Re: [gentoo-dev] [RFC] DIGESTS metadata variable for cache validation

[Tiziano Müller]

Am Samstag, den 07.02.2009, 15:23 -0800 schrieb Zac Medico:
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> Tiziano Müller wrote:
> > Am Montag, den 02.02.2009, 12:34 -0800 schrieb Zac Medico:
> >> For the digest format, I suggest that we use the leftmost 10
> >> hexadecimal digits of the SHA-1 digest. The rationale for limiting
> >> it to 10 digits (out of 40) is to save space. Due to the avalanche
> >> effect [2], 10 digits should be sufficient to ensure that problems
> >> resulting from hash collisions are extremely unlikely.
> > I'd recommend to prefix the digest with a "{TYPE}" (like for hashed
> > passwords) to be able to change the digest algorithm as needed
> > (especially in regards to the current SHA successor competition).
> > This allows a future package manager which might use SHA-3 for hashing
> > (once it's released) to still check old digests. Furthermore it would
> > allow for easier transition and only needs a definition of allowed
> > hashes instead of a specific one.
>
> I like that idea. That way it's not necessary to bump the EAPI in
> order to change the hash function. So, a typical DIGESTS value might
> look like this:
>
> SHA1 02021be38b a28b191904 3992945426 6ec21b29a3

Sleeping over it again I don't think that truncating a hash is a good
idea (truncating it from 40 to 10 digits makes the possibility of
collisions much much higher).
But if you want to go this way, I'd say you should use something like
SHA1t (t for truncated) to make sure we can use full hashes once we feel
it's appropriate.

-- 
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Tiziano Müller
Gentoo Linux Developer, Council Member
Areas of responsibility:
  Samba, PostgreSQL, CPP, Python, sysadmin
E-Mail     : dev-zero@g.o
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