This should be filled in prior to calling the function. The hash algorithm
should already have been called and the hash finalised and the output should
be in a buffer pointed to by the 'digest' member.
Any extra data to be added to the hash by the hash format (eg. PGP) should
have been added by the caller prior to finalising the hash.
It is assumed that the signature is made up of a number of MPI values. If an
algorithm becomes available for which this is not the case, the above structure
will have to change.
It is also assumed that it will have been checked that the signature algorithm
matches the key algorithm.
Signed-off-by: David Howells <dhowells@redhat.com>