MARF Working Group J. Falk, Ed.
Internet-Draft Return Path
Intended status: Standards Track M. Kucherawy, Ed.
Expires: July 6, 2012 Cloudmark
January 3, 2012
Redaction of Potentially Sensitive Data from Mail Abuse Reports
draft-ietf-marf-redaction-04
Abstract
Email messages often contain information that might be considered
private or sensitive, per either regulation or social norms. When
such a message becomes the subject of a report intended to be shared
with other entities, the report generator may wish to redact or elide
the sensitive portions of the message. This memo suggests one method
for doing so effectively.
[NOTE TO EDITOR: Murray Kucherawy is listed as an author only to
enable him to complete the publication process on behalf of J.D.
Falk. Please remove Murray from the author list prior to
publication.]
Status of this Memo
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Copyright Notice
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Recommended Practice . . . . . . . . . . . . . . . . . . . . . 3
3. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
3.1. General . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Digest Collisions . . . . . . . . . . . . . . . . . . . . . 4
3.3. Information Not Redacted . . . . . . . . . . . . . . . . . 4
4. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
6.1. Normative References . . . . . . . . . . . . . . . . . . . 5
6.2. Informative References . . . . . . . . . . . . . . . . . . 5
Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . . 5
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6
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1. Introduction
[ARF] defines a message format for sending reports of abuse in the
messaging infrastructure, with an eye toward automating both the
generating and consumption of those reports.
For privacy considerations it might be the policy of a report
generator to redact, or obscure, portions of the report that might
identify an end user who caused the report to be generated.
Precisely how this is done is unspecified in [ARF] as it will
generally be a matter of local policy. That specification does
admonish generators against being too over-zealous with this
practice, as obscuring too much data makes the report non-actionable.
Previous redaction practices, such as replacing local-parts of
addresses with a uniform string like "xxxxxxxx", often frustrates any
kind of prioritizing or grouping of reports.
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 that data could theoretically be correlated against log
files or other resources to determine the intended recipient of the
message.
2. Recommended Practice
To enable correlation of reports that might refer to a common but
anonymous source, the following redaction practice is RECOMMENDED:
1. Select an arbitrary string that will be used by an Administrative
Management Domain (ADMD) that generates reports. This string
will not be changed except according to a key rotation policy or
similar. Call this the "redaction key".
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, construct a new string that is a
copy of the redaction key with the private data concatenated to
it.
4. Compute a digest of each composite string with any hashing/digest
algorithm such as one defined in [FIPS-180-3-2008].
5. Encode each digest with the base64 algorithm as defined in
[BASE64].
6. Replace each instance of private data with the corresponding
base64-encoded hash when generating the report.
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This has the effect of obscuring the data in an 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.
3. Security Considerations
3.1. General
General security issues with respect to these reports are found in
[ARF].
3.2. 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.
3.3. Information Not Redacted
Although the identity of a report generator can be redacted 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. It is incumbent on the report generator to anticipate and
redact or otherwise obscure such data, or accept that such recovery
is possible.
[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
when establishing such agreements.
4. 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
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relevant entities to contain private data as much as possible.
5. IANA Considerations
This memo includes no request to IANA.
[RFC Editor note: This section may be removed prior to publication.]
6. References
6.1. 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.
6.2. Informative References
[FBL-BCP] Falk, J., "Complaint Feedback Loop Operational
Recommendations", RFC 6449, November 2011.
[FIPS-180-3-2008]
U.S. Department of Commerce, "Secure Hash Standard", FIPS
PUB 180-3, October 2008.
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
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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. It has 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=".
Thus, 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 by 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.
Appendix B. Acknowledgements
Much of the text in this document was initially moved from other MARF
working group documents, crafted by Murray S. Kucherawy with
contributions from Monica Chew, Tim Draegen, Michael Adkins, and
myself. Additional feedback was provided by S. Moonesamy, Alessandro
Vesely, and Mykyta Yevstifeyev.
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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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