Date: Mon, 4 Jan 2021 11:32:37 -0800 From: John-Mark Gurney <jmg@funkthat.com> To: RW <rwmaillists@googlemail.com> Cc: freebsd-current@freebsd.org Subject: Re: HEADS UP: FreeBSD src repo transitioning to git this weekend Message-ID: <20210104193237.GD31099@funkthat.com> In-Reply-To: <20210101165651.7319af5a@gumby.homeunix.com> References: <20201218182820.1P0tK%steffen@sdaoden.eu> <20201223023242.GG31099@funkthat.com> <20201223162417.v7Ce6%steffen@sdaoden.eu> <20201229011939.GU31099@funkthat.com> <20201229210454.Lh4y_%steffen@sdaoden.eu> <20201230004620.GB31099@funkthat.com> <CAD2Ti2-4xS5n0%2B1oLOHyFh4%2BOCnwtNAAwMkkWzwRVDnt-xmb1Q@mail.gmail.com> <20201231193908.GC31099@funkthat.com> <CAD2Ti2-dKMOx2-k71UyZs1kAGCXPuVwO9ee861oRFNV=aCfuqA@mail.gmail.com> <20210101165651.7319af5a@gumby.homeunix.com>
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RW wrote this message on Fri, Jan 01, 2021 at 16:56 +0000: > On Thu, 31 Dec 2020 21:25:08 -0500 > grarpamp wrote: > > > > Is there any reason to think [bittorrent] insecure? > > > > Cost under $50k of compute to break sha-1, > > AFAIK you cannot break SHA-1 in the sense of creating data that > matches a specific hash. What you can do is create a collision between > two blocks of data, varying both blocks in the process. This makes > SHA-1 unsuitable for digital signatures. TL;DR: No, SHA-1 is broken. PERIOD. SHAttered[1] (2017) created two valid PDF documents which had the same SHA-1 hash. The issue was that they were able to choose the entire document. The paper released in 2019[2] and implemented in 2020 allows for chosen-prefix attacks[3] which means that SHA-1 is broken. It allows the creation of a different message prefix, but results in the same hash. Before this, git was "secure" in that objects were prepended w/ length and size, but w/ this latest attack, those can now be changed as well, allowing someone to wholesale replace a file in git w/ a different length, and the SHA-1 hash being the same. > A *third-party* attacker cannot create a bogus torrent using a > collision attack against SHA-1 because the attacker would need to match > a specific hash value. Wrong, see above. > What may be possible is that the creator of the legitimate torrent > might create two torrents with the same hash, but this seems very > contrived and not very useful. It has all sorts of problems as a way of > delivering targeted malware. Again, wrong. [1] https://en.wikipedia.org/wiki/SHA-1#SHAttered_%E2%80%93_first_public_collision [2] https://en.wikipedia.org/wiki/SHA-1#Birthday-Near-Collision_Attack_%E2%80%93_first_practical_chosen-prefix_attack [3] https://en.wikipedia.org/wiki/Collision_attack#Chosen-prefix_collision_attack -- John-Mark Gurney Voice: +1 415 225 5579 "All that I will do, has been done, All that I have, has not."
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