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A comment on Whatever happened to SHA-256 support in Git?. Parent comment

Re:Whatever happened to SHA-256 support in Git? by stingraycharles on Thursday June 23, 2022 @02:04PM (#31852915)
I found this, which says that the SHA algorithm allows for truncation: https://csrc.nist.gov/publications/detail/sp/800-107/rev-1/f...
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Re:Whatever happened to SHA-256 support in Git? by Dylan16807 on Thursday June 23, 2022 @02:35PM (#31853366)
Not just allows, it becomes more secure when you truncate.
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Re:Whatever happened to SHA-256 support in Git? by tatersolid on Thursday June 23, 2022 @02:59PM (#31853678)
Truncated SHA-* hashes are more secure against length-extension attacks, but are very much less secure against collision and pre-image attacks (which are more important in most scenarios).
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Re:Whatever happened to SHA-256 support in Git? by Dylan16807 on Thursday June 23, 2022 @03:20PM (#31853973)
But also, 256 is overkill for collisions and pre-image.

There's a point where truncating starts to make it weaker, but when you first start chopping off bytes the benefits outweigh the drawbacks.

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Re:Whatever happened to SHA-256 support in Git? by neon_electro on Thursday June 23, 2022 @02:50PM (#31853559)
Care to elaborate? This is not something I would've intuited.
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Re:Whatever happened to SHA-256 support in Git? by bawolff on Thursday June 23, 2022 @02:56PM (#31853630)
Presumably they are referring to length extension attacks. You can't pull them off if you truncate. https://en.m.wikipedia.org/wiki/Length_extension_attack

Generally though length ext attacks have a solution - HMAC, which is much more secure than truncate.

The more you truncate, the more vulnerable you are to birthday attacks (practically speaking you would have to truncate quite a lot)

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Re:Whatever happened to SHA-256 support in Git? by kzrdude on Thursday June 23, 2022 @03:15PM (#31853905)
The canonical solution is SHA-512/256 i.e 512 truncated to 256 bits where "nothing is lost" compared to SHA-256 and something is gained. It might even be faster (due to the 64-bit word formulation of SHA-512) in some implementations.
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Re:Whatever happened to SHA-256 support in Git? by NovemberWhiskey on Thursday June 23, 2022 @03:35PM (#31854168)
Generally is faster (fewer rounds per byte). If you have 256 bits available for your hash and you're on a 64 bit architecture, I've yet to see a case where you're not better off for performance and security choosing SHA-512/256 over SHA-256, assuming you have the choice.
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Re:Whatever happened to SHA-256 support in Git? by ztorkelson on Thursday June 23, 2022 @05:41PM (#31855498)
Is this still true? I understood SHA256 to be faster than SHA512 due to hardware acceleration on current CPUs; dedicated instructions exist for the former but not the latter.
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Re:Whatever happened to SHA-256 support in Git? by NovemberWhiskey on Friday June 24, 2022 @01:19PM (#31866026)
It seems like it's not true on a MacBook with the Apple M1 processor; SHA-256 is now significantly faster. It still seems to be true on a couple of Intel machines I have access to.
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Re:Whatever happened to SHA-256 support in Git? by bawolff on Friday June 24, 2022 @08:02PM (#31871042)
Googling i noticed there seems to be a bug in openssl where it does not use optimized sha-512 on m1 (but does for sha256) - https://github.com/openssl/openssl/issues/14897 so that might be the explanation.

Also i think the length of the input matters when comparing sha256 vs sha512.

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