It appears that Hannah Stern  <[email protected]> said:
>Hi!
>
>On 7/20/25 21:10, John R Levine wrote:
>> On Sun, 20 Jul 2025, Wei Chuang wrote:
>>> There are two problems: first, the keys that a sender supports are 
>>> obscured by the selectors.
>
>> I don't see what the problem is.  Every signature has the selector and 
>> algorithm so the verifier knows what to look for, right?
>
>Only if we implicitly assume the decision to mandatorily use the same 
>selector for all algorithms.

No, that depends how we do it.  Several of the proposals have separate
selectors for each signature.

>> Hmm.  I want to think some more about whether the rule is that ALL the 
>> signatures have to be valid (give or take ones the verifier doesn't 
>> support) or ANY signature is adequate.
>
>For the still newer PQC algorithms, it could make sense to require that 
>at least one PQC and at least one preQC algorithm yield a valid 
>signature. So in case the chosen PQC algorithm turns out to be weak, 
>we'd be at least still secure-enough against non-quantum attackers.

I fear this is a swamp we do not want to enter, trying to say which
signatures are "better" than others.  If recipient systems want to
apply their own heuristics they can do that, but I do not believe that
we can guess now what sort of heuristics will be useful or which will
be useless or even worse, counterproductive.

R's,
John

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