Returns the sha1 hash of the modulus of a public key in a cert
    Used for generating subject key identifiers

def _get_pubkey_hash(cert):
    '''
    Returns the sha1 hash of the modulus of a public key in a cert
    Used for generating subject key identifiers
    '''
    sha_hash = hashlib.sha1(cert.get_pubkey().get_modulus()).hexdigest()
    return _pretty_hex(sha_hash)