add rsa_padding_supported interface to RSABackend and OpenSSL backend

3 files changed, 22 insertions, 0 deletions

diff --git a/cryptography/hazmat/backends/interfaces.py b/cryptography/hazmat/backends/interfaces.py
index 264c5afb..11b13788 100644
--- a/cryptography/hazmat/backends/interfaces.py
+++ b/cryptography/hazmat/backends/interfaces.py
@@ -129,6 +129,12 @@ class RSABackend(object):
         Returns encrypted bytes.
         """
 
+    @abc.abstractmethod
+    def rsa_padding_supported(self, padding):
+        """
+        Returns True if the backend supports the given padding options.
+        """
+
 
 @six.add_metaclass(abc.ABCMeta)
 class DSABackend(object):
diff --git a/cryptography/hazmat/backends/multibackend.py b/cryptography/hazmat/backends/multibackend.py
index 21d307cf..21630ba8 100644
--- a/cryptography/hazmat/backends/multibackend.py
+++ b/cryptography/hazmat/backends/multibackend.py
@@ -164,6 +164,12 @@ class MultiBackend(object):
         raise UnsupportedAlgorithm("RSA is not supported by the backend.",
                                    _Reasons.UNSUPPORTED_PUBLIC_KEY_ALGORITHM)
 
+    def rsa_padding_supported(self, padding):
+        for b in self._filtered_backends(RSABackend):
+            return b.rsa_padding_supported(padding)
+        raise UnsupportedAlgorithm("RSA is not supported by the backend.",
+                                   _Reasons.UNSUPPORTED_PUBLIC_KEY_ALGORITHM)
+
     def generate_dsa_parameters(self, key_size):
         for b in self._filtered_backends(DSABackend):
             return b.generate_dsa_parameters(key_size)
diff --git a/cryptography/hazmat/backends/openssl/backend.py b/cryptography/hazmat/backends/openssl/backend.py
index 5d9626d0..e5d6eaa1 100644
--- a/cryptography/hazmat/backends/openssl/backend.py
+++ b/cryptography/hazmat/backends/openssl/backend.py
@@ -534,6 +534,16 @@ class Backend(object):
         else:
             return isinstance(algorithm, hashes.SHA1)
 
+    def rsa_padding_supported(self, padding):
+        if isinstance(padding, PKCS1v15):
+            return True
+        elif isinstance(padding, PSS) and isinstance(padding._mgf, MGF1):
+            return self.mgf1_hash_supported(padding._mgf._algorithm)
+        elif isinstance(padding, OAEP) and isinstance(padding._mgf, MGF1):
+            return isinstance(padding._mgf._algorithm, hashes.SHA1)
+        else:
+            return False
+
     def generate_dsa_parameters(self, key_size):
         if key_size not in (1024, 2048, 3072):
             raise ValueError(