RSA OAEP label support for OpenSSL 1.0.2+ (#3897)

* RSA OAEP label support for OpenSSL 1.0.2+

* changelog

* move around tests, address review feedback, use backend supported method

* unsupported padding catches this now

2 files changed, 20 insertions, 4 deletions

diff --git a/src/cryptography/hazmat/backends/openssl/backend.py b/src/cryptography/hazmat/backends/openssl/backend.py
index 6c9ef84f..d9a5bdf2 100644
--- a/src/cryptography/hazmat/backends/openssl/backend.py
+++ b/src/cryptography/hazmat/backends/openssl/backend.py
@@ -547,7 +547,11 @@ class Backend(object):
         elif isinstance(padding, OAEP) and isinstance(padding._mgf, MGF1):
             return (
                 self._oaep_hash_supported(padding._mgf._algorithm) and
-                self._oaep_hash_supported(padding._algorithm)
+                self._oaep_hash_supported(padding._algorithm) and
+                (
+                    (padding._label is None or len(padding._label) == 0) or
+                    self._lib.Cryptography_HAS_RSA_OAEP_LABEL == 1
+                )
             )
         else:
             return False
diff --git a/src/cryptography/hazmat/backends/openssl/rsa.py b/src/cryptography/hazmat/backends/openssl/rsa.py
index 839ef147..05b4e9dc 100644
--- a/src/cryptography/hazmat/backends/openssl/rsa.py
+++ b/src/cryptography/hazmat/backends/openssl/rsa.py
@@ -57,9 +57,6 @@ def _enc_dec_rsa(backend, key, data, padding):
                 _Reasons.UNSUPPORTED_PADDING
             )
 
-        if padding._label is not None and padding._label != b"":
-            raise ValueError("This backend does not support OAEP labels.")
-
     else:
         raise UnsupportedAlgorithm(
             "{0} is not supported by this backend.".format(
@@ -106,6 +103,21 @@ def _enc_dec_rsa_pkey_ctx(backend, key, data, padding_enum, padding):
         res = backend._lib.EVP_PKEY_CTX_set_rsa_oaep_md(pkey_ctx, oaep_md)
         backend.openssl_assert(res > 0)
 
+    if (
+        isinstance(padding, OAEP) and
+        padding._label is not None and
+        len(padding._label) > 0
+    ):
+        # set0_rsa_oaep_label takes ownership of the char * so we need to
+        # copy it into some new memory
+        labelptr = backend._lib.OPENSSL_malloc(len(padding._label))
+        backend.openssl_assert(labelptr != backend._ffi.NULL)
+        backend._ffi.memmove(labelptr, padding._label, len(padding._label))
+        res = backend._lib.EVP_PKEY_CTX_set0_rsa_oaep_label(
+            pkey_ctx, labelptr, len(padding._label)
+        )
+        backend.openssl_assert(res == 1)
+
     outlen = backend._ffi.new("size_t *", buf_size)
     buf = backend._ffi.new("unsigned char[]", buf_size)
     res = crypt(pkey_ctx, buf, outlen, data, len(data))