Inplace mode for _int_to_bn

Mostly useful for working with (EC)?DSA_SIG right now

2 files changed, 25 insertions, 2 deletions

diff --git a/cryptography/hazmat/backends/openssl/backend.py b/cryptography/hazmat/backends/openssl/backend.py
index 021ce8c4..6bf3787a 100644
--- a/cryptography/hazmat/backends/openssl/backend.py
+++ b/cryptography/hazmat/backends/openssl/backend.py
@@ -293,7 +293,7 @@ class Backend(object):
             self._lib.OPENSSL_free(hex_cdata)
             return int(hex_str, 16)
 
-    def _int_to_bn(self, num):
+    def _int_to_bn(self, num, bn=None):
         """
         Converts a python integer to a BIGNUM. The returned BIGNUM will not
         be garbage collected (to support adding them to structs that take
@@ -301,11 +301,14 @@ class Backend(object):
         be discarded after use.
         """
 
+        if bn is None:
+            bn = self._ffi.NULL
+
         if six.PY3:
             # Python 3 has constant time to_bytes, so use that.
 
             binary = num.to_bytes(int(num.bit_length() / 8.0 + 1), "big")
-            bn_ptr = self._lib.BN_bin2bn(binary, len(binary), self._ffi.NULL)
+            bn_ptr = self._lib.BN_bin2bn(binary, len(binary), bn)
             assert bn_ptr != self._ffi.NULL
             return bn_ptr
 
@@ -314,6 +317,7 @@ class Backend(object):
 
             hex_num = hex(num).rstrip("L").lstrip("0x").encode("ascii") or b"0"
             bn_ptr = self._ffi.new("BIGNUM **")
+            bn_ptr[0] = bn
             res = self._lib.BN_hex2bn(bn_ptr, hex_num)
             assert res != 0
             assert bn_ptr[0] != self._ffi.NULL
diff --git a/tests/hazmat/backends/test_openssl.py b/tests/hazmat/backends/test_openssl.py
index 4f682f66..43d28c33 100644
--- a/tests/hazmat/backends/test_openssl.py
+++ b/tests/hazmat/backends/test_openssl.py
@@ -233,6 +233,25 @@ class TestOpenSSLRandomEngine(object):
         e = backend._lib.ENGINE_get_default_RAND()
         assert e == backend._ffi.NULL
 
+    def test_int_to_bn(self):
+        value = (2 ** 4242) - 4242
+        bn = backend._int_to_bn(value)
+        assert bn != backend._ffi.NULL
+        bn = backend._ffi.gc(bn, backend._lib.BN_free)
+
+        assert bn
+        assert backend._bn_to_int(bn) == value
+
+    def test_int_to_bn_inplace(self):
+        value = (2 ** 4242) - 4242
+        bn_ptr = backend._lib.BN_new()
+        assert bn_ptr != backend._ffi.NULL
+        bn_ptr = backend._ffi.gc(bn_ptr, backend._lib.BN_free)
+        bn = backend._int_to_bn(value, bn_ptr)
+
+        assert bn == bn_ptr
+        assert backend._bn_to_int(bn_ptr) == value
+
 
 class TestOpenSSLRSA(object):
     @pytest.mark.skipif(