Fixes #4830 -- handle negative serial numbers (#4843)

2 files changed, 12 insertions, 1 deletions

diff --git a/src/cryptography/hazmat/backends/openssl/backend.py b/src/cryptography/hazmat/backends/openssl/backend.py
index 74dedbe0..ee864137 100644
--- a/src/cryptography/hazmat/backends/openssl/backend.py
+++ b/src/cryptography/hazmat/backends/openssl/backend.py
@@ -334,7 +334,10 @@ class Backend(object):
             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")
+            val = int.from_bytes(self._ffi.buffer(bin_ptr)[:bin_len], "big")
+            if self._lib.BN_is_negative(bn):
+                val = -val
+            return val
         else:
             # Under Python 2 the best we can do is hex()
             hex_cdata = self._lib.BN_bn2hex(bn)
diff --git a/tests/x509/test_x509.py b/tests/x509/test_x509.py
index 72cd49e7..afca9c5b 100644
--- a/tests/x509/test_x509.py
+++ b/tests/x509/test_x509.py
@@ -597,6 +597,14 @@ class TestRSACertificate(object):
             cert.signature_algorithm_oid == SignatureAlgorithmOID.RSA_WITH_SHA1
         )
 
+    def test_negative_serial_number(self, backend):
+        cert = _load_cert(
+            os.path.join("x509", "custom", "negative_serial.pem"),
+            x509.load_pem_x509_certificate,
+            backend,
+        )
+        assert cert.serial_number == -18008675309
+
     def test_alternate_rsa_with_sha1_oid(self, backend):
         cert = _load_cert(
             os.path.join("x509", "alternate-rsa-sha1-oid.pem"),