MARF Working Group J. Falk, Ed.
Internet-Draft Return Path
Intended status: Standards Track M. Kucherawy, Ed.
Expires: July 21, 2012 Cloudmark
January 18, 2012
Redaction of Potentially Sensitive Data from Mail Abuse Reports
draft-ietf-marf-redaction-05
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.
Status of this Memo
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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
3.4. Key Management . . . . . . . . . . . . . . . . . . . . . . 5
3.5. Algorithm Vulnerabilities . . . . . . . . . . . . . . . . . 5
4. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6.1. Normative References . . . . . . . . . . . . . . . . . . . 6
6.2. Informative References . . . . . . . . . . . . . . . . . . 6
Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . . 6
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7
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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 anonymize, or obscure, portions of the report that might
identify an end user who caused the report to be generated. This has
come to be known in feedback loop parlance as "redaction". 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 be correlated against log files or other
resources to determine the intended recipient of the message.
2. 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:
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". The redaction key
SHOULD be based on at least 64 bits of pseudo-random input. (See
Section 3.3 and Section 3.4 for additional discussion.)
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; a secure hash such as one defined in [FIPS-180-3-2008]
or a secure message digest algorithm based on a secure hash is
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suggested. (See Section 3.3 for discussion.)
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.
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 by locating them in the message logs of the originating system
or 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 minimum lengths for the redaction
key or the selection of particularly strong hashes. Given the
ultimate recoverability of the redacted information, the
cryptographic strength of the hash and particularly long or
unguessable keys are not critical security measures.
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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
when establishing such agreements.
3.4. Key Management
As with any application that uses secret keys, care must be taken to
guard the redaction key against compromise. If the key is no longer
a secret, recovering the redacted information becomes a simple brute
force attack.
Also, periodically changing the key is a means of limiting the
quantity of redacted information that would be exposed by the
compromise of a single redaction key, and hence is advised. However,
a consideration when developing a key rotation policy is that
correlation of the redacted form of the obscured information cannot
occur across use of different redaction keys.
3.5. Algorithm Vulnerabilities
The simple key-message hash method described in Section 2 is
vulnerable to some well known attacks that can be used to recover the
redaction key. This is especially important to consider if obtaining
the redaction key also somehow creates an exposure for the report
generator in other ways. Although stronger mechanisms like [HMAC]
close these loopholes, it is believed that such extra hardening is
unnecessary given the discussion above.
Future work that seeks to obscure private data in some way should not
presume that this mechanism is sufficient. It solves a simple policy
requirement for this specific use case, and is not a reliable
security mechanism for general use.
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.
[HMAC] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997.
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
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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. 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 John Levine, 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)
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128 King St., 2nd Floor
San Francisco, CA 94107
US
Email: msk@cloudmark.com
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