Re: [Uta] Robert Wilton's Discuss on draft-ietf-uta-rfc7525bis-09: (with DISCUSS and COMMENT)

[Thomas Fossati <[e-mail]>]

Hi Rob, Peter,

On Thursday, 14 July 2022 at 16:07, Peter Saint-Andre <[e-mail]> wrote:
> On 7/14/22 3:37 AM, Robert Wilton via Datatracker wrote:
> > (4)
> >     When using RSA, servers MUST authenticate using certificates
> >     with at least a 2048-bit modulus for the public key.  In
> >     addition, the use of the SHA-256 hash algorithm is RECOMMENDED
> >     and SHA-1 or MD5 MUST NOT be used ([RFC9155], and see
> >     [CAB-Baseline] for more details).
> >
> > So, for clarity, this would presumably mean that SHA-256 is also
> > preferred over say SHA-512?  Is that the intention?  Or would it be
> > better if the SHOULD allowed stronger ciphers?
>
> I think we should probably say "SHA-256 or stronger", but again I'd
> like to see what my co-authors think.

My two cents on this point.

Readers are always free to choose stronger algorithms if they want to.
However, in this case I don't see a good reason for doing so: if your
threat model involves an adversary with a quantum computer, 256 is as
good as 384 or 512, but it's more concise.  So, if your cert is
short-lived, going higher than 256 does not provide any real advantage.

If one's cert's notAfter is distant enough that they should start
worrying about the reality of a quantum adversary, then I think the whole
advice in this section would need a massive revamp :-)

cheers, t


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