add decode_rfc6979_signature helper for DSA/ECDSA
diff --git a/docs/hazmat/primitives/asymmetric/dsa.rst b/docs/hazmat/primitives/asymmetric/dsa.rst
index 43741ed..df3c99f 100644
--- a/docs/hazmat/primitives/asymmetric/dsa.rst
+++ b/docs/hazmat/primitives/asymmetric/dsa.rst
@@ -80,7 +80,8 @@
>>> 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 fd20cbb..13ab7c1 100644
--- a/docs/hazmat/primitives/asymmetric/ec.rst
+++ b/docs/hazmat/primitives/asymmetric/ec.rst
@@ -53,7 +53,9 @@
>>> 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 6a5228b..24f0f5b 100644
--- a/docs/hazmat/primitives/asymmetric/index.rst
+++ b/docs/hazmat/primitives/asymmetric/index.rst
@@ -11,3 +11,4 @@
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 0000000..9749b6d
--- /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 0000000..5e35b3f
--- /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)