Re: [dmarc-ietf] third party authorization, not, was non-mailing list

[Dotzero <[e-mail]>]

On Sat, Aug 29, 2020 at 3:43 PM Douglas E. Foster <fosterd=
[e-mail]> wrote:

> To elaborate on my question and Michael Hammer's answer:
>
> To be unique, a signature needs a unique dataset from which the hash is
> computed.   The weak signature will not be unique because it will be
> computed on non-random content such as From, To, and Date.
>

There are additional ways of introducing complexity and randomness.

>
> However, the signature can only be used by the designated domain.   So the
> worst possible "misuse" would be for the designated domain to use the
> signature on other messages.   This seems unlikely, and the worst-case use
> is no different than what ATSP would authorize.   But the weak signature
> has less information leakage, since nothing is published in DNS about the
> signature technique.   So I agree that the approach is a good one for those
> who want to provide mailing-list authorization.
>
> The remaining challenge is to communicate between recipient domains and
> mailing lists so that the list knows whether the recipient will honor the
> weak signature system.
>
> Doug Foster
>
>
>
>
> ------------------------------
> *From*: Jim Fenton <[e-mail]>
> *Sent*: 8/26/20 5:01 PM
> *To*: Dotzero <[e-mail]>
> *Cc*: IETF DMARC WG <[e-mail]>
> *Subject*: Re: [dmarc-ietf] third party authorization, not, was
> non-mailing list
> On 8/26/20 10:54 AM, Dotzero wrote:
>
>
>
> On Wed, Aug 26, 2020 at 1:32 PM Doug Foster <fosterd=
> [e-mail]> wrote:
>
>> Are the weak signatures vulnerable to a replay attack?    I thought that
>> one of the reasons that DKIM signatures included the whole body was to
>> prevent the signature from being reused.
>>
>>
>>
>> DF
>>
>
> Not particularly vulnerable. The requirement is that you have the "weak
> signature" plus the intermediary full DKIM signature. This let's the
> validator/receiver know that the originating domain knew that the
> intermediary might break the originating domains DKIM signature but the
> validator/receiver would have the DKIM signature of the intermediary. The
> "weak signature" is only validated against that specific message and
> headers it signed and that specific intermediary. It's not a
> generic/general signature.
>
>
> It sounds like the weak signature is just a regular DKIM signature plus
> the designation of the intermediary, and the "weak" part is that you don't
> check the body hash against the body. Have I got that right?
>
> -Jim
>
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