Re: [openpgp] SHA3 algorithm ids.

Paul Wouters <paul@nohats.ca> Tue, 11 August 2015 12:30 UTC

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Date: Tue, 11 Aug 2015 08:30:42 -0400 (EDT)
From: Paul Wouters <paul@nohats.ca>
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Cc: Phillip Hallam-Baker <phill@hallambaker.com>
Subject: Re: [openpgp] SHA3 algorithm ids.
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On Tue, 11 Aug 2015, Werner Koch wrote:

> We have a lot of experience in how to deploy new algorithms and we are
> very conservative here.  My request for adding SHA3 algo ids does not
> mean in any way that I endorse its use or would even suggest that
> 4880bis should contain a SHOULD or MAY for implementing such an
> algorithm.  When we come to the point on deciding on algorithms I would
> suggest something like this:
>
> - Implementations MUST implement SHA-1.  Implementations MAY implement
> - other algorithms.  MD5 is deprecated.
> + Implementations MUST implement SHA-1 and SHA2-FIXME.  Implementations
> + MUST NOT implement MD5.  Implementations SHOULD NOT implement
> + SHA3-xxxx.  Implementations MAY implement other algorithms.

openpgp is unique in that there is a _very_ long validity time required
for some algorithms, so one could verify a 20 year old message, even if
that security 20 years later is questionable (eg breakable)

I would like to see (and maybe the documents already do that but the
above bullet points don't indicicate this) a difference in support for
verifying signatures (eg we should implement MD5 and MUST implement
SHA1) and creating new signatures (MUST NOT use MD5 or SHA1)

> The algo ids are a different case and I would be fine with the RFC-7120
> method.  Iff the unexpected case happens that a severe weakness in SHA2
> is found, the pre-allocated SHA3 ids will allow us to quickly switch to
> SHA3.  Isn't that the whole point of SHA3 being plugin-in replacements
> for SHA2?

Yes, but I don't see why we need to have 6 versions of SHA3 on standbye.
openpgp validity / security is measured in years, and as such,
performance don't really come to play when considering algorithms.

Paul