Re: [openpgp] Fingerprint requirements for OpenPGP
Daniel Kahn Gillmor <dkg@fifthhorseman.net> Tue, 12 April 2016 18:38 UTC
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From: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
To: KellerFuchs <KellerFuchs@hashbang.sh>
In-Reply-To: <20160412172246.GE9034@hashbang.sh>
References: <87vb3nslqh.fsf@alice.fifthhorseman.net> <20160412083409.GA16775@littlepip.fritz.box> <87egaarj74.fsf@alice.fifthhorseman.net> <20160412172246.GE9034@hashbang.sh>
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Date: Tue, 12 Apr 2016 14:38:05 -0400
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Cc: IETF OpenPGP <openpgp@ietf.org>, Vincent Breitmoser <look@my.amazin.horse>
Subject: Re: [openpgp] Fingerprint requirements for OpenPGP
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On Tue 2016-04-12 13:22:46 -0400, KellerFuchs <KellerFuchs@hashbang.sh> wrote: > I think it is sane here to require collision resistence, because that's what > multi-target preimages devolves to: an attacker might not want to target > one specific key, but one amongst a large set of key (says, the developers > of a popular software, or perhaps any “widely signed” key in the WoT). > > In that case, the attacker gets a non-trivial speedup, and as the set of > targets grows larger, the hardness of the problem devolves into that of > finding a collision. The size of the set of possible targets is relevant here, though, right? if the pool of available targets is 2^16, you're not going to get more than a 2^16 speedup by targeting them. Even if we had a separate key for every IPv4 address, that'd only be a 32-bit reduction in work factor. by comparison, a pre-image brute force costs 2^(2X) when a collision attack costs 2^X due to the "birthday paradox". We're talking about fingerprints in the range of 2^160 (the OpenPGPv4 fingerprint) and 2^256 (probably the upper bound of what we can expect humans to be able to deal with). so you'd need a pool of keys to attack on the order of 2^80 or 2^128 to get a comparable speedup. > Moreover, colision resistence implies second-preimage resistence, and hash > functions are usually considered “broken” by cryptographers once there is > an attack for colisions, so it seems OK to be somewhat cautious here and > require collisions to be hard. I think we're in quite a bit of trouble if we actually need both collision resistance and any sort of plausible human-accessibility for high-entropy strings. --dkg
- [openpgp] Fingerprint requirements for OpenPGP Daniel Kahn Gillmor
- Re: [openpgp] Fingerprint requirements for OpenPGP Vincent Breitmoser
- Re: [openpgp] Fingerprint requirements for OpenPGP Joseph Lorenzo Hall
- Re: [openpgp] Fingerprint requirements for OpenPGP Vincent Breitmoser
- Re: [openpgp] Fingerprint requirements for OpenPGP Daniel Kahn Gillmor
- Re: [openpgp] Fingerprint requirements for OpenPGP Derek Atkins
- Re: [openpgp] Fingerprint requirements for OpenPGP Daniel Kahn Gillmor
- Re: [openpgp] Fingerprint requirements for OpenPGP Derek Atkins
- Re: [openpgp] Fingerprint requirements for OpenPGP Daniel Kahn Gillmor
- Re: [openpgp] Fingerprint requirements for OpenPGP Salz, Rich
- Re: [openpgp] Fingerprint requirements for OpenPGP Daniel Kahn Gillmor
- Re: [openpgp] Fingerprint requirements for OpenPGP Werner Koch
- Re: [openpgp] Fingerprint requirements for OpenPGP Werner Koch
- Re: [openpgp] Fingerprint requirements for OpenPGP KellerFuchs
- Re: [openpgp] Fingerprint requirements for OpenPGP Derek Atkins
- Re: [openpgp] Fingerprint requirements for OpenPGP Jon Callas
- [openpgp] proof-of-work fingerprints [was: Re: Fi… Daniel Kahn Gillmor
- Re: [openpgp] Fingerprint requirements for OpenPGP Daniel Kahn Gillmor
- Re: [openpgp] Fingerprint requirements for OpenPGP Werner Koch
- Re: [openpgp] Fingerprint requirements for OpenPGP Bill Frantz
- Re: [openpgp] Fingerprint requirements for OpenPGP Werner Koch
- Re: [openpgp] Fingerprint requirements for OpenPGP Derek Atkins
- Re: [openpgp] Fingerprint requirements for OpenPGP Derek Atkins
- Re: [openpgp] Fingerprint requirements for OpenPGP Derek Atkins
- Re: [openpgp] Fingerprint requirements for OpenPGP Derek Atkins
- Re: [openpgp] Fingerprint requirements for OpenPGP Joseph Lorenzo Hall
- Re: [openpgp] Fingerprint requirements for OpenPGP Werner Koch
- Re: [openpgp] Fingerprint requirements for OpenPGP Werner Koch
- Re: [openpgp] Fingerprint requirements for OpenPGP Vincent Breitmoser
- Re: [openpgp] Fingerprint requirements for OpenPGP Joseph Lorenzo Hall
- Re: [openpgp] Fingerprint requirements for OpenPGP Derek Atkins
- Re: [openpgp] Fingerprint requirements for OpenPGP Derek Atkins
- Re: [openpgp] Fingerprint requirements for OpenPGP Joseph Lorenzo Hall
- Re: [openpgp] Fingerprint requirements for OpenPGP Phillip Hallam-Baker
- Re: [openpgp] proof-of-work fingerprints [was: Re… Phillip Hallam-Baker
- Re: [openpgp] proof-of-work fingerprints [was: Re… Phillip Hallam-Baker