From: Thomas Ackermann <th.acker@xxxxxxxx> Move rationale for new hash function to beginning of document so that it appears before the concrete move to SHA-256 is described. Remove details about SHA-1 weaknesses. Instead add references to the details of how the new hash function was chosen. Signed-off-by: Thomas Ackermann <th.acker@xxxxxxxx> --- .../technical/hash-function-transition.txt | 62 +++++-------------- 1 file changed, 16 insertions(+), 46 deletions(-) diff --git a/Documentation/technical/hash-function-transition.txt b/Documentation/technical/hash-function-transition.txt index 86b09ea0f21..475f2f501a6 100644 --- a/Documentation/technical/hash-function-transition.txt +++ b/Documentation/technical/hash-function-transition.txt @@ -27,22 +27,12 @@ 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 -(https://shattered.io) demonstrated a practical SHA-1 hash collision. +Over time some flaws in SHA-1 have been discovered by security researchers. 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, 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 -happens to produce exactly the same output for all known inputs, -except two PDFs published by the SHAttered researchers, and the new -implementation (written by those researchers) claims to detect future -cryptanalytic collision attacks. - -Regardless, it's considered prudent to move past any variant of SHA-1 +Thus it's considered prudent to move past any variant of SHA-1 to a new hash. There's no guarantee that future attacks on SHA-1 won't be published in the future, and those attacks may not have viable mitigations. @@ -57,6 +47,19 @@ 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. +Choice of Hash +-------------- +There were several contenders for a successor hash to SHA-1, including +SHA-256, SHA-512/256, SHA-256x16, K12, and BLAKE2bp-256. + +In late 2018 the project picked SHA-256 as its successor hash. + +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. + Goals ----- 1. The transition to SHA-256 can be done one local repository at a time. @@ -601,39 +604,6 @@ 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. - Transition plan --------------- Some initial steps can be implemented independently of one another: -- gitgitgadget
