Raise ve on bad gt (#4180)

* Raise a ValueError when conversion to generalizedtime fails

* added test for badasn1time value error

* pep8 compliance

* Addressing code review

+ VE now raises with ```{!r}``` formatting
+ Test now checks that the bad string made it into the VE message

* using ValueError.match

1 files changed, 7 insertions, 1 deletions

diff --git a/src/cryptography/hazmat/backends/openssl/decode_asn1.py b/src/cryptography/hazmat/backends/openssl/decode_asn1.py
index 24eb55b1..31fb8cfc 100644
--- a/src/cryptography/hazmat/backends/openssl/decode_asn1.py
+++ b/src/cryptography/hazmat/backends/openssl/decode_asn1.py
@@ -734,7 +734,13 @@ def _parse_asn1_time(backend, asn1_time):
     generalized_time = backend._lib.ASN1_TIME_to_generalizedtime(
         asn1_time, backend._ffi.NULL
     )
-    backend.openssl_assert(generalized_time != backend._ffi.NULL)
+    if generalized_time == backend._ffi.NULL:
+        raise ValueError(
+            "Couldn't parse ASN.1 time as generalizedtime {!r}".format(
+                _asn1_string_to_bytes(backend, asn1_time)
+            )
+        )
+
     generalized_time = backend._ffi.gc(
         generalized_time, backend._lib.ASN1_GENERALIZEDTIME_free
     )