scripts/sign_encrypt.py

#!/usr/bin/env python3
# SPDX-License-Identifier: BSD-2-Clause
#
# Copyright (c) 2015, 2017, 2019, Linaro Limited
#

import sys
import math


algo = {'TEE_ALG_RSASSA_PKCS1_PSS_MGF1_SHA256': 0x70414930,
        'TEE_ALG_RSASSA_PKCS1_V1_5_SHA256': 0x70004830}

enc_key_type = {'SHDR_ENC_KEY_DEV_SPECIFIC': 0x0,
                'SHDR_ENC_KEY_CLASS_WIDE': 0x1}

SHDR_BOOTSTRAP_TA = 1
SHDR_ENCRYPTED_TA = 2
SHDR_MAGIC = 0x4f545348
SHDR_SIZE = 20


def uuid_parse(s):
    from uuid import UUID
    return UUID(s)


def int_parse(str):
    return int(str, 0)


def get_args(logger):
    from argparse import ArgumentParser, RawDescriptionHelpFormatter
    import textwrap
    command_base = ['sign-enc', 'digest', 'stitch', 'verify']
    command_aliases_digest = ['generate-digest']
    command_aliases_stitch = ['stitch-ta']
    command_aliases = command_aliases_digest + command_aliases_stitch
    command_choices = command_base + command_aliases

    dat = '[' + ', '.join(command_aliases_digest) + ']'
    sat = '[' + ', '.join(command_aliases_stitch) + ']'

    parser = ArgumentParser(
        description='Sign and encrypt (optional) a Trusted Application for' +
        ' OP-TEE.',
        usage='\n   %(prog)s command [ arguments ]\n\n'

        '   command:\n' +
        '     sign-enc    Generate signed and optionally encrypted loadable' +
        ' TA image file.\n' +
        '                 Takes arguments --uuid, --ta-version, --in, --out,' +
        ' --key,\n' +
        '                 --enc-key (optional) and' +
        ' --enc-key-type (optional).\n' +
        '     digest      Generate loadable TA binary image digest' +
        ' for offline\n' +
        '                 signing. Takes arguments --uuid, --ta-version,' +
        ' --in, --key,\n'
        '                 --enc-key (optional), --enc-key-type (optional),' +
        ' --algo (optional) and --dig.\n' +
        '     stitch      Generate loadable signed and encrypted TA binary' +
        ' image file from\n' +
        '                 TA raw image and its signature. Takes' +
        ' arguments --uuid, --in, --key, --out,\n' +
        '                 --enc-key (optional), --enc-key-type (optional),\n' +
        '                 --algo (optional) and --sig.\n' +
        '     verify      Verify signed TA binary\n' +
        '                 Takes arguments --uuid, --in, --key\n\n' +
        '   %(prog)s --help  show available commands and arguments\n\n',
        formatter_class=RawDescriptionHelpFormatter,
        epilog=textwrap.dedent('''\
            If no command is given, the script will default to "sign-enc".

            command aliases:
              The command \'digest\' can be aliased by ''' + dat + '''
              The command \'stitch\' can be aliased by ''' + sat + '\n' + '''
            example offline signing command using OpenSSL for algorithm
            TEE_ALG_RSASSA_PKCS1_PSS_MGF1_SHA256:
              base64 -d <UUID>.dig | \\
              openssl pkeyutl -sign -inkey <KEYFILE>.pem \\
                  -pkeyopt digest:sha256 -pkeyopt rsa_padding_mode:pss \\
                  -pkeyopt rsa_pss_saltlen:digest \\
                  -pkeyopt rsa_mgf1_md:sha256 | \\
              base64 > <UUID>.sig\n
            example offline signing command using OpenSSL for algorithm
            TEE_ALG_RSASSA_PKCS1_V1_5_SHA256:
              base64 -d <UUID>.dig | \\
              openssl pkeyutl -sign -inkey <KEYFILE>.pem \\
                  -pkeyopt digest:sha256 -pkeyopt rsa_padding_mode:pkcs1 | \\
              base64 > <UUID>.sig
            '''))

