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Re: [ietf-dkim] Base issue: multiple linked signatures

2007-01-03 23:10:29
Stephen Farrell wrote:

Well, happy new year all.

Having looked through this, I see no consensus for this
addition/change other than perhaps to weaken the "MUST NOT"
on the "z=". Correct me if I'm wrong there.

Was there a suggestion for new text for the "z=" that we can
consider? (Sorry if I missed it.)

My input on this.

Based on the earlier discussions about this, I found it to be a wasted tag with meaningless value because of the idea it was "acceptable" that the contents of the tags could be different what the actual header values are, including having other header values that were not necessarily part of the hashing.

Under those considerations, it could be *only* be used for some diagnostic reason only understood by the signer and if there is any good reason, I would think it would be for analyzing changed header values.

So if there was an *official* way to determine if a header change did take place, then maybe the Z= tag might be useful here for explaining a possible signature failure. We already have the body hash, so if that turns out to be ok, then "possibly" a signature failure was due to a header change. But that only be done if a copy of the headers used for signing are stored in the z= tag.

If this becomes acceptable, then its quite possible for a verifier to short circuit the verification hashing overhead if the h= header values do not match z= values. That would be a big plus in the verifier world.

It is quite possible to use this for checking DKIM signed mail that have arrived from list servers. A specific verifier may wish to do a brute force checking concept, 1) first check the h= headers, then if that fails, 2) replace the header values with the z= copy and check again.

By far, the typical list server minimum mutations would be:

  1) optionally adding a [listname] tag to the subject line
  2) optionally adding a footer to the body.

Its quite possible for the verifier to resolve the body with the l= length, and the subject change, if hashed, with the z= copy.

So if there any proposal for text change here, I would suggest that the z= tag, if used, "SHOULD" include a copy of the hash header values or at the very least, the copy of the header "most likely" to change for the DOMAIN intentional chosen DKIM MAIL ROUTE of this mail that may suffer mail integrity issues.

If anything, change this text:

    Verifiers MUST NOT use the header field names or copied values
    for checking the signature in any way.  Copied header field
    values are for diagnostic use only.

to:

    Verifiers MAY use the header field names or copied values
    for checking the signature and comparing expected header
    values.


---
HLS






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