v2f.py

#!/usr/bin/env python3

import atexit
import os
import signal
import sys

import u2fraw
import u2fhid


PID = os.getpid()


def print_usage_then_exit():
    print('''Usage:
\t%s [<directory>]

The optional directory argument specify the location to store
all information about this virtual U2F authenticator.  The
default value is "~/.v2f".
''' % sys.argv[0])
    sys.exit(1)


def main():
    # set up SIGINT handler
    def SIGINT_handler(*_, **__):
        print('\nA SIGINT (CTRL-C) signal is detected')
        sys.exit(0)
    signal.signal(signal.SIGINT, SIGINT_handler)

    # determine V2F_DIR from argument list
    args = sys.argv[1:]
    if len(args) == 0:
        V2F_DIR = os.path.expanduser('~/.v2f')
    elif len(args) == 1:
        V2F_DIR = args[0]
    else:
        print_usage_then_exit()

    # make sure V2F_DIR is a directory
    os.makedirs(V2F_DIR, exist_ok=True)

    # make sure V2F_DIR/lock does not exist
    try:
        with open(V2F_DIR + '/lock', 'x') as f:
            f.write(str(PID))
    except FileExistsError:
        print('There is already another process using %s to run v2f' % V2F_DIR)
        print('because %s lock file exists' % (V2F_DIR + '/lock'))
        sys.exit(1)

    print('v2f (V2F_DIR=%s PID=%d) started' % (V2F_DIR, PID))

    # remember to clean up
    def clean_up_before_exiting():
        print()
        print('v2f (V2F_DIR=%s PID=%d) ended' % (V2F_DIR, PID))
        os.remove(V2F_DIR + '/lock')
    atexit.register(clean_up_before_exiting)

    # make sure we have V2F_DIR/device_master_secret_key (64-byte)
    # make sure we have V2F_DIR/number_of_signatures_generated (4-byte)
    if not os.path.exists(V2F_DIR + '/device_master_secret_key'):
        with open(V2F_DIR + '/device_master_secret_key', 'wb') as f:
            f.write(os.urandom(64))
    if not os.path.exists(V2F_DIR + '/number_of_signatures_generated'):
        with open(V2F_DIR + '/number_of_signatures_generated', 'wb') as f:
            f.write(b'\x00\x00\x00\x00')

    # read device master secret key
    with open(V2F_DIR + '/device_master_secret_key', 'rb') as f:
        DEVICE_MASTER_SECRET_KEY = f.read()

    # construct a closure that gives you latest counter value
    def update_counter():
        with open(V2F_DIR + '/number_of_signatures_generated', 'rb') as f:
            DEVICE_COUNTER = f.read()
        old_value = int.from_bytes(DEVICE_COUNTER[:4], 'big')
        new_value = old_value + 1
        with open(V2F_DIR + '/number_of_signatures_generated', 'wb') as f:
            f.write(new_value.to_bytes(4, 'big'))
        return old_value

    u2fraw.initialize(DEVICE_MASTER_SECRET_KEY, update_counter, V2F_DIR)

    u2fhid.run_uhid_event_loop()


if __name__ == '__main__':
    main()