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Re: [openpgp] Version 5 key and fingerprint proposal

Jon Callas <joncallas@icloud.com> Fri, 10 March 2017 22:13 UTC

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From: Jon Callas <joncallas@icloud.com>
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Date: Fri, 10 Mar 2017 14:13:35 -0800
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References: <87varlou5m.fsf@wheatstone.g10code.de> <20170307230605.GA2@hashbang.sh> <87efy8ntcx.fsf@wheatstone.g10code.de> <20170309174531.GB2@hashbang.sh> <20170309184745.GC2@hashbang.sh> <CABcZeBMhpXy-e9Mtp8LwfqfAVW_ks3JBw1H2N3H_0c4gpQBqpg@mail.gmail.com> <DAC23A62-14BF-4AAA-8E52-09029B279E8F@icloud.com> <87varhculg.fsf@wheatstone.g10code.de>
To: Werner Koch <wk@gnupg.org>
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Cc: IETF OpenPGP <openpgp@ietf.org>, Jon Callas <joncallas@icloud.com>
Subject: Re: [openpgp] Version 5 key and fingerprint proposal
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> On Mar 10, 2017, at 2:08 AM, Werner Koch <wk@gnupg.org> wrote:
> 
> On Thu,  9 Mar 2017 23:01, joncallas@icloud.com said:
>> At the risk of sending this into a rathole, a viable alternative would
>> be to use SHA512/t as a truncation function. It's got a well-defined
> 
> We had a discussion here on the merits of SHA-256 over SHA-512 with the
> two arguments I already mentioned:
> 
>  - SHA-256 is much faster on smaller 32 bit systems
>  - SHA-256 is anyway required to verify existing signatures.
> 
> An advantage of SHA-512 is that this would benefit an X25519-only based
> implementation because that requires SHA-512 anyway.

This is a different suggestion, one about SHA512/t, which has an output length of 't' bits. It's a cute little hack that NIST put on top of SHA-512 to get a variable-output hash function.

I didn't bring in performance discussions because this is about fingerprints where it doesn't matter so much one way or the other. But since you did, you're right, that on a 32-bit system, SHA256 is faster. But on a 64-bit system, SHA-512 is faster, often like 1.5x faster.

But anyway, the suggestion is because if you're going to generate a 200-bit fingerprint, using a variable output hash function solves the problem of having to figure out how to truncate, as well as any issues in truncation.

	Jon

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