid replacement
       in the original context.  For example, replacing a local-part in
       an email address with transformation output containing an "@"
       character (ASCII 0x40) or a space character (ASCII 0x20) is not
       permitted by the specification for local-part ([SMTP]), so the
       transformation output needs to be encoded as described.
   5.  Replace each instance of private data with the corresponding
       (possibly encoded) transformation when generating the report.
       Note that the replaced text could also be in a context that has
       constraints such as length limits that need to be observed.

   This has the effect of obscuring the data (in a potentially
   irreversible way) while still allowing the report recipient to
   observe that numerous reports are about one particular end user.
   Such detection enables the receiver to prioritize its reactions based
   on problems that appear to be focused on specific end users that may
   be under attack.

4.  Transformation Mechanisms

   This memo does not specify a particular transformation mechanism as a
   requirement.  The interoperability that this memo seeks to provide is
   enabled by the consistency of the transformation.

   The issue of the security of the transformation, frustrating attempts
   to reverse the transformation, is a matter of local policy.  A
   continuum of possible transformations exists, from trivial ones such
   as rot13, CRC32 and base64, through strong cryptographic encodings
   such as [HMAC] and even full encryption, or private transformations
   such as mapping an email address to an internal customer number.  An
   operator wishing to perform report redaction needs to select a
   consistent transformation that obscures the private data and is
   resilient to attempts to extract the original data to the extent
   required by local policy, keeping in mind that the environment in

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   which the transformation is operating is not a highly secure one.
   See Section 5.3 for further details of this issue.

   An implementation MAY choose any transformation that has a reasonably
   low likelihood of collision.

5.  Security Considerations

5.1.  General

   General security issues with respect to these reports are found in
   [ARF].

5.2.  Digest Collisions

   Message digest collisions are a well-understood issue.  Their
   application here involves a report receiver improperly concluding
   that two pieces of redacted information were originally the same when
   in fact they are not.  This can lead to a denial of service, where
   the inadvertently improper application of complaint data causes
   unjustified corrective action.  Such cases are sufficiently unlikely
   as to be of little concern.

5.3.  Information Not Redacted

   Although the identity of the user causing a report to be generated
   can be obscured using this mechanism, other properties of a message
   (such as the Message-ID field) that are not redacted could be used to
   recover the original data by locating them in the message logs of the
   originating system or via other data correlation techniques.  It is
   incumbent on the report generator to anticipate and redact or
   otherwise obscure such data, or accept that such recovery is possible
   even from the very simplest kinds of feedback.

   It is for this reason that the normative portions of this memo do not
   include stronger assertions about cryptography used in the
   transformation.  Given the ultimate recoverability of the redacted
   information, the cryptographic strength of the transformation is not
   a critical security measure.

   The process of redacting a feedback report satisfies a privacy
   requirement established by local policy, and is not meant to provide
   strong security properties.

   [FBL-BCP] and Section 8 of [ARF] discuss topics related to
   establishment of bilateral agreements between report producers and
   consumers.  The issues raised here are also things to be considered

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   when establishing such agreements.

6.  Privacy Considerations

   While the method of redaction described in this document may reduce
   the likelihood of some types of private data from leaking between
   ADMDs, it is extremely unlikely that report generation software could
   ever be created to recognize all of the different ways that private
   information could be expressed through human written language.  If
   further protections are required, implementers may wish to consider
   establishing some sort of out-of-band arrangements between the
   relevant entities to contain private data as much as possible.

7.  IANA Considerations

   This memo includes no request to IANA.

   [RFC Editor note: This section may be removed prior to publication.]

8.  References

8.1.  Normative References

   [ARF]      Shafranovich, Y., Levine, J., and M. Kucherawy, "An
              Extensible Format for Email Feedback Reports", RFC 5965,
              August 2010.

   [BASE64]   Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

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

8.2.  Informative References

   [FBL-BCP]  Falk, J., "Complaint Feedback Loop Operational
              Recommendations", RFC 6449, November 2011.

   [HMAC]     Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              February 1997.

   [SMTP]     Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

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Appendix A.  Example

   Assume the following input message:

     From: alice@example.com
     To: bob@example.net
     Subject: Make money fast!
     Message-ID: <123456789@mailer.example.com>
     Date: Thu, 17 Nov 2011 22:19:40 -0500

     Want to make a lot of money really fast?  Check it out!
     http://www.example.com/scam/0xd0d0cafe

   On receipt, bob@example.net reports this message as abusive through
   whatever mechanism his mailbox provider has established.  This causes
   an [ARF] message to be generated.  However, example.net wishes to
   obscure Bob's email address lest it be relayed to the offending
   agent, which could lead to more trouble for Bob.

   Thus, example.net plans to redact the local-part of the recipient
   address in the To: field.  Local policy and security requirements
   suggest the algorithm known as "H" (a hash of a key concatenated with
   the data to be obscured) using SHA1 is adequeate.  It has thus
   selected a redaction key of "potatoes", and the private data in this
   case is the string "bob".  The concatenation of "potatoesbob" is
   digested with SHA1 and then base64-encoded to the string
   "rZ8cqXWGiKHzhz1MsFRGTysHia4=".

   Therefore, when constructing the ARF message in response to Bob's
   complaint, the following form of the received message is used in the
   third part of the ARF report:

     From: alice@example.com
     To: rZ8cqXWGiKHzhz1MsFRGTysHia4=@example.net
     Subject: Make money fast!
     Message-ID: <123456789@mailer.example.com>
     Date: Thu, 17 Nov 2011 22:19:40 -0500

     Want to make a lot of money really fast?  Check it out!
     http://www.example.com/scam/0xd0d0cafe

   Note, however, that it is possible the redacted information can be
   recovered by agents at example.com searching their logs for the
   original envelope associated with the message, by correlating with
   the Message-ID contents which were not redacted here.  It is expected
   that feedback loops generating such reports involve senders that have
   been vetted against such information leakage.

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Appendix B.  Acknowledgements

   Much of the text in this document was initially moved from other MARF
   working group documents, with contributions from Monica Chew, Tim
   Draegen, Michael Adkins, and other members of the Messaging Anti-
   Abuse Working Group.  Additional feedback was provided by John
   Levine, S. Moonesamy, Alessandro Vesely, and Mykyta Yevstifeyev.

Authors' Addresses

   J.D. Falk (editor)
   Return Path
   100 Mathilda Place, Suite 100
   Sunnyvale, CA  94086
   US

   Email: ietf@cybernothing.org
   URI:   http://www.returnpath.net/

   M. Kucherawy (editor)
   Cloudmark
   128 King St., 2nd Floor
   San Francisco, CA  94107
   US

   Email: msk@cloudmark.com

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