Merge pull request #1502 from reaperhulk/fix-1285

Support decoding RFC 6979 signatures to (r, s)

1 files changed, 46 insertions, 0 deletions

diff --git a/src/cryptography/hazmat/primitives/asymmetric/utils.py b/src/cryptography/hazmat/primitives/asymmetric/utils.py
new file mode 100644
index 00000000..71f4ff8e
--- /dev/null
+++ b/src/cryptography/hazmat/primitives/asymmetric/utils.py
@@ -0,0 +1,46 @@
+# This file is dual licensed under the terms of the Apache License, Version
+# 2.0, and the BSD License. See the LICENSE file in the root of this repository
+# for complete details.
+
+from __future__ import absolute_import, division, print_function
+
+from pyasn1.codec.der import decoder, encoder
+from pyasn1.error import PyAsn1Error
+from pyasn1.type import namedtype, univ
+
+import six
+
+
+class _DSSSigValue(univ.Sequence):
+    componentType = namedtype.NamedTypes(
+        namedtype.NamedType('r', univ.Integer()),
+        namedtype.NamedType('s', univ.Integer())
+    )
+
+
+def decode_rfc6979_signature(signature):
+    try:
+        data, remaining = decoder.decode(signature, asn1Spec=_DSSSigValue())
+    except PyAsn1Error:
+        raise ValueError("Invalid signature data. Unable to decode ASN.1")
+
+    if remaining:
+        raise ValueError(
+            "The signature contains bytes after the end of the ASN.1 sequence."
+        )
+    r = int(data.getComponentByName('r'))
+    s = int(data.getComponentByName('s'))
+    return (r, s)
+
+
+def encode_rfc6979_signature(r, s):
+    if (
+        not isinstance(r, six.integer_types) or
+        not isinstance(s, six.integer_types)
+    ):
+        raise ValueError("Both r and s must be integers")
+
+    sig = _DSSSigValue()
+    sig.setComponentByName('r', r)
+    sig.setComponentByName('s', s)
+    return encoder.encode(sig)