create & use _evp_md_from_algorithm and _evp_md_non_null_from_algorithm (#4542)

* create & use _evp_md_from_algorithm and _evp_md_non_null_from_algorithm

* remove unused import

1 files changed, 14 insertions, 13 deletions

diff --git a/src/cryptography/hazmat/backends/openssl/rsa.py b/src/cryptography/hazmat/backends/openssl/rsa.py
index 9a7bfaa5..00f5e377 100644
--- a/src/cryptography/hazmat/backends/openssl/rsa.py
+++ b/src/cryptography/hazmat/backends/openssl/rsa.py
@@ -92,14 +92,11 @@ def _enc_dec_rsa_pkey_ctx(backend, key, data, padding_enum, padding):
         isinstance(padding, OAEP) and
         backend._lib.Cryptography_HAS_RSA_OAEP_MD
     ):
-        mgf1_md = backend._lib.EVP_get_digestbyname(
-            padding._mgf._algorithm.name.encode("ascii"))
-        backend.openssl_assert(mgf1_md != backend._ffi.NULL)
+        mgf1_md = backend._evp_md_non_null_from_algorithm(
+            padding._mgf._algorithm)
         res = backend._lib.EVP_PKEY_CTX_set_rsa_mgf1_md(pkey_ctx, mgf1_md)
         backend.openssl_assert(res > 0)
-        oaep_md = backend._lib.EVP_get_digestbyname(
-            padding._algorithm.name.encode("ascii"))
-        backend.openssl_assert(oaep_md != backend._ffi.NULL)
+        oaep_md = backend._evp_md_non_null_from_algorithm(padding._algorithm)
         res = backend._lib.EVP_PKEY_CTX_set_rsa_oaep_md(pkey_ctx, oaep_md)
         backend.openssl_assert(res > 0)
 
@@ -189,15 +186,21 @@ def _rsa_sig_determine_padding(backend, key, padding, algorithm):
 
 def _rsa_sig_setup(backend, padding, algorithm, key, data, init_func):
     padding_enum = _rsa_sig_determine_padding(backend, key, padding, algorithm)
-    evp_md = backend._lib.EVP_get_digestbyname(algorithm.name.encode("ascii"))
-    backend.openssl_assert(evp_md != backend._ffi.NULL)
+    evp_md = backend._evp_md_non_null_from_algorithm(algorithm)
     pkey_ctx = backend._lib.EVP_PKEY_CTX_new(key._evp_pkey, backend._ffi.NULL)
     backend.openssl_assert(pkey_ctx != backend._ffi.NULL)
     pkey_ctx = backend._ffi.gc(pkey_ctx, backend._lib.EVP_PKEY_CTX_free)
     res = init_func(pkey_ctx)
     backend.openssl_assert(res == 1)
     res = backend._lib.EVP_PKEY_CTX_set_signature_md(pkey_ctx, evp_md)
-    backend.openssl_assert(res > 0)
+    if res == 0:
+        backend._consume_errors()
+        raise UnsupportedAlgorithm(
+            "{0} is not supported by this backend for RSA signing.".format(
+                algorithm.name
+            ),
+            _Reasons.UNSUPPORTED_HASH
+        )
     res = backend._lib.EVP_PKEY_CTX_set_rsa_padding(pkey_ctx, padding_enum)
     backend.openssl_assert(res > 0)
     if isinstance(padding, PSS):
@@ -206,10 +209,8 @@ def _rsa_sig_setup(backend, padding, algorithm, key, data, init_func):
         )
         backend.openssl_assert(res > 0)
 
-        mgf1_md = backend._lib.EVP_get_digestbyname(
-            padding._mgf._algorithm.name.encode("ascii")
-        )
-        backend.openssl_assert(mgf1_md != backend._ffi.NULL)
+        mgf1_md = backend._evp_md_non_null_from_algorithm(
+            padding._mgf._algorithm)
         res = backend._lib.EVP_PKEY_CTX_set_rsa_mgf1_md(pkey_ctx, mgf1_md)
         backend.openssl_assert(res > 0)