gpg-shadow-key-from-x509-cert-req/create-gpg-shadow-key-from-x509-cert-req.py

#!/usr/bin/env python3

import binascii
import cryptography.hazmat.backends
import cryptography.hazmat.primitives.serialization
import cryptography.x509
import ctypes
import math

DEFAULT_KEY_OUTPUT_PATH_PATTERN = '{keygrip_hex}.key'
DEFAULT_SMARTCARD_APP_ID_HEX = 'D2760001240102010001234567890000'

gcrypt = ctypes.cdll.LoadLibrary('libgcrypt.so')
gcrypt.gcry_pk_get_keygrip.restype = ctypes.POINTER(ctypes.c_char)


def convert_to_sexp(data):
    if isinstance(data, int):
        return convert_to_sexp(data.to_bytes(
            math.ceil(data.bit_length() / 8),
            'big',
        ))
    elif isinstance(data, str):
        return convert_to_sexp(data.encode())
    elif isinstance(data, bytes):
        return str(len(data)).encode() + b':' + data
    else:
        return b'(' + b''.join(convert_to_sexp(i) for i in data) + b')'


def keygrip_from_key_sexp(key_sexp_data):
    sexp = ctypes.c_void_p()
    assert not gcrypt.gcry_sexp_new(
        ctypes.pointer(sexp),
        key_sexp_data,
        len(key_sexp_data),
        0,
    )
    keygrip = gcrypt.gcry_pk_get_keygrip(sexp, None)[:20]
    gcrypt.gcry_sexp_release(sexp)
    return keygrip


def create_gpg_key(input_path, gpg_key_output_path_pattern, smartcard_app_id_hex):
    backend = cryptography.hazmat.backends.default_backend()
    with open(input_path, 'rb') as f:
        req = cryptography.x509.load_pem_x509_csr(f.read(), backend)
    assert req.is_signature_valid
    pubnums = req.public_key().public_numbers()
    key_data = ['shadowed-private-key', [
        'rsa',
        ['n', pubnums.n],
        ['e', pubnums.e],
        ['shadowed', 't1-v1', [int(smartcard_app_id_hex, 16), 'OPENPGP.1']],
    ]]
    key_sexp_data = convert_to_sexp(key_data)
    keygrip = keygrip_from_key_sexp(key_sexp_data)
    keygrip_hex = binascii.hexlify(keygrip).upper().decode()
    with open(gpg_key_output_path_pattern.format(keygrip_hex=keygrip_hex), 'wb') as f:
        f.write(key_sexp_data)


def _init_argparser():
    import argparse
    argparser = argparse.ArgumentParser(
        description='create a shadowed-private-key in sexp format for gnupg\'s private-keys-v1.d folder'
            + ' containing the public key of a PEM-encoded X.509 certificate signing request (CSR)',
    )
    argparser.add_argument(
        'input_path',
        help='path to PEM-encoded X.509 signing request',
    )
    argparser.add_argument(
        '--gpg-key-output-path',
        dest='gpg_key_output_path_pattern',
        metavar='path-pattern',
        default=DEFAULT_KEY_OUTPUT_PATH_PATTERN,
        help='path to sexp-encoded shadowed-private-key to be created.'
            + ' if specified, {keygrip_hex} will be substituted.'
            + ' (default: "%(default)s")',
    )
    argparser.add_argument(
        '--smartcard-app-id',
        dest='smartcard_app_id_hex',
        metavar='hex-string',
        default=DEFAULT_SMARTCARD_APP_ID_HEX,
        help='default: %(default)s',
    )
    return argparser


def main(argv):
    argparser = _init_argparser()
    args = argparser.parse_args(argv[1:])
    create_gpg_key(**vars(args))
    return 0

if __name__ == '__main__':
    import sys
    sys.exit(main(sys.argv))