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#!/usr/bin/env python3
#
# Libu2f-emu setup directory generator for USB U2F key emulation.
#
# Copyright (c) 2020 César Belley <cesar.belley@lse.epita.fr>
# Written by César Belley <cesar.belley@lse.epita.fr>
#
# This work is licensed under the terms of the GNU GPL, version 2
# or, at your option, any later version. See the COPYING file in
# the top-level directory.
import sys
import os
from random import randint
from typing import Tuple
from cryptography.hazmat.backends import default_backend
from cryptography.hazmat.primitives.asymmetric import ec
from cryptography.hazmat.primitives.serialization import Encoding, \
NoEncryption, PrivateFormat, PublicFormat
from OpenSSL import crypto
def write_setup_dir(dirpath: str, privkey_pem: bytes, cert_pem: bytes,
entropy: bytes, counter: int) -> None:
"""
Write the setup directory.
Args:
dirpath: The directory path.
key_pem: The private key PEM.
cert_pem: The certificate PEM.
entropy: The 48 bytes of entropy.
counter: The counter value.
"""
# Directory
os.mkdir(dirpath)
# Private key
with open(f'{dirpath}/private-key.pem', 'bw') as f:
f.write(privkey_pem)
# Certificate
with open(f'{dirpath}/certificate.pem', 'bw') as f:
f.write(cert_pem)
# Entropy
with open(f'{dirpath}/entropy', 'wb') as f:
f.write(entropy)
# Counter
with open(f'{dirpath}/counter', 'w') as f:
f.write(f'{str(counter)}\n')
def generate_ec_key_pair() -> Tuple[str, str]:
"""
Generate an ec key pair.
Returns:
The private and public key PEM.
"""
# Key generation
privkey = ec.generate_private_key(ec.SECP256R1, default_backend())
pubkey = privkey.public_key()
# PEM serialization
privkey_pem = privkey.private_bytes(encoding=Encoding.PEM,
format=PrivateFormat.TraditionalOpenSSL,
encryption_algorithm=NoEncryption())
pubkey_pem = pubkey.public_bytes(encoding=Encoding.PEM,
format=PublicFormat.SubjectPublicKeyInfo)
return privkey_pem, pubkey_pem
def generate_certificate(privkey_pem: str, pubkey_pem: str) -> str:
"""
Generate a x509 certificate from a key pair.
Args:
privkey_pem: The private key PEM.
pubkey_pem: The public key PEM.
Returns:
The certificate PEM.
"""
# Convert key pair
privkey = crypto.load_privatekey(crypto.FILETYPE_PEM, privkey_pem)
pubkey = crypto.load_publickey(crypto.FILETYPE_PEM, pubkey_pem)
# New x509v3 certificate
cert = crypto.X509()
cert.set_version(0x2)
# Serial number
cert.set_serial_number(randint(1, 2 ** 64))
# Before / After
cert.gmtime_adj_notBefore(0)
cert.gmtime_adj_notAfter(4 * (365 * 24 * 60 * 60))
# Public key
cert.set_pubkey(pubkey)
# Subject name and issueer
cert.get_subject().CN = "U2F emulated"
cert.set_issuer(cert.get_subject())
# Extensions
cert.add_extensions([
crypto.X509Extension(b"subjectKeyIdentifier",
False, b"hash", subject=cert),
])
cert.add_extensions([
crypto.X509Extension(b"authorityKeyIdentifier",
False, b"keyid:always", issuer=cert),
])
cert.add_extensions([
crypto.X509Extension(b"basicConstraints", True, b"CA:TRUE")
])
# Signature
cert.sign(privkey, 'sha256')
return crypto.dump_certificate(crypto.FILETYPE_PEM, cert)
def generate_setup_dir(dirpath: str) -> None:
"""
Generates the setup directory.
Args:
dirpath: The directory path.
"""
# Key pair
privkey_pem, pubkey_pem = generate_ec_key_pair()
# Certificate
certificate_pem = generate_certificate(privkey_pem, pubkey_pem)
# Entropy
entropy = os.urandom(48)
# Counter
counter = 0
# Write
write_setup_dir(dirpath, privkey_pem, certificate_pem, entropy, counter)
def main() -> None:
"""
Main function
"""
# Dir path
if len(sys.argv) != 2:
sys.stderr.write(f'Usage: {sys.argv[0]} <setup_dir>\n')
exit(2)
dirpath = sys.argv[1]
# Dir non existence
if os.path.exists(dirpath):
sys.stderr.write(f'Directory: {dirpath} already exists.\n')
exit(1)
generate_setup_dir(dirpath)
if __name__ == '__main__':
main()
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