src/cryptography/hazmat/backends/openssl/cmac.py

# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#    http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
# implied.
# See the License for the specific language governing permissions and
# limitations under the License.

from __future__ import absolute_import, division, print_function


from cryptography import utils
from cryptography.exceptions import (
    InvalidSignature, UnsupportedAlgorithm, _Reasons
)
from cryptography.hazmat.primitives import constant_time, interfaces
from cryptography.hazmat.primitives.ciphers.modes import CBC


@utils.register_interface(interfaces.MACContext)
class _CMACContext(object):
    def __init__(self, backend, algorithm, ctx=None):
        if not backend.cmac_algorithm_supported(algorithm):
            raise UnsupportedAlgorithm("This backend does not support CMAC.",
                                       _Reasons.UNSUPPORTED_CIPHER)

        self._backend = backend
        self._key = algorithm.key
        self._algorithm = algorithm
        self._output_length = algorithm.block_size // 8

        if ctx is None:
            registry = self._backend._cipher_registry
            adapter = registry[type(algorithm), CBC]

            evp_cipher = adapter(self._backend, algorithm, CBC)

            ctx = self._backend._lib.CMAC_CTX_new()

            assert ctx != self._backend._ffi.NULL
            ctx = self._backend._ffi.gc(ctx, self._backend._lib.CMAC_CTX_free)

            self._backend._lib.CMAC_Init(
                ctx, self._key, len(self._key),
                evp_cipher, self._backend._ffi.NULL
            )

        self._ctx = ctx

    algorithm = utils.read_only_property("_algorithm")

    def update(self, data):
        res = self._backend._lib.CMAC_Update(self._ctx, data, len(data))
        assert res == 1

    def finalize(self):
        buf = self._backend._ffi.new("unsigned char[]", self._output_length)
        length = self._backend._ffi.new("size_t *", self._output_length)
        res = self._backend._lib.CMAC_Final(
            self._ctx, buf, length
        )
        assert res == 1

        self._ctx = None

        return self._backend._ffi.buffer(buf)[:]

    def copy(self):
        copied_ctx = self._backend._lib.CMAC_CTX_new()
        copied_ctx = self._backend._ffi.gc(
            copied_ctx, self._backend._lib.CMAC_CTX_free
        )
        res = self._backend._lib.CMAC_CTX_copy(
            copied_ctx, self._ctx
        )
        assert res == 1
        return _CMACContext(
            self._backend, self._algorithm, ctx=copied_ctx
        )

    def verify(self, signature):
        digest = self.finalize()
        if not constant_time.bytes_eq(digest, signature):
            raise InvalidSignature("Signature did not match digest.")