add decode_rfc6979_signature helper for DSA/ECDSA

5 files changed, 44 insertions, 2 deletions

diff --git a/docs/hazmat/primitives/asymmetric/dsa.rst b/docs/hazmat/primitives/asymmetric/dsa.rst
index 43741ede..df3c99fc 100644
--- a/docs/hazmat/primitives/asymmetric/dsa.rst
+++ b/docs/hazmat/primitives/asymmetric/dsa.rst
@@ -80,7 +80,8 @@ provider.
     >>> signature = signer.finalize()
 
 The ``signature`` is a ``bytes`` object, whose contents is DER encoded as
-described in :rfc:`6979`.
+described in :rfc:`6979`. This can be decoded using
+:func:`~cryptography.hazmat.primitives.asymmetric.utils.decode_rfc6979_signature`.
 
 Verification
 ~~~~~~~~~~~~
diff --git a/docs/hazmat/primitives/asymmetric/ec.rst b/docs/hazmat/primitives/asymmetric/ec.rst
index fd20cbb2..13ab7c11 100644
--- a/docs/hazmat/primitives/asymmetric/ec.rst
+++ b/docs/hazmat/primitives/asymmetric/ec.rst
@@ -53,7 +53,9 @@ Elliptic Curve Signature Algorithms
         >>> signature = signer.finalize()
 
     The ``signature`` is a ``bytes`` object, whose contents is DER encoded as
-    described in :rfc:`6979`.
+    described in :rfc:`6979`. This can be decoded using
+    :func:`~cryptography.hazmat.primitives.asymmetric.utils.decode_rfc6979_signature`.
+
 
 
 .. class:: EllipticCurvePrivateNumbers(private_value, public_numbers)
diff --git a/docs/hazmat/primitives/asymmetric/index.rst b/docs/hazmat/primitives/asymmetric/index.rst
index 6a5228ba..24f0f5b1 100644
--- a/docs/hazmat/primitives/asymmetric/index.rst
+++ b/docs/hazmat/primitives/asymmetric/index.rst
@@ -11,3 +11,4 @@ Asymmetric algorithms
     rsa
     padding
     serialization
+    utils
diff --git a/docs/hazmat/primitives/asymmetric/utils.rst b/docs/hazmat/primitives/asymmetric/utils.rst
new file mode 100644
index 00000000..9749b6d1
--- /dev/null
+++ b/docs/hazmat/primitives/asymmetric/utils.rst
@@ -0,0 +1,16 @@
+.. hazmat::
+
+Asymmetric Utilities
+====================
+
+.. currentmodule:: cryptography.hazmat.primitives.asymmetric.utils
+
+
+.. function:: decode_rfc6979_signature(signature)
+
+    Takes in :rfc:`6979` signatures generated by the DSA/ECDSA signers and
+    returns a tuple ``(r, s)``.
+
+    :param bytes signature: The signature to decode.
+
+    :returns: The decoded tuple ``(r, s)``.
diff --git a/src/cryptography/hazmat/primitives/asymmetric/utils.py b/src/cryptography/hazmat/primitives/asymmetric/utils.py
new file mode 100644
index 00000000..5e35b3f6
--- /dev/null
+++ b/src/cryptography/hazmat/primitives/asymmetric/utils.py
@@ -0,0 +1,22 @@
+# 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
+from pyasn1.type import namedtype, univ
+
+
+class _DSSSigValue(univ.Sequence):
+    componentType = namedtype.NamedTypes(
+        namedtype.NamedType('r', univ.Integer()),
+        namedtype.NamedType('s', univ.Integer())
+    )
+
+
+def decode_rfc6979_signature(signature):
+    data = decoder.decode(signature, asn1Spec=_DSSSigValue())
+    r = int(data[0].getComponentByName('r'))
+    s = int(data[0].getComponentByName('s'))
+    return (r, s)