[dkim-ops] key validation question

[Tony Hansen]

On 4/9/2011 3:17 AM, SM wrote:
> Hi Tony,
> At 19:39 08-04-2011, Tony Hansen wrote:
>> The value of h= does *not* indicate what is *implemented* by the signer
>> -- it *only* indicates what is currently being *used* to sign with.
>> Nowhere does the spec say that h= indicates what has been implemented by
>> the signer.
>
> The text in Section 3.6.1 is confusing due to the MUST.  It mixes 
> implementation and operations.  It can be read as:
>
>  If you have the h= in the DNS record, you MUST include "sha1" and
>  "sha256" in the list of acceptable hash algorithms.
>
> Murray mentioned a case when "h=" may be useful.  Even if you rewrite 
> the "MUST", people reading the RFC might still be confused.  It would 
> be clearer if the following was removed:
>
>   'Signers and Verifiers MUST support the "sha256" hash algorithm.
>    Verifiers MUST also support the "sha1" hash algorithm.'

Interesting suggestion.

The MUSTs *are* redundant with section 3.3's first paragraph. However, 
it's still important.

If this section were rewritten, I'd suggest something like this:

    h=  Acceptable hash algorithms (plain-text; OPTIONAL, defaults to
        allowing all algorithms).  A colon-separated list of hash
        algorithms that might be used.

        As stated in section 3.3, Signers and Verifiers MUST
        support the "sha256" hash algorithm, and Verifiers MUST also support
        the "sha1" hash algorithm. Which algorithms are listed
        in h= is an operational choice by the sender.

Tony Hansen