Merge pull request #931 from Ayrx/cmac-implementation

CMAC implementation and docs

2 files changed, 158 insertions, 2 deletions

diff --git a/cryptography/hazmat/backends/openssl/backend.py b/cryptography/hazmat/backends/openssl/backend.py
index 5e13bfc1..7d73c413 100644
--- a/cryptography/hazmat/backends/openssl/backend.py
+++ b/cryptography/hazmat/backends/openssl/backend.py
@@ -25,8 +25,8 @@ from cryptography.exceptions import (
     UnsupportedAlgorithm, _Reasons
 )
 from cryptography.hazmat.backends.interfaces import (
-    CipherBackend, DSABackend, HMACBackend, HashBackend, PBKDF2HMACBackend,
-    RSABackend
+    CMACBackend, CipherBackend, DSABackend, HMACBackend, HashBackend,
+    PBKDF2HMACBackend, RSABackend
 )
 from cryptography.hazmat.bindings.openssl.binding import Binding
 from cryptography.hazmat.primitives import hashes, interfaces
@@ -47,6 +47,7 @@ _OpenSSLError = collections.namedtuple("_OpenSSLError",
 
 
 @utils.register_interface(CipherBackend)
+@utils.register_interface(CMACBackend)
 @utils.register_interface(DSABackend)
 @utils.register_interface(HashBackend)
 @utils.register_interface(HMACBackend)
@@ -554,6 +555,16 @@ class Backend(object):
 
         return self._ffi.buffer(buf)[:res]
 
+    def cmac_algorithm_supported(self, algorithm):
+        return (
+            backend._lib.Cryptography_HAS_CMAC == 1
+            and backend.cipher_supported(algorithm, CBC(
+                b"\x00" * algorithm.block_size))
+        )
+
+    def create_cmac_ctx(self, algorithm):
+        return _CMACContext(self, algorithm)
+
 
 class GetCipherByName(object):
     def __init__(self, fmt):
@@ -1240,4 +1251,74 @@ class _RSAVerificationContext(object):
             raise InvalidSignature
 
 
+@utils.register_interface(interfaces.CMACContext)
+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")
+
+        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
+            try:
+                adapter = registry[type(algorithm), CBC]
+            except KeyError:
+                raise UnsupportedAlgorithm(
+                    "cipher {0} is not supported by this backend".format(
+                        algorithm.name), _Reasons.UNSUPPORTED_CIPHER
+                )
+
+            evp_cipher = adapter(self._backend, algorithm, CBC)
+            if evp_cipher == self._backend._ffi.NULL:
+                raise UnsupportedAlgorithm(
+                    "cipher {0} is not supported by this backend".format(
+                        algorithm.name), _Reasons.UNSUPPORTED_CIPHER
+                )
+
+            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
+
+    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
+        )
+
+
 backend = Backend()
diff --git a/cryptography/hazmat/primitives/cmac.py b/cryptography/hazmat/primitives/cmac.py
new file mode 100644
index 00000000..7e7f65ab
--- /dev/null
+++ b/cryptography/hazmat/primitives/cmac.py
@@ -0,0 +1,75 @@
+# 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
+
+import six
+
+from cryptography import utils
+from cryptography.exceptions import (
+    AlreadyFinalized, InvalidSignature, UnsupportedAlgorithm, _Reasons
+)
+from cryptography.hazmat.backends.interfaces import CMACBackend
+from cryptography.hazmat.primitives import constant_time, interfaces
+
+
+@utils.register_interface(interfaces.CMACContext)
+class CMAC(object):
+    def __init__(self, algorithm, backend, ctx=None):
+        if not isinstance(backend, CMACBackend):
+            raise UnsupportedAlgorithm(
+                "Backend object does not implement CMACBackend",
+                _Reasons.BACKEND_MISSING_INTERFACE
+            )
+
+        if not isinstance(algorithm, interfaces.BlockCipherAlgorithm):
+            raise TypeError(
+                "Expected instance of interfaces.BlockCipherAlgorithm"
+            )
+        self._algorithm = algorithm
+
+        self._backend = backend
+        if ctx is None:
+            self._ctx = self._backend.create_cmac_ctx(self._algorithm)
+        else:
+            self._ctx = ctx
+
+    def update(self, data):
+        if self._ctx is None:
+            raise AlreadyFinalized("Context was already finalized")
+        if isinstance(data, six.text_type):
+            raise TypeError("Unicode-objects must be encoded before hashing")
+        self._ctx.update(data)
+
+    def finalize(self):
+        if self._ctx is None:
+            raise AlreadyFinalized("Context was already finalized")
+        digest = self._ctx.finalize()
+        self._ctx = None
+        return digest
+
+    def verify(self, signature):
+        if isinstance(signature, six.text_type):
+            raise TypeError("Unicode-objects must be encoded before verifying")
+        digest = self.finalize()
+        if not constant_time.bytes_eq(digest, signature):
+            raise InvalidSignature("Signature did not match digest.")
+
+    def copy(self):
+        if self._ctx is None:
+            raise AlreadyFinalized("Context was already finalized")
+        return CMAC(
+            self._algorithm,
+            backend=self._backend,
+            ctx=self._ctx.copy()
+        )