Work around pyasn1's willingness to return endOfOctets in DER parsing

See #1838 for discussion.

1 files changed, 7 insertions, 0 deletions

diff --git a/src/cryptography/hazmat/primitives/asymmetric/utils.py b/src/cryptography/hazmat/primitives/asymmetric/utils.py
index 71f4ff8e..f04eb66e 100644
--- a/src/cryptography/hazmat/primitives/asymmetric/utils.py
+++ b/src/cryptography/hazmat/primitives/asymmetric/utils.py
@@ -4,6 +4,7 @@
 
 from __future__ import absolute_import, division, print_function
 
+from pyasn1.codec.ber import eoo
 from pyasn1.codec.der import decoder, encoder
 from pyasn1.error import PyAsn1Error
 from pyasn1.type import namedtype, univ
@@ -28,6 +29,12 @@ def decode_rfc6979_signature(signature):
         raise ValueError(
             "The signature contains bytes after the end of the ASN.1 sequence."
         )
+    # pyasn1 can erroneously return this from top-level DER decoding.
+    # It's intended as a sentinel in recursive BER decoding, so it's
+    # returned even though an asn1Spec is provided.
+    if data == eoo.endOfOctets:
+        raise ValueError("Invalid signature data. Unable to decode ASN.1")
+
     r = int(data.getComponentByName('r'))
     s = int(data.getComponentByName('s'))
     return (r, s)