From 5988eb631a3a3a42f82c1442fae79001ad2b90e7 Mon Sep 17 00:00:00 2001 From: =?utf8?q?=C3=86var=20Arnfj=C3=B6r=C3=B0=20Bjarmason?= Date: Mon, 26 Mar 2018 18:27:08 +0000 Subject: [PATCH] doc hash-function-transition: clarify what SHAttered means MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit Attempt to clarify what the SHAttered attack means in practice for Git. The previous version of the text made no mention whatsoever of Git already having a mitigation for this specific attack, which the SHAttered researchers claim will detect cryptanalytic collision attacks. I may have gotten some of the nuances wrong, but as far as I know this new text accurately summarizes the current situation with SHA-1 in git. I.e. git doesn't really use SHA-1 anymore, it uses Hardened-SHA-1 (they just so happen to produce the same outputs 99.99999999999...% of the time). Thus the previous text was incorrect in asserting that: [...]As a result [of SHAttered], SHA-1 cannot be considered cryptographically secure any more[...] That's not the case. We have a mitigation against SHAttered, *however* we consider it prudent to move to work towards a NewHash should future vulnerabilities in either SHA-1 or Hardened-SHA-1 emerge. Signed-off-by: Ævar Arnfjörð Bjarmason Signed-off-by: Junio C Hamano --- .../technical/hash-function-transition.txt | 29 ++++++++++++++++++---- 1 file changed, 24 insertions(+), 5 deletions(-) diff --git a/Documentation/technical/hash-function-transition.txt b/Documentation/technical/hash-function-transition.txt index 34396f13ec..4ab6cd1012 100644 --- a/Documentation/technical/hash-function-transition.txt +++ b/Documentation/technical/hash-function-transition.txt @@ -28,11 +28,30 @@ advantages: address stored content. Over time some flaws in SHA-1 have been discovered by security -researchers. https://shattered.io demonstrated a practical SHA-1 hash -collision. As a result, SHA-1 cannot be considered cryptographically -secure any more. This impacts the communication of hash values because -we cannot trust that a given hash value represents the known good -version of content that the speaker intended. +researchers. 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. + +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 +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. + +If SHA-1 and its variants were to be truly broken, Git's hash function +could not be considered cryptographically secure any more. This would +impact the communication of hash values because we could not trust +that a given hash value represented the known good version of content +that the speaker intended. SHA-1 still possesses the other properties such as fast object lookup and safe error checking, but other hash functions are equally suitable -- 2.11.4.GIT