Simplify code slightly by adding a new binding

2 files changed, 3 insertions, 4 deletions

diff --git a/src/_cffi_src/openssl/bignum.py b/src/_cffi_src/openssl/bignum.py
index ae035007..455afdc1 100644
--- a/src/_cffi_src/openssl/bignum.py
+++ b/src/_cffi_src/openssl/bignum.py
@@ -71,6 +71,8 @@ int BN_mask_bits(BIGNUM *, int);
 """
 
 MACROS = """
+int BN_num_bytes(const BIGNUM *);
+
 int BN_zero(BIGNUM *);
 int BN_one(BIGNUM *);
 int BN_mod(BIGNUM *, const BIGNUM *, const BIGNUM *, BN_CTX *);
diff --git a/src/cryptography/hazmat/backends/openssl/backend.py b/src/cryptography/hazmat/backends/openssl/backend.py
index 0dd9a2e3..c0c9ebe2 100644
--- a/src/cryptography/hazmat/backends/openssl/backend.py
+++ b/src/cryptography/hazmat/backends/openssl/backend.py
@@ -919,17 +919,14 @@ class Backend(object):
         assert bn != self._ffi.NULL
         if six.PY3:
             # Python 3 has constant time from_bytes, so use that.
-
-            bn_num_bytes = (self._lib.BN_num_bits(bn) + 7) // 8
+            bn_num_bytes = self._lib.BN_num_bytes(bn)
             bin_ptr = self._ffi.new("unsigned char[]", bn_num_bytes)
             bin_len = self._lib.BN_bn2bin(bn, bin_ptr)
             # A zero length means the BN has value 0
             self.openssl_assert(bin_len >= 0)
             return int.from_bytes(self._ffi.buffer(bin_ptr)[:bin_len], "big")
-
         else:
             # Under Python 2 the best we can do is hex()
-
             hex_cdata = self._lib.BN_bn2hex(bn)
             self.openssl_assert(hex_cdata != self._ffi.NULL)
             hex_str = self._ffi.string(hex_cdata)