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Re: [openpgp] Fingerprint requirements for OpenPGP

Vincent Breitmoser <look@my.amazin.horse> Tue, 12 April 2016 08:34 UTC

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Date: Tue, 12 Apr 2016 10:34:09 +0200
From: Vincent Breitmoser <look@my.amazin.horse>
To: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
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Subject: Re: [openpgp] Fingerprint requirements for OpenPGP
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Daniel Kahn Gillmor(dkg@fifthhorseman.net)@Mon, Apr 11, 2016 at 08:40:22PM -0400:
>  * it should be as close as possible to the human attention span -- this
>    is Vincent's point, i think.  Humans really can't reliably deal with
>    512 bits of entropy when doing any kind of data entry or comparison.

Yes.

>  * it should be cheap to compute from a given key -- you shouldn't need
>    a gig of RAM or a minute of CPU to calculate the fingerprints of any
>    key.

Strictly speaking, we can be slightly less restrictive: It must be cheap
to verify, given a fingerprint, that it's the correct one for a key.
This distinction does not make a difference unless we store the
fingerprints as part of the data format (which we probably shouldn't),
so this is more of an academic point.

>  * it should be strong enough that we do not believe anyone can create a
>    key with a fingerprint that collides with another key's fingerprint

Quite importantly, this should be "another *independent* key's
fingerprint", i.e. the requirement is preimage resistance, not collision
resistance.  Creating two keys with colliding fingerprints is fine, at
least noone could come up with a attack scenario where it mattered.

 - V

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