Arthur de Jong

Open Source / Free Software developer

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authorArthur de Jong <arthur@arthurdejong.org>2014-05-29 18:39:35 +0200
committerArthur de Jong <arthur@arthurdejong.org>2014-05-30 13:23:16 +0200
commit5720fe55f7549feb6f0319b720cded777e8f6d9f (patch)
tree5379164adfb85d0f99371a95d86df50a217affa4
parent7164d892734dd2cf7a4358ddb31f532e1f781721 (diff)
Provide an RFC 3394 AES key wrapping algorithm
This also introduces an EncryptionError exception.
Diffstat
-rw-r--r--pskc/aeskw.py73
-rw-r--r--pskc/exceptions.py5
-rw-r--r--tests/test_aeskw.doctest101
3 files changed, 179 insertions, 0 deletions
diff --git a/pskc/aeskw.py b/pskc/aeskw.py
new file mode 100644
index 0000000..da75a18
--- /dev/null
+++ b/pskc/aeskw.py
@@ -0,0 +1,73 @@
+# aeskw.py - implementation of AES key wrapping
+# coding: utf-8
+#
+# Copyright (C) 2014 Arthur de Jong
+#
+# This library is free software; you can redistribute it and/or
+# modify it under the terms of the GNU Lesser General Public
+# License as published by the Free Software Foundation; either
+# version 2.1 of the License, or (at your option) any later version.
+#
+# This library is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+# Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public
+# License along with this library; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+# 02110-1301 USA
+
+"""Implement key wrapping as described in RFC 3394."""
+
+from Crypto.Cipher import AES
+from Crypto.Util.number import long_to_bytes
+from Crypto.Util.strxor import strxor
+
+from pskc.exceptions import EncryptionError, DecryptionError
+
+
+def _split(value):
+ return value[:8], value[8:]
+
+
+RFC3394_IV = 'a6a6a6a6a6a6a6a6'.decode('hex')
+
+
+def wrap(plaintext, key):
+ """Apply the AES key wrap algorithm to the plaintext."""
+
+ if len(plaintext) % 8 != 0 or len(plaintext) < 16:
+ raise EncryptionError('Plaintext length wrong')
+
+ encrypt = AES.new(key).encrypt
+ n = len(plaintext) / 8
+ A = RFC3394_IV
+ R = [plaintext[i * 8:i * 8 + 8]
+ for i in range(n)]
+ for j in range(6):
+ for i in range(n):
+ A, R[i] = _split(encrypt(A + R[i]))
+ A = strxor(A, long_to_bytes(n * j + i + 1, 8))
+ return A + ''.join(R)
+
+
+def unwrap(ciphertext, key):
+ """Apply the AES key unwrap algorithm to the ciphertext."""
+
+ if len(ciphertext) % 8 != 0 or len(ciphertext) < 24:
+ raise DecryptionError('Ciphertext length wrong')
+
+ decrypt = AES.new(key).decrypt
+ n = len(ciphertext) / 8 - 1
+ A = ciphertext[:8]
+ R = [ciphertext[(i + 1) * 8:(i + 2) * 8]
+ for i in range(n)]
+ for j in reversed(range(6)):
+ for i in reversed(range(n)):
+ A = strxor(A, long_to_bytes(n * j + i + 1, 8))
+ A, R[i] = _split(decrypt(A + R[i]))
+
+ if A == RFC3394_IV:
+ return ''.join(R)
+ raise DecryptionError('IV does not match')
diff --git a/pskc/exceptions.py b/pskc/exceptions.py
index 7fde416..19d801e 100644
--- a/pskc/exceptions.py
+++ b/pskc/exceptions.py
@@ -36,6 +36,11 @@ class ParseError(PSKCError):
pass
+class EncryptionError(PSKCError):
+ """There was a problem encrypting the value."""
+ pass
+
+
class DecryptionError(PSKCError):
"""There was a problem decrypting the value.
diff --git a/tests/test_aeskw.doctest b/tests/test_aeskw.doctest
new file mode 100644
index 0000000..c0fc449
--- /dev/null
+++ b/tests/test_aeskw.doctest
@@ -0,0 +1,101 @@
+test_keywrap.doctest - test keywrap functions
+
+Copyright (C) 2014 Arthur de Jong
+
+This library is free software; you can redistribute it and/or
+modify it under the terms of the GNU Lesser General Public
+License as published by the Free Software Foundation; either
+version 2.1 of the License, or (at your option) any later version.
