Add _Reasons.UNSUPPORTED_MGF

3 files changed, 7 insertions, 4 deletions

diff --git a/cryptography/exceptions.py b/cryptography/exceptions.py
index 4b4d4c37..d2782be6 100644
--- a/cryptography/exceptions.py
+++ b/cryptography/exceptions.py
@@ -19,6 +19,7 @@ class _Reasons(object):
     UNSUPPORTED_HASH = object()
     UNSUPPORTED_CIPHER = object()
     UNSUPPORTED_PADDING = object()
+    UNSUPPORTED_MGF = object()
 
 
 class UnsupportedAlgorithm(Exception):
diff --git a/cryptography/hazmat/backends/openssl/backend.py b/cryptography/hazmat/backends/openssl/backend.py
index 753717d4..3293741c 100644
--- a/cryptography/hazmat/backends/openssl/backend.py
+++ b/cryptography/hazmat/backends/openssl/backend.py
@@ -731,7 +731,8 @@ class _RSASignatureContext(object):
         elif isinstance(padding, PSS):
             if not isinstance(padding._mgf, MGF1):
                 raise UnsupportedAlgorithm(
-                    "Only MGF1 is supported by this backend"
+                    "Only MGF1 is supported by this backend",
+                    _Reasons.UNSUPPORTED_MGF
                 )
 
             # Size of key in bytes - 2 is the maximum
@@ -915,7 +916,8 @@ class _RSAVerificationContext(object):
         elif isinstance(padding, PSS):
             if not isinstance(padding._mgf, MGF1):
                 raise UnsupportedAlgorithm(
-                    "Only MGF1 is supported by this backend"
+                    "Only MGF1 is supported by this backend",
+                    _Reasons.UNSUPPORTED_MGF
                 )
 
             # Size of key in bytes - 2 is the maximum
diff --git a/tests/hazmat/primitives/test_rsa.py b/tests/hazmat/primitives/test_rsa.py
index 5d94e790..32f54945 100644
--- a/tests/hazmat/primitives/test_rsa.py
+++ b/tests/hazmat/primitives/test_rsa.py
@@ -630,7 +630,7 @@ class TestRSASignature(object):
             key_size=512,
             backend=backend
         )
-        with pytest.raises(UnsupportedAlgorithm):
+        with raises_unsupported_algorithm(_Reasons.UNSUPPORTED_MGF):
             private_key.signer(padding.PSS(mgf=DummyMGF()), hashes.SHA1(),
                                backend)
 
@@ -881,7 +881,7 @@ class TestRSAVerification(object):
             backend=backend
         )
         public_key = private_key.public_key()
-        with pytest.raises(UnsupportedAlgorithm):
+        with raises_unsupported_algorithm(_Reasons.UNSUPPORTED_MGF):
             public_key.verifier(b"sig", padding.PSS(mgf=DummyMGF()),
                                 hashes.SHA1(), backend)