    parser.add_argument(
        'command', choices=command_choices, nargs='?',
        default='sign-enc',
        help='Command, one of [' + ', '.join(command_base) + ']')
    parser.add_argument('--uuid', required=True,
                        type=uuid_parse, help='String UUID of the TA')
    parser.add_argument('--key', required=True,
                        help='Name of signing key file (PEM format)')
    parser.add_argument('--enc-key', required=False,
                        help='Encryption key string')
    parser.add_argument(
        '--enc-key-type', required=False, default='SHDR_ENC_KEY_DEV_SPECIFIC',
        choices=list(enc_key_type.keys()),
        help='Encryption key type.\n' +
        '(SHDR_ENC_KEY_DEV_SPECIFIC or SHDR_ENC_KEY_CLASS_WIDE).\n' +
        'Defaults to SHDR_ENC_KEY_DEV_SPECIFIC.')
    parser.add_argument(
        '--ta-version', required=False, type=int_parse, default=0,
        help='TA version stored as a 32-bit unsigned integer and used for\n' +
        'rollback protection of TA install in the secure database.\n' +
        'Defaults to 0.')
    parser.add_argument(
        '--sig', required=False, dest='sigf',
        help='Name of signature input file, defaults to <UUID>.sig')
    parser.add_argument(
        '--dig', required=False, dest='digf',
        help='Name of digest output file, defaults to <UUID>.dig')
    parser.add_argument(
        '--in', required=True, dest='inf',
        help='Name of application input file, defaults to <UUID>.stripped.elf')
    parser.add_argument(
        '--out', required=False, dest='outf',
        help='Name of application output file, defaults to <UUID>.ta')
    parser.add_argument('--algo', required=False, choices=list(algo.keys()),
                        default='TEE_ALG_RSASSA_PKCS1_PSS_MGF1_SHA256',
                        help='The hash and signature algorithm, ' +
                        'defaults to TEE_ALG_RSASSA_PKCS1_PSS_MGF1_SHA256. ' +
                        'Allowed values are: ' +
                        ', '.join(list(algo.keys())), metavar='')

    parsed = parser.parse_args()

    # Check parameter combinations

    if parsed.digf is None and \
       parsed.outf is not None and \
       parsed.command in ['digest'] + command_aliases_digest:
        logger.error('A digest was requested, but argument --out was given.' +
                     '  Did you mean:\n  ' +
                     parser.prog+' --dig ' + parsed.outf + ' ...')
        sys.exit(1)

    if parsed.digf is not None \
       and parsed.outf is not None \
       and parsed.command in ['digest'] + command_aliases_digest:
        logger.warn('A digest was requested, but arguments --dig and ' +
                    '--out were given.\n' +
                    '  --out will be ignored.')

    # Set defaults for optional arguments.

    if parsed.sigf is None:
        parsed.sigf = str(parsed.uuid)+'.sig'
    if parsed.digf is None:
        parsed.digf = str(parsed.uuid)+'.dig'
    if parsed.inf is None:
        parsed.inf = str(parsed.uuid)+'.stripped.elf'
    if parsed.outf is None:
        parsed.outf = str(parsed.uuid)+'.ta'

    return parsed


def main():
    from cryptography import exceptions
    from cryptography.hazmat.backends import default_backend
    from cryptography.hazmat.primitives import serialization
    from cryptography.hazmat.primitives import hashes
    from cryptography.hazmat.primitives.asymmetric import padding
    from cryptography.hazmat.primitives.asymmetric import rsa
    from cryptography.hazmat.primitives.asymmetric import utils
    import base64
    import logging
    import os
    import struct

    logging.basicConfig()
    logger = logging.getLogger(os.path.basename(__file__))

    args = get_args(logger)

    with open(args.key, 'rb') as f:
        data = f.read()

        try:
            key = serialization.load_pem_private_key(data, password=None,
                                                     backend=default_backend())
        except ValueError:
            key = serialization.load_pem_public_key(data,
                                                    backend=default_backend())

    with open(args.inf, 'rb') as f:
        img = f.read()

    chosen_hash = hashes.SHA256()
    h = hashes.Hash(chosen_hash, default_backend())

    digest_len = chosen_hash.digest_size
    sig_len = math.ceil(key.key_size / 8)

    img_size = len(img)

    hdr_version = args.ta_version  # struct shdr_bootstrap_ta::ta_version

    magic = SHDR_MAGIC
    if args.enc_key:
        img_type = SHDR_ENCRYPTED_TA
    else:
        img_type = SHDR_BOOTSTRAP_TA

    shdr = struct.pack('<IIIIHH',
                       magic, img_type, img_size, algo[args.algo],
                       digest_len, sig_len)
    shdr_uuid = args.uuid.bytes
    shdr_version = struct.pack('<I', hdr_version)

    if args.enc_key:
        from cryptography.hazmat.primitives.ciphers.aead import AESGCM
        cipher = AESGCM(bytes.fromhex(args.enc_key))
        # Use 12 bytes for nonce per recommendation
        nonce = os.urandom(12)
        out = cipher.encrypt(nonce, img, None)
        ciphertext = out[:-16]
        # Authentication Tag is always the last 16 bytes
        tag = out[-16:]

        enc_algo = 0x40000810      # TEE_ALG_AES_GCM
        flags = enc_key_type[args.enc_key_type]
        ehdr = struct.pack('<IIHH',
                           enc_algo, flags, len(nonce), len(tag))

    h.update(shdr)
    h.update(shdr_uuid)
    h.update(shdr_version)
    if args.enc_key:
        h.update(ehdr)
        h.update(nonce)
        h.update(tag)
    h.update(img)
    img_digest = h.finalize()

