Add verify function to hmac and hashes.

4 files changed, 28 insertions, 4 deletions

diff --git a/cryptography/exceptions.py b/cryptography/exceptions.py
index e9d88199..44363c24 100644
--- a/cryptography/exceptions.py
+++ b/cryptography/exceptions.py
@@ -30,3 +30,7 @@ class NotYetFinalized(Exception):
 
 class InvalidTag(Exception):
     pass
+
+
+class InvalidSignature(Exception):
+    pass
diff --git a/cryptography/hazmat/primitives/hashes.py b/cryptography/hazmat/primitives/hashes.py
index bee188b3..b3c626d4 100644
--- a/cryptography/hazmat/primitives/hashes.py
+++ b/cryptography/hazmat/primitives/hashes.py
@@ -16,8 +16,8 @@ from __future__ import absolute_import, division, print_function
 import six
 
 from cryptography import utils
-from cryptography.exceptions import AlreadyFinalized
-from cryptography.hazmat.primitives import interfaces
+from cryptography.exceptions import AlreadyFinalized, InvalidSignature
+from cryptography.hazmat.primitives import constant_time, interfaces
 
 
 @utils.register_interface(interfaces.HashContext)
@@ -55,6 +55,13 @@ class Hash(object):
         self._ctx = None
         return digest
 
+    def verify(self, sig):
+        if isinstance(sig, six.text_type):
+            raise TypeError("Unicode-objects must be encoded before verifying")
+        digest = self.finalize()
+        if not constant_time.bytes_eq(digest, sig):
+            raise InvalidSignature("Signature did not match digest.")
+
 
 @utils.register_interface(interfaces.HashAlgorithm)
 class SHA1(object):
diff --git a/cryptography/hazmat/primitives/hmac.py b/cryptography/hazmat/primitives/hmac.py
index 618bccc5..8ade84aa 100644
--- a/cryptography/hazmat/primitives/hmac.py
+++ b/cryptography/hazmat/primitives/hmac.py
@@ -16,8 +16,8 @@ from __future__ import absolute_import, division, print_function
 import six
 
 from cryptography import utils
-from cryptography.exceptions import AlreadyFinalized
-from cryptography.hazmat.primitives import interfaces
+from cryptography.exceptions import AlreadyFinalized, InvalidSignature
+from cryptography.hazmat.primitives import constant_time, interfaces
 
 
 @utils.register_interface(interfaces.HashContext)
@@ -57,3 +57,10 @@ class HMAC(object):
         digest = self._ctx.finalize()
         self._ctx = None
         return digest
+
+    def verify(self, sig):
+        if isinstance(sig, six.text_type):
+            raise TypeError("Unicode-objects must be encoded before verifying")
+        digest = self.finalize()
+        if not constant_time.bytes_eq(digest, sig):
+            raise InvalidSignature("Signature did not match digest.")
diff --git a/cryptography/hazmat/primitives/interfaces.py b/cryptography/hazmat/primitives/interfaces.py
index e3f4f586..76dc9339 100644
--- a/cryptography/hazmat/primitives/interfaces.py
+++ b/cryptography/hazmat/primitives/interfaces.py
@@ -152,3 +152,9 @@ class HashContext(six.with_metaclass(abc.ABCMeta)):
         """
         return a HashContext that is a copy of the current context.
         """
+
+    @abc.abstractmethod
+    def verify(self, sig):
+        """
+        compare digest to sig and raise exception if not equal.
+        """