add generate_rsa_parameters_supported to RSABackend

3 files changed, 19 insertions, 0 deletions

diff --git a/cryptography/hazmat/backends/interfaces.py b/cryptography/hazmat/backends/interfaces.py
index 11b13788..97a7a4fd 100644
--- a/cryptography/hazmat/backends/interfaces.py
+++ b/cryptography/hazmat/backends/interfaces.py
@@ -135,6 +135,13 @@ class RSABackend(object):
         Returns True if the backend supports the given padding options.
         """
 
+    @abc.abstractmethod
+    def generate_rsa_parameters_supported(self, public_exponent, key_size):
+        """
+        Returns True if the backend supports the given parameters for key
+        generation.
+        """
+
 
 @six.add_metaclass(abc.ABCMeta)
 class DSABackend(object):
diff --git a/cryptography/hazmat/backends/multibackend.py b/cryptography/hazmat/backends/multibackend.py
index 21630ba8..b4cb6889 100644
--- a/cryptography/hazmat/backends/multibackend.py
+++ b/cryptography/hazmat/backends/multibackend.py
@@ -132,6 +132,14 @@ class MultiBackend(object):
         raise UnsupportedAlgorithm("RSA is not supported by the backend.",
                                    _Reasons.UNSUPPORTED_PUBLIC_KEY_ALGORITHM)
 
+    def generate_rsa_parameters_supported(self, public_exponent, key_size):
+        for b in self._filtered_backends(RSABackend):
+            return b.generate_rsa_parameters_supported(
+                public_exponent, key_size
+            )
+        raise UnsupportedAlgorithm("RSA is not supported by the backend.",
+                                   _Reasons.UNSUPPORTED_PUBLIC_KEY_ALGORITHM)
+
     def create_rsa_signature_ctx(self, private_key, padding, algorithm):
         for b in self._filtered_backends(RSABackend):
             return b.create_rsa_signature_ctx(private_key, padding, algorithm)
diff --git a/cryptography/hazmat/backends/openssl/backend.py b/cryptography/hazmat/backends/openssl/backend.py
index e5d6eaa1..8d76160d 100644
--- a/cryptography/hazmat/backends/openssl/backend.py
+++ b/cryptography/hazmat/backends/openssl/backend.py
@@ -378,6 +378,10 @@ class Backend(object):
 
         return self._rsa_cdata_to_private_key(ctx)
 
+    def generate_rsa_parameters_supported(self, public_exponent, key_size):
+        return (public_exponent >= 3 and public_exponent & 1 != 0 and
+                key_size >= 512)
+
     def _new_evp_pkey(self):
         evp_pkey = self._lib.EVP_PKEY_new()
         assert evp_pkey != self._ffi.NULL