xref: /openbmc/qemu/scripts/u2f-setup-gen.py (revision 7f6c3d1a) 
1#!/usr/bin/env python3
2#
3# Libu2f-emu setup directory generator for USB U2F key emulation.
4#
5# Copyright (c) 2020 César Belley <cesar.belley@lse.epita.fr>
6# Written by César Belley <cesar.belley@lse.epita.fr>
7#
8# This work is licensed under the terms of the GNU GPL, version 2
9# or, at your option, any later version.  See the COPYING file in
10# the top-level directory.
11
12import sys
13import os
14from random import randint
15from typing import Tuple
16
17from cryptography.hazmat.backends import default_backend
18from cryptography.hazmat.primitives.asymmetric import ec
19from cryptography.hazmat.primitives.serialization import Encoding, \
20    NoEncryption, PrivateFormat, PublicFormat
21from OpenSSL import crypto
22
23
24def write_setup_dir(dirpath: str, privkey_pem: bytes, cert_pem: bytes,
25                    entropy: bytes, counter: int) -> None:
26    """
27    Write the setup directory.
28
29    Args:
30        dirpath: The directory path.
31        key_pem: The private key PEM.
32        cert_pem: The certificate PEM.
33        entropy: The 48 bytes of entropy.
34        counter: The counter value.
35    """
36    # Directory
37    os.mkdir(dirpath)
38
39    # Private key
40    with open(f'{dirpath}/private-key.pem', 'bw') as f:
41        f.write(privkey_pem)
42
43    # Certificate
44    with open(f'{dirpath}/certificate.pem', 'bw') as f:
45        f.write(cert_pem)
46
47    # Entropy
48    with open(f'{dirpath}/entropy', 'wb') as f:
49        f.write(entropy)
50
51    # Counter
52    with open(f'{dirpath}/counter', 'w') as f:
53        f.write(f'{str(counter)}\n')
54
55
56def generate_ec_key_pair() -> Tuple[str, str]:
57    """
58    Generate an ec key pair.
59
60    Returns:
61        The private and public key PEM.
62    """
63    # Key generation
64    privkey = ec.generate_private_key(ec.SECP256R1, default_backend())
65    pubkey = privkey.public_key()
66
67    # PEM serialization
68    privkey_pem = privkey.private_bytes(encoding=Encoding.PEM,
69                                        format=PrivateFormat.TraditionalOpenSSL,
70                                        encryption_algorithm=NoEncryption())
71    pubkey_pem = pubkey.public_bytes(encoding=Encoding.PEM,
72                                     format=PublicFormat.SubjectPublicKeyInfo)
73    return privkey_pem, pubkey_pem
74
75
76def generate_certificate(privkey_pem: str, pubkey_pem: str) -> str:
77    """
78    Generate a x509 certificate from a key pair.
79
80    Args:
81        privkey_pem: The private key PEM.
82        pubkey_pem: The public key PEM.
83
84    Returns:
85        The certificate PEM.
86    """
87    # Convert key pair
88    privkey = crypto.load_privatekey(crypto.FILETYPE_PEM, privkey_pem)
89    pubkey = crypto.load_publickey(crypto.FILETYPE_PEM, pubkey_pem)
90
91    # New x509v3 certificate
92    cert = crypto.X509()
93    cert.set_version(0x2)
94
95    # Serial number
96    cert.set_serial_number(randint(1, 2 ** 64))
97
98    # Before / After
99    cert.gmtime_adj_notBefore(0)
100    cert.gmtime_adj_notAfter(4 * (365 * 24 * 60 * 60))
101
102    # Public key
103    cert.set_pubkey(pubkey)
104
105    # Subject name and issueer
106    cert.get_subject().CN = "U2F emulated"
107    cert.set_issuer(cert.get_subject())
108
109    # Extensions
110    cert.add_extensions([
111        crypto.X509Extension(b"subjectKeyIdentifier",
112                             False, b"hash", subject=cert),
113    ])
114    cert.add_extensions([
115        crypto.X509Extension(b"authorityKeyIdentifier",
116                             False, b"keyid:always", issuer=cert),
117    ])
118    cert.add_extensions([
119        crypto.X509Extension(b"basicConstraints", True, b"CA:TRUE")
120    ])
121
122    # Signature
123    cert.sign(privkey, 'sha256')
124
125    return crypto.dump_certificate(crypto.FILETYPE_PEM, cert)
126
127
128def generate_setup_dir(dirpath: str) -> None:
129    """
130    Generates the setup directory.
131
132    Args:
133        dirpath: The directory path.
134    """
135    # Key pair
136    privkey_pem, pubkey_pem = generate_ec_key_pair()
137
138    # Certificate
139    certificate_pem = generate_certificate(privkey_pem, pubkey_pem)
140
141    # Entropy
142    entropy = os.urandom(48)
143
144    # Counter
145    counter = 0
146
147    # Write
148    write_setup_dir(dirpath, privkey_pem, certificate_pem, entropy, counter)
149
150
151def main() -> None:
152    """
153    Main function
154    """
155    # Dir path
156    if len(sys.argv) != 2:
157        sys.stderr.write(f'Usage: {sys.argv[0]} <setup_dir>\n')
158        exit(2)
159    dirpath = sys.argv[1]
160
161    # Dir non existence
162    if os.path.exists(dirpath):
163        sys.stderr.write(f'Directory: {dirpath} already exists.\n')
164        exit(1)
165
166    generate_setup_dir(dirpath)
167
168
169if __name__ == '__main__':
170    main()
171