+
+This library is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+Lesser General Public License for more details.
+
+You should have received a copy of the GNU Lesser General Public
+License along with this library; if not, write to the Free Software
+Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+02110-1301 USA
+
+
+>>> from pskc.aeskw import wrap, unwrap
+
+
+Wrap 128 bits of Key Data with a 128-bit KEK (test vector 4.1 from RFC 3394).
+
+>>> key = '000102030405060708090A0B0C0D0E0F'.decode('hex')
+>>> plaintext = '00112233445566778899AABBCCDDEEFF'.decode('hex')
+>>> ciphertext = '1FA68B0A8112B447AEF34BD8FB5A7B829D3E862371D2CFE5'.decode('hex')
+>>> wrap(plaintext, key) == ciphertext
+True
+>>> unwrap(ciphertext, key) == plaintext
+True
+
+
+Wrap 128 bits of Key Data with a 192-bit KEK (test vector 4.2 from RFC 3394).
+
+>>> key = '000102030405060708090A0B0C0D0E0F1011121314151617'.decode('hex')
+>>> plaintext = '00112233445566778899AABBCCDDEEFF'.decode('hex')
+>>> ciphertext = '96778B25AE6CA435F92B5B97C050AED2468AB8A17AD84E5D'.decode('hex')
+>>> wrap(plaintext, key) == ciphertext
+True
+>>> unwrap(ciphertext, key) == plaintext
+True
+
+
+Wrap 128 bits of Key Data with a 256-bit KEK (test vector 4.3 from RFC 3394).
+
+>>> key = '000102030405060708090A0B0C0D0E0F101112131415161718191A1B1C1D1E1F'.decode('hex')
+>>> plaintext = '00112233445566778899AABBCCDDEEFF'.decode('hex')
+>>> ciphertext = '64E8C3F9CE0F5BA263E9777905818A2A93C8191E7D6E8AE7'.decode('hex')
+>>> wrap(plaintext, key) == ciphertext
+True
+>>> unwrap(ciphertext, key) == plaintext
+True
+
+
+Wrap 192 bits of Key Data with a 192-bit KEK (test vector 4.4 from RFC 3394).
+
+>>> key = '000102030405060708090A0B0C0D0E0F1011121314151617'.decode('hex')
+>>> plaintext = '00112233445566778899AABBCCDDEEFF0001020304050607'.decode('hex')
+>>> ciphertext = '031D33264E15D33268F24EC260743EDCE1C6C7DDEE725A936BA814915C6762D2'.decode('hex')
+>>> wrap(plaintext, key) == ciphertext
+True
+>>> unwrap(ciphertext, key) == plaintext
+True
+
+
+Wrap 192 bits of Key Data with a 256-bit KEK (test vector 4.5 from RFC 3394).
+
+>>> key = '000102030405060708090A0B0C0D0E0F101112131415161718191A1B1C1D1E1F'.decode('hex')
+>>> plaintext = '00112233445566778899AABBCCDDEEFF0001020304050607'.decode('hex')
+>>> ciphertext = 'A8F9BC1612C68B3FF6E6F4FBE30E71E4769C8B80A32CB8958CD5D17D6B254DA1'.decode('hex')
+>>> wrap(plaintext, key) == ciphertext
+True
+>>> unwrap(ciphertext, key) == plaintext
+True
+
+
+Wrap 256 bits of Key Data with a 256-bit KEK (test vector 4.6 from RFC 3394).
+
+>>> key = '000102030405060708090A0B0C0D0E0F101112131415161718191A1B1C1D1E1F'.decode('hex')
+>>> plaintext = '00112233445566778899AABBCCDDEEFF000102030405060708090A0B0C0D0E0F'.decode('hex')
+>>> ciphertext = '28C9F404C4B810F4CBCCB35CFB87F8263F5786E2D80ED326CBC7F0E71A99F43BFB988B9B7A02DD21'.decode('hex')
+>>> wrap(plaintext, key) == ciphertext
+True
+>>> unwrap(ciphertext, key) == plaintext
+True
+
+
+Mangling the ciphertext and unwrapping results in an exception:
+
+>>> ciphertext = 'XX' + ciphertext[2:]
+>>> unwrap(ciphertext, key)
+Traceback (most recent call last):
+ ...
+DecryptionError: IV does not match
+>>> ciphertext = ciphertext[2:]
+>>> unwrap(ciphertext, key)
+Traceback (most recent call last):
+ ...
+DecryptionError: Ciphertext length wrong