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

# This file is dual licensed under the terms of the Apache License, Version
# 2.0, and the BSD License. See the LICENSE file in the root of this repository
# for complete details.

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
from cryptography.hazmat.primitives.ciphers.modes import CBC


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()

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

            key_ptr = self._backend._ffi.from_buffer(self._key)
            res = self._backend._lib.CMAC_Init(
                ctx,
                key_ptr,
                len(self._key),
                evp_cipher,
                self._backend._ffi.NULL,
            )
            self._backend.openssl_assert(res == 1)

        self._ctx = ctx

    algorithm = utils.read_only_property("_algorithm")

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