Re: including the entire fingerprint of the issuer in an OpenPGP certification

David Shaw <dshaw@jabberwocky.com> Tue, 18 January 2011 22:05 UTC

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Subject: Re: including the entire fingerprint of the issuer in an OpenPGP certification
From: David Shaw <dshaw@jabberwocky.com>
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Date: Tue, 18 Jan 2011 17:05:55 -0500
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On Jan 18, 2011, at 4:07 PM, Daniel Kahn Gillmor wrote:

> On 01/18/2011 12:48 PM, Jon Callas wrote:
>> If we combine it with a hash-independent fingerprint -- e.g., first byte is an algorithm ID, others are the actual hash -- then we can put it in now and then run with it.
> 
> Daniel Nagy suggests that we also change the material being hashed in
> the fingerprint (he wants to leave out the creation date).  If that
> proposal ends up achieving consensus then your proposal here will need
> further modification.
> 
> Does anyone feel strongly about Nagy's proposal, by the way?  i'm not
> sure what the tradeoffs are.
> 
> Even without that concern, if we encourage super-flexible use like this,
> user agents who wanted to use it to test for the presence of a given key
> in an indexed keystore would need to index their keys with every
> possible digest that might be used -- that seems excessive somehow.
> (and unlikely that keyserver implementations would want a half-dozen
> indexes, for that matter)  Wouldn't it be better to just implement it
> for today's fingerprint, and then when a new fingerprint is agreed upon,
> determine (by subpacket length maybe?) whether it's the new fingerprint
> or the old one.  Compliant user agents would keep the two indexes around
> until the v4 fingerprint goes away, and then drop the old one.
> 
> Alternately, we could embed the algorithm ID as you suggest, and SHOULD
> people into generating them using only the consensus fingerprint
> algorithms so that reasonable user agents only need to create indexes
> over SHA1 (now) and SHA3 (whenever that happens).

I don't think we want people using other than the consensus fingerprint algorithms and methods.  I suggest we make the first byte a version field, which can be set to '4' today for the current fingerprint, '5' for v5 keys, etc.  I suppose we could skip that field and detect version based on size, but why use heuristics when we can know for sure with a version byte?

David