Subject: Re: [ietf-dkim] Mailing lists and s/mime & dkim signatures - mua considerations

From: "Sonneveld, Rolf" <[email-masked]>
Date: Wed, 25 Aug 2010 14:13:11 +0200

On 25-08-10, Hector Santos <[email-masked]> wrote:

Rolf E. Sonneveld wrote:

> Although DKIM does not specify (as far as I know) what to do with DKIM
> signatures in inner bodyparts, I think DKIM signatures should never be
> removed without a good reason.

If you believe this, then you have to advocate the removal of the RFC
4871 mandate regarding invalid signatures changing to no-signature
status as if it never existed and the message was never signed.

No, absolutely not! It seems you state here that a broken signature is worse than no signature. It isn't. They're to be treated equally.

What this means is that if a MLM keeps broken tracings of signatures,
the only existing trace signature is the valid one.

No, this is not what it means. It's quite simple: ANY verifier can encounter ZERO, ONE or MORE DKIM signatures, some of them can be broken (by MLM's or by other mail agents), some of them can proof to verify correctly. The only conclusion a verifier can make for EACH VALID signature is, that the domain that's in the d= value of THAT signature takes (some) responsibility for that message (in the incarnation of the message of the moment, when that domain signed the message). All other (non-valid) signatures are to be ignored.

All others must
be ignored.

And ANY non-valid signature must be treated as if it were not present in the message at all. The fact that an MLM breaks a signature is not unique for MLMs. Any agent in the path between signer(s) and verifier(s) can break a signature. Let's keep it clear: a broken signature is to be ignored (base DKIM spec). But removing signatures without a good reason is wrong.

/rolf

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From: Hector Santos <[email-masked]>
Date: Wed, 25 Aug 2010 08:32:40 -0400

Sonneveld, Rolf wrote:

> Let's keep it clear: a broken signature is to be ignored 
> (base DKIM spec). But removing signatures without a good reason 
> is wrong.

A good reason is to lower the confusion of an unknown assessment 
world, especially when the LAST SIGNER is taking responsibility and is 
the presumably the only "vounch-able" entity but the unknown 
non-standard reputation filtering engines (RFE) advocates.

What is your reason for keeping a broken signature?  Do you have an 
RFE that can utilize this information?

Look, systems that support policy are in a better position to filter 
these faults in the system.  But in lieu of a policy standard, I can 
probably see where the MLM or any resigner blindly stamps its DNA 
ignoring any previous single or collection of signatures.

But we have a WG Policy concept in place. One way for the MLM to lower 
the confusion is to honor it first.  After that, it can really do what 
it whats because it is all now in an indeterminate state that can only 
be possibly handled by special signing arrangements.

-- 
Hector Santos, CTO
http://www.santronics.com
http://santronics.blogspot.com

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From: Douglas Otis <[email-masked]>
Date: Wed, 25 Aug 2010 09:16:18 -0700

> non-standard reputation filtering engines (RFE) advocates.
Agreed.  When a mailing list manager (MLM) flattens messages, modifies 
subject lines, and appends unsubscribe information, the mailing list 
manager and _not_ the author should be considered to represent the 
entity introducing the message into the mail stream.  As such, broken 
signatures represent an unnecessary consumption of resources.

Mike Thomas advocated use of the -l body length option with header 
copies to recover most, but not all, signatures using various unknown 
heuristics.  Unfortunately, the -l option can lead to an exploitable 
situation when a message is accepted because it was signed, but 
recipients remain unaware of unsigned portions.  Since not all 
signatures can be recovered, this either exposes recipients to possible 
exploitation, or it creates support issues when messages or portions 
thereof are dropped when recovery fails.  It also seems unrealistic to 
expect adoption of a message display convention to highlight which 
portion of the message body was signed by various signatures.  Use of 
the -l parameter should be considered a bad practice that should be 
deprecated, and too much to ask of MUAs to properly support.
> RFE that can utilize this information?
None that I can imagine, beyond the practice advocated by Mike Thomas.

-Doug

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From: <[email-masked]>
Date: Wed, 25 Aug 2010 08:23:37 -0400

/rolf
<ATT00001..txt>
If I have an email message in my possession and wish to send it on for any reason whatsoever I can remove mangle or otherwise any portion of the message for any reason at all. Why should I keep any forensic information before sending the message on? I am taking responsibility for sending my messages, no one else.