ietf-openpgp
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Re: [openpgp] Fingerprint requirements for OpenPGP

2016-04-12 03:34:28
Daniel Kahn Gillmor(dkg(_at_)fifthhorseman(_dot_)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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