    def write_image_with_signature(sig):
        with open(args.outf, 'wb') as f:
            f.write(shdr)
            f.write(img_digest)
            f.write(sig)
            f.write(shdr_uuid)
            f.write(shdr_version)
            if args.enc_key:
                f.write(ehdr)
                f.write(nonce)
                f.write(tag)
                f.write(ciphertext)
            else:
                f.write(img)

    def sign_encrypt_ta():
        if not isinstance(key, rsa.RSAPrivateKey):
            logger.error('Provided key cannot be used for signing, ' +
                         'please use offline-signing mode.')
            sys.exit(1)
        else:
            if args.algo == 'TEE_ALG_RSASSA_PKCS1_PSS_MGF1_SHA256':
                sig = key.sign(
                    img_digest,
                    padding.PSS(
                        mgf=padding.MGF1(chosen_hash),
                        salt_length=digest_len
                    ),
                    utils.Prehashed(chosen_hash)
                )
            elif args.algo == 'TEE_ALG_RSASSA_PKCS1_V1_5_SHA256':
                sig = key.sign(
                    img_digest,
                    padding.PKCS1v15(),
                    utils.Prehashed(chosen_hash)
                )

            if len(sig) != sig_len:
                raise Exception(("Actual signature length is not equal to ",
                                 "the computed one: {} != {}").
                                format(len(sig), sig_len))
            write_image_with_signature(sig)
            logger.info('Successfully signed application.')

    def generate_digest():
        with open(args.digf, 'wb+') as digfile:
            digfile.write(base64.b64encode(img_digest))

    def stitch_ta():
        try:
            with open(args.sigf, 'r') as sigfile:
                sig = base64.b64decode(sigfile.read())
        except IOError:
            if not os.path.exists(args.digf):
                generate_digest()
            logger.error('No signature file found. Please sign\n %s\n' +
                         'offline and place the signature at \n %s\n' +
                         'or pass a different location ' +
                         'using the --sig argument.\n',
                         args.digf, args.sigf)
            sys.exit(1)
        else:
            try:
                if args.algo == 'TEE_ALG_RSASSA_PKCS1_PSS_MGF1_SHA256':
                    key.verify(
                        sig,
                        img_digest,
                        padding.PSS(
                            mgf=padding.MGF1(chosen_hash),
                            salt_length=digest_len
                        ),
                        utils.Prehashed(chosen_hash)
                    )
                elif args.algo == 'TEE_ALG_RSASSA_PKCS1_V1_5_SHA256':
                    key.verify(
                        sig,
                        img_digest,
                        padding.PKCS1v15(),
                        utils.Prehashed(chosen_hash)
                    )
            except exceptions.InvalidSignature:
                logger.error('Verification failed, ignoring given signature.')
                sys.exit(1)

            write_image_with_signature(sig)
            logger.info('Successfully applied signature.')

    def verify_ta():
        # Extract header
        [magic,
         img_type,
         img_size,
         algo_value,
         digest_len,
         sig_len] = struct.unpack('<IIIIHH', img[:SHDR_SIZE])

        # Extract digest and signature
        start, end = SHDR_SIZE, SHDR_SIZE + digest_len
        digest = img[start:end]

        start, end = end, SHDR_SIZE + digest_len + sig_len
        signature = img[start:end]

        # Extract UUID and TA version
        start, end = end, end + 16 + 4
        [uuid, ta_version] = struct.unpack('<16sI', img[start:end])

        if magic != SHDR_MAGIC:
            raise Exception("Unexpected magic: 0x{:08x}".format(magic))

        if img_type != SHDR_BOOTSTRAP_TA:
            raise Exception("Unsupported image type: {}".format(img_type))

        if algo_value not in algo.values():
            raise Exception('Unrecognized algorithm: 0x{:08x}'
                            .format(algo_value))

        # Verify signature against hash digest
        if algo_value == 0x70414930:
            key.verify(
                signature,
                digest,
                padding.PSS(
                    mgf=padding.MGF1(chosen_hash),
                    salt_length=digest_len
                ),
                utils.Prehashed(chosen_hash)
            )
        else:
            key.verify(
                signature,
                digest,
                padding.PKCS1v15(),
                utils.Prehashed(chosen_hash)
            )

        h = hashes.Hash(chosen_hash, default_backend())

        # sizeof(struct shdr)
        h.update(img[:SHDR_SIZE])

        # sizeof(struct shdr_bootstrap_ta)
        h.update(img[start:end])

        # raw image
        start = end
        end += img_size
        h.update(img[start:end])

        if digest != h.finalize():
            raise Exception('Hash digest does not match')

        logger.info('Trusted application is correctly verified.')

    # dispatch command
    {
        'sign-enc': sign_encrypt_ta,
        'digest': generate_digest,
        'generate-digest': generate_digest,
        'stitch': stitch_ta,
        'stitch-ta': stitch_ta,
        'verify': verify_ta,
    }.get(args.command, 'sign_encrypt_ta')()


if __name__ == "__main__":
    main()