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Re: [openpgp] Followup on fingerprints

Derek Atkins <warlord@MIT.EDU> Tue, 04 August 2015 13:17 UTC

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From: Derek Atkins <warlord@MIT.EDU>
To: Nicholas Cole <nicholas.cole@gmail.com>
References: <87twsn2wcz.fsf@vigenere.g10code.de> <CAMm+LwgRJX-SvydmpUAJMmN3yysi4zzGSpO2yY4JAMhD-9xLgQ@mail.gmail.com> <87zj2ecmv8.fsf@alice.fifthhorseman.net> <CAMm+LwgKmcTes=V7uS3MjCQixWCo-i7PY=VE7eCHSqt3Ho3OSg@mail.gmail.com> <87a8udd4u6.fsf@alice.fifthhorseman.net> <sjm61503182.fsf@securerf.ihtfp.org> <CAMm+LwgEVySpfL-iN2uzX-4tu7R+isDkHE9D8uAeLTxxd4VxqQ@mail.gmail.com> <sjmwpxc1kbv.fsf@securerf.ihtfp.org> <CAAS2fgR6LYck+km5Ze6S9z65ZgsR61d8md2CqojDaceZ0OrZrw@mail.gmail.com> <9c2c8c5df67c83925d7e3c21fe943483.squirrel@mail2.ihtfp.org> <20150803173231.GG3067@straylight.m.ringlet.net> <2439a89a6c4eb70044e144406a732482.squirrel@mail2.ihtfp.org> <87io8v7uqt.fsf@littlepip.fritz.box> <87h9of7p0e.fsf@littlepip.fritz.box> <87wpxbtuwk.fsf@vigenere.g10code.de> <CAAu18hez49oVhTwRLqv=3rifbg5q5+EqsSvBO0c-ezq+M_Qmyw@mail.gmail.com>
Date: Tue, 04 Aug 2015 09:16:52 -0400
In-Reply-To: <CAAu18hez49oVhTwRLqv=3rifbg5q5+EqsSvBO0c-ezq+M_Qmyw@mail.gmail.com> (Nicholas Cole's message of "Tue, 4 Aug 2015 09:05:03 +0100")
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Cc: IETF OpenPGP <openpgp@ietf.org>
Subject: Re: [openpgp] Followup on fingerprints
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Nicholas Cole <nicholas.cole@gmail.com> writes:

> On Tuesday, 4 August 2015, Werner Koch <wk@gnupg.org> wrote:
>
>     On Tue,  4 Aug 2015 04:42, look@my.amazin.horse said:
>    
>     > And the actual attack is "slightly weaker non-repudiation"?
>    
>     ... when using a truncated fingerprint.
>    
>     Why should anyone truncate a fingerprint from 20 bytes to 13 bytes?
>     This is an arbitrary value in between the known weak 8 byte keyids and
>     the full 20 byte fingerprints for which we expect that in our lifetime
>     collisions can be 
>
> I'm really struggling to follow what is going on with this whole discussion! 
> Fingerprints need to be robust enough that creating aritrary collisions is not
> feasible. That has always been central to OpenPGP.  If that creates headaches
> for user interfaces then we will have to find ways to deal with that, but that
> is a separate discussion. 
>
> I thought that there were some well established, secure as far as anyone
> knows, hash algorithms. We've many years experience of the problems of
> including or not including various extra bits of information along with the key
> material itself, so doesn't the WG just need to pick one of the candidate
> algorithms and have done with it?  

Every hash algorithm is going to have collisions.  In an ideal hash you
can find a collision in 2^(N/2) trials where N is the number of bits in
the hash.  If you truncate the hash then that reduces N.  In non-ideal
hashes it's less effort.

> openpgp mailing list
> openpgp@ietf.org
> https://www.ietf.org/mailman/listinfo/openpgp

-derek
-- 
       Derek Atkins, SB '93 MIT EE, SM '95 MIT Media Laboratory
       Member, MIT Student Information Processing Board  (SIPB)
       URL: http://web.mit.edu/warlord/    PP-ASEL-IA     N1NWH
       warlord@MIT.EDU                        PGP key available

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