#!/usr/bin/env python """ Python implementation of RFC 5649 AES Key Wrap With Padding, using PyCrypto to supply the AES code. """ # Terminology mostly follows the RFC, including variable names. # # Block sizes get confusing: AES Key Wrap uses 64-bit blocks, not to # be confused with AES, which uses 128-bit blocks. In practice, this # is less confusing than when reading the description, because we # concatenate two 64-bit blocks just prior to performing an AES ECB # operation, then immediately split the result back into a pair of # 64-bit blocks. class AESKeyWrapWithPadding(object): """ Implementation of AES Key Wrap With Padding from RFC 5649. """ class UnwrapError(Exception): "Something went wrong during unwrap." def __init__(self, key): from Crypto.Cipher import AES self.ctx = AES.new(key, AES.MODE_ECB) def _encrypt(self, b1, b2): aes_block = self.ctx.encrypt(b1 + b2) return aes_block[:8], aes_block[8:] def _decrypt(self, b1, b2): aes_block = self.ctx.decrypt(b1 + b2) return aes_block[:8], aes_block[8:] @staticmethod def _start_stop(start, stop): # Syntactic sugar step = -1 if start > stop else 1 return xrange(start, stop + step, step) def wrap_key(self, Q): """ Wrap a key according to RFC 5649 section 4.1. Q is the plaintext to be wrapped, a byte string. Returns C, the wrapped ciphertext. """ from struct import pack, unpack m = len(Q) # Plaintext length if m % 8 != 0: # Pad Q if needed Q += "\x00" * (8 - (m % 8)) R = [pack(">LL", 0xa65959a6, m)] # Magic MSB(32,A), build LSB(32,A) R.extend(Q[i : i + 8] # Append Q for i in xrange(0, len(Q), 8)) n = len(R) - 1 if n == 1: R[0], R[1] = self._encrypt(R[0], R[1]) else: # RFC 3394 section 2.2.1 for j in self._start_stop(0, 5): for i in self._start_stop(1, n): R[0], R[i] = self._encrypt(R[0], R[i]) W0, W1 = unpack(">LL", R[0]) W1 ^= n * j + i R[0] = pack(">LL", W0, W1) assert len(R) == (n + 1) and all(len(r) == 8 for r in R) return "".join(R) def unwrap_key(self, C): """ Unwrap a key according to RFC 5649 section 4.2. C is the ciphertext to be unwrapped, a byte string Returns Q, the unwrapped plaintext. """ from struct import pack, unpack if len(C) % 8 != 0: raise self.UnwrapError("Ciphertext length {} is not an integral number of blocks" .format(len(C))) n = (len(C) / 8) - 1 R = [C[i : i + 8] for i in xrange(0, len(C), 8)] if n == 1: R[0], R[1] = self._decrypt(R[0], R[1]) else: # RFC 3394 section 2.2.2 steps (1), (2), and part of (3) for j in self._start_stop(5, 0): for i in self._start_stop(n, 1): W0, W1 = unpack(">LL", R[0]) W1 ^= n * j + i R[0] = pack(">LL", W0, W1) R[0], R[i] = self._decrypt(R[0], R[i]) magic, m = unpack(">LL", R[0]) if magic != 0xa65959a6: raise self.UnwrapError("Magic value in AIV should have been 0xa65959a6, was 0x{:02x}" .format(magic)) if m <= 8 * (n - 1) or m > 8 * n: raise self.UnwrapError("Length encoded in AIV out of range: m {}, n {}".format(m, n)) R = "".join(R[1:]) assert len(R) == 8 * n if any(r != "\x00" for r in R[m:]): raise self.UnwrapError("Nonzero trailing bytes {}".format(R[m:].encode("hex"))) return R[:m] if __name__ == "__main__": # Test code from here down import unittest class TestAESKeyWrapWithPadding(unittest.TestCase): @staticmethod def bin2hex(bytes, sep = ":"): return sep.join("{:02x}".format(ord(b)) for b in bytes) @staticmethod def hex2bin(text): return text.translate(None, ": \t\n\r").decode("hex") def loopback_test(self, I): K = AESKeyWrapWithPadding(self.hex2bin("00:01:02:03:04:05:06:07:08:09:0a:0b:0c:0d:0e:0f")) C = K.wrap_key(I) O = K.unwrap_key(C) self.assertEqual(I, O, "Input and output plaintext did not match: {!r} <> {!r}".format(I, O)) def rfc5649_test(self, K, Q, C): K = AESKeyWrapWithPadding(key = self.hex2bin(K)) Q = self.hex2bin(Q) C = self.hex2bin(C) c = K.wrap_key(Q) q = K.unwrap_key(C) self.assertEqual(q, Q, "Input and output plaintext did not match: {} <> {}".format(self.bin2hex(Q), self.bin2hex(q))) self.assertEqual(c, C, "Input and output ciphertext did not match: {} <> {}".format(self.bin2hex(C), self.bin2hex(c))) def test_rfc5649_1(self): self.rfc5649_test(K = "5840df6e29b02af1 ab493b705bf16ea1 ae8338f4dcc176a8", Q = "c37b7e6492584340 bed1220780894115 5068f738", C = "138bdeaa9b8fa7fc 61f97742e72248ee 5ae6ae5360d1ae6a 5f54f373fa543b6a") def test_rfc5649_2(self): self.rfc5649_test(K = "5840df6e29b02af1 ab493b705bf16ea1 ae8338f4dcc176a8", Q = "466f7250617369", C = "afbeb0f07dfbf541 9200f2ccb50bb24f") def test_mangled_1(self): self.assertRaises(AESKeyWrapWithPadding.UnwrapError, self.rfc5649_test, K = "5840df6e29b02af0 ab493b705bf16ea1 ae8338f4dcc176a8", Q = "466f7250617368", C = "afbeb0f07dfbf541 9200f2ccb50bb24f") def test_mangled_2(self): self.assertRaises(AESKeyWrapWithPadding.UnwrapError, self.rfc5649_test, K = "5840df6e29b02af0 ab493b705bf16ea1 ae8338f4dcc176a8", Q = "466f7250617368", C = "afbeb0f07dfbf541 9200f2ccb50bb24f 0123456789abcdef") def test_mangled_3(self): self.assertRaises(AESKeyWrapWithPadding.UnwrapError, self.rfc5649_test, K = "5840df6e29b02af1 ab493b705bf16ea1 ae8338f4dcc176a8", Q = "c37b7e6492584340 bed1220780894115 5068f738", C = "138bdeaa9b8fa7fc 61f97742e72248ee 5ae6ae5360d1ae6a") def test_loopback_1(self): self.loopback_test("!") def test_loopback_2(self): self.loopback_test("Yo!") def test_loopback_3(self): self.loopback_test("Hi, Mom") def test_loopback_4(self): self.loopback_test("1" * (64 / 8)) def test_loopback_5(self): self.loopback_test("2" * (128 / 8)) def test_loopback_6(self): self.loopback_test("3" * (256 / 8)) def test_loopback_7(self): self.loopback_test("3.14159265358979323846264338327950288419716939937510") def test_loopback_8(self): self.loopback_test("3.14159265358979323846264338327950288419716939937510") def test_loopback_9(self): self.loopback_test("Hello! My name is Inigo Montoya. You killed my AES key wrapper. Prepare to die.") def test_joachim_loopback(self): from os import urandom I = "31:32:33" K = AESKeyWrapWithPadding(urandom(256/8)) C = K.wrap_key(I) O = K.unwrap_key(C) self.assertEqual(I, O, "Input and output plaintext did not match: {!r} <> {!r}".format(I, O)) unittest.main(verbosity = 9)