#!/usr/bin/env python3

"""
Somewhere, the HSM has to have a last-gasp default PIN, even if it's
only the null string, because there has to be **some** way to
initialize the poor thing.  Absent a better plan (feel free to
suggest one!), this last-gasp default is compiled in.

The normal value of this last-gasp PIN is deliberately chosen to be
annoying, so that people will change it, but since the derevation
requires running PBKDF2 and you might want a different default if
you're compiling this for yourself, we provide the script that
generates the default.
"""

# Author: Rob Austein
# Copyright (c) 2016, NORDUnet A/S
# All rights reserved.
#
# Redistribution and use in source and binary forms, with or without
# modification, are permitted provided that the following conditions are
# met:
# - Redistributions of source code must retain the above copyright notice,
#   this list of conditions and the following disclaimer.
#
# - Redistributions in binary form must reproduce the above copyright
#   notice, this list of conditions and the following disclaimer in the
#   documentation and/or other materials provided with the distribution.
#
# - Neither the name of the NORDUnet nor the names of its contributors may
#   be used to endorse or promote products derived from this software
#   without specific prior written permission.
#
# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
# IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
# TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
# PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
# HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED
# TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
# PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
# LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
# NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
# SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

from argparse                   import ArgumentParser, ArgumentDefaultsHelpFormatter
from binascii                   import hexlify
from os                         import urandom
from Crypto.Protocol.KDF        import PBKDF2
from Crypto.Hash                import SHA256, HMAC

parser = ArgumentParser(description = __doc__, formatter_class = ArgumentDefaultsHelpFormatter)
parser.add_argument("-p", "--pin",
                    default = "YouReallyNeedToChangeThisPINRightNowWeAreNotKidding",
                    help    = "PIN plaintext before PBKDF2 processing")
parser.add_argument("-i", "--iterations",
                    type    = int,
                    default = 1000,
                    help    = "PBKDF2 iteration count")
parser.add_argument("-d", "--derived-key-length",
                    type    = int,
                    default = 64,
                    help    = "length of PBKDF2 output (must match libhal)")
args = parser.parse_args()

def HMAC_SHA256(pin, salt):
    return HMAC.new(pin, salt, SHA256).digest()

def uint8(value):
    value = hexlify(value).decode("ascii")
    return ", ".join("0x" + value[i : i + 2] for i in range(0, len(value), 2))

salt = urandom(16)

pin  = PBKDF2(password = args.pin,
              salt     = salt,
              dkLen    = args.derived_key_length,
              count    = args.iterations,
              prf      = HMAC_SHA256)

print('''\
/*
 * Automatically generated by a script, do not edit.
 */

static const hal_ks_pin_t hal_last_gasp_pin = {{
  {iterations},
  {{{pin}}},
  {{{salt}}}
}};'''.format(iterations = args.iterations,
              pin        = uint8(pin),
              salt       = uint8(salt)))