Re: [ietf-dkim] Core algorithm support/use, draft text v2

[Jim Fenton]

Eric Rescorla wrote: Jim Fenton <[EMAIL PROTECTED]> writes: The "figuring out" of which algorithm to use is the central problem here. There just isn't a good way to do it, which is why my text suggested the use of two signatures. The verifier only needs to verify one of them, and can decide which hash algorithm to use for the verification early in the receipt of the message. I'm less concerned with burdening signers than verifiers. Certainly, I agree that when transitioning from algorithm A to algorithm B you need to temporarily sign with both A and B. The question is to what extent we are transitioning from A to B. E.g., if we do anything to break existing signatures, then there's less point in running SHA-1 in parallel since the signatures won't work anyway and we can just tell people to deploy SHA-256 verification with the new canonicalization or whatever. I can't think of any reason that we would break existing signatures without some observable change in the DKIM-Signature header field.  Either a change in some parameter (such as the change in canonicalization algorithm names between allman-00 and allman-01) or an increment in the version number.  I consider such a change equivalent to a change in the hash algorithm from a compatibility standpoint:  we could run the old and new versions in parallel as separate signatures. That said, why are you less concerned with signers than verifiers? RSA signatures are much faster to verify than generate (about 20x) so even messages destined for a fair number of people tend to burden the signer more. And, of course, the verification load is likely to be more spread out.... A single message destined for a fair number of people would only be signed once, since we aren't signing the envelope addresses.  The reason I'm more sensitive to verification load is the potential make-work attack from an attacker who generates valid-looking but invalid signatures (which are effectively free in terms of computation).  This is described in more detail in section 4.1.6 of the threat document. -Jim

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