Re: DSA GnuPG signatures

[Andrey Borzenkov <arvidjaar@gmail.com>]

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В Fri, 11 Jan 2013 21:54:22 +0100
Vladimir 'φ-coder/phcoder' Serbinenko <phcoder@gmail.com> пишет:

> Hello, all. I've just committed import of libgcrypt and implementation
> of related code to check signatures. Short usage:
> verify_detached FILE FILE.sig [pubkey.gpg]
> trust KEY.gpg
> distruct KEYID
> check_signatures=[enforce|no]
> 
> grub-mkimage -k KEY gcry_dsa verify [...]
> 
> When check_signatures=enforce every time anthing tries to open a file
> its signature (file.sig) is looked for and the open fails if signature
> is absent or invalid.

There is no protection against file modification after signature was
verified. Is it intentional (i.e. it is not considered as viable
threat)? One possibility is to cache file in memory on first open,
which ensures it cannot be modified externally.

> Some limitations:
> 1) DSA keys only. RSA is more tricky since it needs padding and RSA
> should be progressively phased out, not put into new places due to some
> vulnerabilities (large classes of semiprimes are factorisable up to the
> point when a lot of care has to be taken to avoid them).
> 2) Not efficient. Checking every file is slow. Some hashlists should be
> implemented.
> 3) Not efficient. File is read twice though it's avoidable in many cases.


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