translate the UnexpectedDER exception to BadSignatureError

the users of VerifyingKey.verify() and VerifyingKey.verify_digest()
should not need to use multiple exceptions to correctly catch invalid
signatures

src/ecdsa/keys.py

@@ -136,11 +136,14 @@ def verify_digest(self, signature, digest, sigdecode=sigdecode_string):
                                  "for your digest (%d)" % (self.curve.name,
                                                            8 * len(digest)))
         number = string_to_number(digest)
-        r, s = sigdecode(signature, self.pubkey.order)
+        try:
+            r, s = sigdecode(signature, self.pubkey.order)
+        except der.UnexpectedDER as e:
+            raise BadSignatureError("Malformed formatting of signature", e)
         sig = ecdsa.Signature(r, s)
         if self.pubkey.verifies(number, sig):
             return True
-        raise BadSignatureError
+        raise BadSignatureError("Signature verification failed")
 
 
 class SigningKey:

src/ecdsa/test_malformed_sigs.py

@@ -0,0 +1,68 @@
+from __future__ import with_statement, division
+
+import unittest
+import os
+import time
+import shutil
+import subprocess
+import pytest
+from binascii import hexlify, unhexlify
+from hashlib import sha1, sha256, sha512
+import hashlib
+
+from six import b, binary_type
+from .keys import SigningKey, VerifyingKey
+from .keys import BadSignatureError
+from . import util
+from .util import sigencode_der, sigencode_strings
+from .util import sigdecode_der, sigdecode_strings
+from .curves import curves, NIST256p, NIST521p
+from .ellipticcurve import Point
+from . import der
+from . import rfc6979
+import copy
+
+sigs = []
+example_data = b("some data to sign")
+
+# Just NIST256p with SHA256 is 560 test cases, all curves with all hashes is
+# few thousand slow test cases; execute the most interesting only
+
+#for curve in curves:
+for curve in [NIST256p]:
+    #for hash_alg in ["md5", "sha1", "sha224", "sha256", "sha384", "sha512"]:
+    for hash_alg in ["sha256"]:
+        key = SigningKey.generate(curve)
+        signature = key.sign(example_data, hashfunc=getattr(hashlib, hash_alg),
+                             sigencode=sigencode_der)
+        for pos in range(len(signature)):
+            for xor in (1<<i for i in range(8)):
+                sigs.append(pytest.param(
+                    key.verifying_key, hash_alg,
+                    signature, pos, xor,
+                    id="{0}-{1}-pos-{2}-xor-{3}".format(
+                        curve.name, hash_alg, pos, xor)))
+
+
+@pytest.mark.parametrize("verifying_key,hash_alg,signature,pos,xor", sigs)
+def test_fuzzed_der_signatures(verifying_key, hash_alg, signature, pos, xor):
+    # check if a malformed DER encoded signature causes the same exception
+    # to be raised irrespective of the type of error
+    sig = bytearray(signature)
+    sig[pos] ^= xor
+    sig = binary_type(sig)
+
+    try:
+        verifying_key.verify(sig, example_data, getattr(hashlib, hash_alg),
+                             sigdecode_der)
+        assert False
+    except BadSignatureError:
+        assert True
+
+
+def __main__():
+    unittest.main()
+
+
+if __name__ == "__main__":
+    __main__()