On 08/03/2010 06:40 PM, Murray S. Kucherawy wrote:
-----Original Message-----
From: Michael Thomas [mailto:mike(_at_)mtcc(_dot_)com]
Sent: Tuesday, August 03, 2010 9:21 AM
To: Murray S. Kucherawy
Cc: Rolf E. Sonneveld; ietf-dkim(_at_)mipassoc(_dot_)org
Subject: Re: [ietf-dkim] MLMs and the use of multipart/alternative to
preserve original DKIM signature and at the same time add a new DKIM
signature
But didn't we also say that such reverified signatures don't get any
additional meaning with 'z=' reprocessing?
Sorry, I don't understand.
I guess I don't either. You're saying use of "l=" and "z=" got your mail-through-lists signature
verification statistics up to 95%. However, RFC4871 says "Copied header field values are for diagnostic use" which I
interpret to mean (and I think discussion on the list back then also agreed) that the information in a "z=" tag isn't
supposed to contribute to the canonicalization algorithms, but instead can only be used for diagnostic purposes (i.e., "This
signature failed, and via the 'z=' we know why... but it still failed.").
Furthermore, the use of "l=" is discouraged in RFC4871 and in the MLM draft:
par. 3.5:
<quote>
INFORMATIVE IMPLEMENTATION WARNING: Use of the "l=" tag might
allow display of fraudulent content without appropriate
warning to end users. The "l=" tag is intended for
increasing signature robustness when sending to mailing lists
that both modify their content and do not sign their
messages. However, using the "l=" tag enables attacks in
which an intermediary with malicious intent modifies a
message to include content that solely benefits the attacker.
It is possible for the appended content to completely replace
the original content in the end recipient's eyes and to
defeat duplicate message detection algorithms. Examples are
described in Security Considerations (Section 8
<http://tools.ietf.org/html/rfc4871#section-8>). To avoid
this attack, signers should be extremely wary of using this
tag, and verifiers might wish to ignore the tag or remove
text that appears after the specified content length.
</quote>
and
<quote>
A possible mitigation to this incompatibility is use of the "l=" tag
to bound the portion of the body covered by the body hash, but this
not workable for [MIME] messages and moreover has security
considerations (see Section 3.5 of [DKIM]). Its use is therefore
discouraged.
</quote>
/rolf
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