gpg_sig_documentation

Data:
    (a) 1 byte : 0x99
    (b) 2 bytes: big-endian zero-padded length of c-end
    (c) 1 byte : version (0x04)
    (d) 4 bytes: timestamp (unixtime, big-endian)
    (e) 1 byte : algorithm (0x01 for RSA)
    (f) N bytes: modulus (as an MPI)
    (g) M bytes: exponent (as an mpi)
N = 2 + bitlen(modulus) / 8
M = 2 + bitlen(exponent) / 8

fingerprint: SHA-1(Data).hexdigest()

message is (13 + bytelen(modulus) + bytelen(exponent)) bytes long
sha-1 uses 64 byte blocks
sha-1 final block padded with 100000000000000000...0000(bitlen) (verify!)

exponent always starts at 13 bytes into data