Raise UnsupportedAlgorithm when initing Hash()

Instead of just an AssertionError.

3 files changed, 20 insertions, 3 deletions

diff --git a/cryptography/hazmat/backends/openssl/backend.py b/cryptography/hazmat/backends/openssl/backend.py
index 588a4273..5b7cb3de 100644
--- a/cryptography/hazmat/backends/openssl/backend.py
+++ b/cryptography/hazmat/backends/openssl/backend.py
@@ -400,7 +400,11 @@ class _HashContext(object):
                                        self._backend.lib.EVP_MD_CTX_destroy)
             evp_md = self._backend.lib.EVP_get_digestbyname(
                 algorithm.name.encode("ascii"))
-            assert evp_md != self._backend.ffi.NULL
+            if evp_md == self._backend.ffi.NULL:
+                raise UnsupportedAlgorithm(
+                    "{0} is not a supported hash on this backend".format(
+                        algorithm.name)
+                )
             res = self._backend.lib.EVP_DigestInit_ex(ctx, evp_md,
                                                       self._backend.ffi.NULL)
             assert res != 0
diff --git a/docs/hazmat/primitives/cryptographic-hashes.rst b/docs/hazmat/primitives/cryptographic-hashes.rst
index 90ca198a..38347378 100644
--- a/docs/hazmat/primitives/cryptographic-hashes.rst
+++ b/docs/hazmat/primitives/cryptographic-hashes.rst
@@ -28,6 +28,9 @@ Message Digests
         >>> digest.finalize()
         'l\xa1=R\xcap\xc8\x83\xe0\xf0\xbb\x10\x1eBZ\x89\xe8bM\xe5\x1d\xb2\xd29%\x93\xafj\x84\x11\x80\x90'
 
+    If the backend doesn't support the requested ``algorithm`` an
+    :class:`~cryptography.exceptions.UnsupportedAlgorithm` will be raised.
+
     Keep in mind that attacks against cryptographic hashes only get stronger
     with time, and that often algorithms that were once thought to be strong,
     become broken. Because of this it's important to include a plan for
diff --git a/tests/hazmat/primitives/test_hashes.py b/tests/hazmat/primitives/test_hashes.py
index ff42e8f4..72bc3e27 100644
--- a/tests/hazmat/primitives/test_hashes.py
+++ b/tests/hazmat/primitives/test_hashes.py
@@ -19,12 +19,18 @@ import pytest
 
 import six
 
-from cryptography.exceptions import AlreadyFinalized
-from cryptography.hazmat.primitives import hashes
+from cryptography import utils
+from cryptography.exceptions import AlreadyFinalized, UnsupportedAlgorithm
+from cryptography.hazmat.primitives import hashes, interfaces
 
 from .utils import generate_base_hash_test
 
 
+@utils.register_interface(interfaces.HashAlgorithm)
+class UnsupportedDummyHash(object):
+    name = "unsupported-dummy-hash"
+
+
 class TestHashContext(object):
     def test_hash_reject_unicode(self, backend):
         m = hashes.Hash(hashes.SHA1(), backend=backend)
@@ -57,6 +63,10 @@ class TestHashContext(object):
         with pytest.raises(AlreadyFinalized):
             h.finalize()
 
+    def test_unsupported_hash(self, backend):
+        with pytest.raises(UnsupportedAlgorithm):
+            hashes.Hash(UnsupportedDummyHash(), backend)
+
 
 class TestSHA1(object):
     test_SHA1 = generate_base_hash_test(