Can't check signature, DSA key 9C973C92 requires a 256 bit or larger hash

[David Shaw]

On Mar 15, 2014, at 3:53 PM, Juha Heljoranta <juha.heljoranta at iki.fi> wrote:

> Hi,
> 
> I am not able to get the gpg to verify a signature.
> 
> Any advice how to fix this?
> Or could the key 9C973C92 be invalid/broken?

The key may be fine, but the signature is invalid.  DSA keys specify how many bits of hash are necessary to make a signature.  This key says it needs a 256-bit hash:

> gpg: requesting key 9C973C92 from hkp server pgp.mit.edu
> gpg: armor: BEGIN PGP PUBLIC KEY BLOCK
> gpg: armor header: Version: SKS 1.1.4
> gpg: armor header: Comment: Hostname: pgp.mit.edu
> :public key packet:
>        version 4, algo 17, created 1330048372, expires 0
>        pkey[0]: [2048 bits]
>        pkey[1]: [256 bits]

                    ^^^^^^^^^^^^

But the signature is strange.  It claims to be SHA-1:

> :signature packet: algo 17, keyid 04918EA99C973C92
>        version 4, created 1352169028, md5len 0, sigclass 0x00
>        digest algo 2, begin of digest 6f 81

          ^^^^^^^^^^^^^

But is way too large:

>        hashed subpkt 2 len 4 (sig created 2012-11-06)
>        subpkt 16 len 8 (issuer key ID 04918EA99C973C92)
>        data: [255 bits]

         ^^^^^^^^^^^^^^^

Basically, the signature failed verification because it's mangled somehow.  I'm not sure how they managed to create it, but it's broken.

David