On Sun, Jan 31 2021, Thomas Ackermann via GitGitGadget wrote: > diff --git a/Documentation/technical/hash-function-transition.txt b/Documentation/technical/hash-function-transition.txt > index dc0c4976a62..c9b57a125e2 100644 > --- a/Documentation/technical/hash-function-transition.txt > +++ b/Documentation/technical/hash-function-transition.txt > @@ -27,13 +27,17 @@ advantages: > methods have a short reliable string that can be used to reliably > address stored content. > > -Over time some flaws in SHA-1 have been discovered by security > -researchers. On 23 February 2017 the SHAttered attack > +Over time some flaws in SHA-1 have been discovered by security researchers. > +In early 2005, around the time that Git was written, Xiaoyun Wang, > +Yiqun Lisa Yin, and Hongbo Yu announced an attack finding SHA-1 > +collisions in 2^69 operations. In August they published details. > +Luckily, no practical demonstrations of a collision in full SHA-1 were > +published until 10 years later: on 23 February 2017 the SHAttered attack > (https://shattered.io) demonstrated a practical SHA-1 hash collision. > > Git v2.13.0 and later subsequently moved to a hardened SHA-1 > -implementation by default, which isn't vulnerable to the SHAttered > -attack. > +implementation by default that mitigates the SHAttered attack, but > +SHA-1 is still believed to be weak. > > Thus Git has in effect already migrated to a new hash that isn't SHA-1 > and doesn't share its vulnerabilities, its new hash function just > @@ -57,6 +61,29 @@ SHA-1 still possesses the other properties such as fast object lookup > and safe error checking, but other hash functions are equally suitable > that are believed to be cryptographically secure. I don't think this is an improvement, why does someone trying to learn about Git's SHA-256 transition care about early SHA-1 flaws that didn't prompt the transition. I'm probably biased since the current intro is mine from 5988eb631a3 (doc hash-function-transition: clarify what SHAttered means, 2018-03-26), but this really feels too much like going into the weeds. I think the document would be improved by just removing the whole mention of early 2005 and mentioning several researchers by name. I think the current prose of "Over time some flaws in SHA-1 have been discovered by security researchers" suffices, if people are curious about SHA-1's vulnerability history there's plenty of good easily found sources for that. > +Choice of Hash > +-------------- > +The hash to replace the hardened SHA-1 should be stronger than SHA-1 > +was: we would like it to be trustworthy and useful in practice for at > +least 10 years. > + > +Some other relevant properties: > + > +1. A 256-bit hash (long enough to match common security practice; not > + excessively long to hurt performance and disk usage). > + > +2. High quality implementations should be widely available (e.g., in > + OpenSSL and Apple CommonCrypto). > + > +3. The hash function's properties should match Git's needs (e.g. Git > + requires collision and 2nd preimage resistance and does not require > + length extension resistance). > + > +4. As a tiebreaker, the hash should be fast to compute (fortunately > + many contenders are faster than SHA-1). > + > +We choose SHA-256. > + > Goals > ----- > 1. The transition to SHA-256 can be done one local repository at a time. > @@ -601,39 +628,6 @@ example: > > git --output-format=sha1 log abac87a^{sha1}..f787cac^{sha256} > > -Choice of Hash > --------------- > -In early 2005, around the time that Git was written, Xiaoyun Wang, > -Yiqun Lisa Yin, and Hongbo Yu announced an attack finding SHA-1 > -collisions in 2^69 operations. In August they published details. > -Luckily, no practical demonstrations of a collision in full SHA-1 were > -published until 10 years later, in 2017. > - > -Git v2.13.0 and later subsequently moved to a hardened SHA-1 > -implementation by default that mitigates the SHAttered attack, but > -SHA-1 is still believed to be weak. > - > -The hash to replace this hardened SHA-1 should be stronger than SHA-1 > -was: we would like it to be trustworthy and useful in practice for at > -least 10 years. > - > -Some other relevant properties: > - > -1. A 256-bit hash (long enough to match common security practice; not > - excessively long to hurt performance and disk usage). > - > -2. High quality implementations should be widely available (e.g., in > - OpenSSL and Apple CommonCrypto). > - > -3. The hash function's properties should match Git's needs (e.g. Git > - requires collision and 2nd preimage resistance and does not require > - length extension resistance). > - > -4. As a tiebreaker, the hash should be fast to compute (fortunately > - many contenders are faster than SHA-1). > - > -We choose SHA-256. > - Same here. We're going into the weeds about what hashes we didn't pick before talking about what we're going to do with SHA-256? Much of that wording is just historical, and pre-dates the SHA-256 pick. I think something like this would be much better at this point: diff --git a/Documentation/technical/hash-function-transition.txt b/Documentation/technical/hash-function-transition.txt index 6fd20ebbc25..a4222eb0a6c 100644 --- a/Documentation/technical/hash-function-transition.txt +++ b/Documentation/technical/hash-function-transition.txt @@ -602,36 +602,17 @@ git --output-format=sha1 log abac87a^{sha1}..f787cac^{sha256} Choice of Hash -------------- -In early 2005, around the time that Git was written, Xiaoyun Wang, -Yiqun Lisa Yin, and Hongbo Yu announced an attack finding SHA-1 -collisions in 2^69 operations. In August they published details. -Luckily, no practical demonstrations of a collision in full SHA-1 were -published until 10 years later, in 2017. -Git v2.13.0 and later subsequently moved to a hardened SHA-1 -implementation by default that mitigates the SHAttered attack, but -SHA-1 is still believed to be weak. - -The hash to replace this hardened SHA-1 should be stronger than SHA-1 -was: we would like it to be trustworthy and useful in practice for at -least 10 years. - -Some other relevant properties: - -1. A 256-bit hash (long enough to match common security practice; not - excessively long to hurt performance and disk usage). - -2. High quality implementations should be widely available (e.g., in - OpenSSL and Apple CommonCrypto). - -3. The hash function's properties should match Git's needs (e.g. Git - requires collision and 2nd preimage resistance and does not require - length extension resistance). +There were several contenders for a successor hash to SHA-1, including +SHA-256, SHA-512/256, SHA-256x16, K12, and BLAKE2bp-256. -4. As a tiebreaker, the hash should be fast to compute (fortunately - many contenders are faster than SHA-1). +In late 2018 the project picked SHA-256 as its successor hash. -We choose SHA-256. +See 0ed8d8da374 (doc hash-function-transition: pick SHA-256 as +NewHash, 2018-08-04) and numerous mailing list threads at the time, +particularly the one starting at +https://lore.kernel.org/git/20180609224913.GC38834@xxxxxxxxxxxxxxxxxxxxxxxxxx/ +for more information. Transition plan ---------------
