chacha20.py raw
1 #!/usr/bin/env python3
2 # Copyright (c) 2022-present The Bitcoin Core developers
3 # Distributed under the MIT software license, see the accompanying
4 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
5
6 """Test-only implementation of ChaCha20 cipher and FSChaCha20 for BIP 324
7
8 It is designed for ease of understanding, not performance.
9
10 WARNING: This code is slow and trivially vulnerable to side channel attacks. Do not use for
11 anything but tests.
12 """
13
14 import unittest
15
16 CHACHA20_INDICES = (
17 (0, 4, 8, 12), (1, 5, 9, 13), (2, 6, 10, 14), (3, 7, 11, 15),
18 (0, 5, 10, 15), (1, 6, 11, 12), (2, 7, 8, 13), (3, 4, 9, 14)
19 )
20
21 CHACHA20_CONSTANTS = (0x61707865, 0x3320646e, 0x79622d32, 0x6b206574)
22 REKEY_INTERVAL = 224 # packets
23
24
25 def rotl32(v, bits):
26 """Rotate the 32-bit value v left by bits bits."""
27 bits %= 32 # Make sure the term below does not throw an exception
28 return ((v << bits) & 0xffffffff) | (v >> (32 - bits))
29
30
31 def chacha20_doubleround(s):
32 """Apply a ChaCha20 double round to 16-element state array s.
33 See https://cr.yp.to/chacha/chacha-20080128.pdf and https://tools.ietf.org/html/rfc8439
34 """
35 for a, b, c, d in CHACHA20_INDICES:
36 s[a] = (s[a] + s[b]) & 0xffffffff
37 s[d] = rotl32(s[d] ^ s[a], 16)
38 s[c] = (s[c] + s[d]) & 0xffffffff
39 s[b] = rotl32(s[b] ^ s[c], 12)
40 s[a] = (s[a] + s[b]) & 0xffffffff
41 s[d] = rotl32(s[d] ^ s[a], 8)
42 s[c] = (s[c] + s[d]) & 0xffffffff
43 s[b] = rotl32(s[b] ^ s[c], 7)
44
45
46 def chacha20_block(key, nonce, cnt):
47 """Compute the 64-byte output of the ChaCha20 block function.
48 Takes as input a 32-byte key, 12-byte nonce, and 32-bit integer counter.
49 """
50 # Initial state.
51 init = [0] * 16
52 init[:4] = CHACHA20_CONSTANTS[:4]
53 init[4:12] = [int.from_bytes(key[i:i+4], 'little') for i in range(0, 32, 4)]
54 init[12] = cnt
55 init[13:16] = [int.from_bytes(nonce[i:i+4], 'little') for i in range(0, 12, 4)]
56 # Perform 20 rounds.
57 state = list(init)
58 for _ in range(10):
59 chacha20_doubleround(state)
60 # Add initial values back into state.
61 for i in range(16):
62 state[i] = (state[i] + init[i]) & 0xffffffff
63 # Produce byte output
64 return b''.join(state[i].to_bytes(4, 'little') for i in range(16))
65
66 class FSChaCha20:
67 """Rekeying wrapper stream cipher around ChaCha20."""
68 def __init__(self, initial_key, rekey_interval=REKEY_INTERVAL):
69 self._key = initial_key
70 self._rekey_interval = rekey_interval
71 self._block_counter = 0
72 self._chunk_counter = 0
73 self._keystream = b''
74
75 def _get_keystream_bytes(self, nbytes):
76 while len(self._keystream) < nbytes:
77 nonce = ((0).to_bytes(4, 'little') + (self._chunk_counter // self._rekey_interval).to_bytes(8, 'little'))
78 self._keystream += chacha20_block(self._key, nonce, self._block_counter)
79 self._block_counter += 1
80 ret = self._keystream[:nbytes]
81 self._keystream = self._keystream[nbytes:]
82 return ret
83
84 def crypt(self, chunk):
85 ks = self._get_keystream_bytes(len(chunk))
86 ret = bytes([ks[i] ^ chunk[i] for i in range(len(chunk))])
87 if ((self._chunk_counter + 1) % self._rekey_interval) == 0:
88 self._key = self._get_keystream_bytes(32)
89 self._block_counter = 0
90 self._keystream = b''
91 self._chunk_counter += 1
92 return ret
93
94
95 # Test vectors from RFC7539/8439 consisting of 32 byte key, 12 byte nonce, block counter
96 # and 64 byte output after applying `chacha20_block` function
97 CHACHA20_TESTS = [
98 ["000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f", [0x09000000, 0x4a000000], 1,
99 "10f1e7e4d13b5915500fdd1fa32071c4c7d1f4c733c068030422aa9ac3d46c4e"
100 "d2826446079faa0914c2d705d98b02a2b5129cd1de164eb9cbd083e8a2503c4e"],
101 ["0000000000000000000000000000000000000000000000000000000000000000", [0, 0], 0,
102 "76b8e0ada0f13d90405d6ae55386bd28bdd219b8a08ded1aa836efcc8b770dc7"
103 "da41597c5157488d7724e03fb8d84a376a43b8f41518a11cc387b669b2ee6586"],
104 ["0000000000000000000000000000000000000000000000000000000000000000", [0, 0], 1,
105 "9f07e7be5551387a98ba977c732d080dcb0f29a048e3656912c6533e32ee7aed"
106 "29b721769ce64e43d57133b074d839d531ed1f28510afb45ace10a1f4b794d6f"],
107 ["0000000000000000000000000000000000000000000000000000000000000001", [0, 0], 1,
108 "3aeb5224ecf849929b9d828db1ced4dd832025e8018b8160b82284f3c949aa5a"
109 "8eca00bbb4a73bdad192b5c42f73f2fd4e273644c8b36125a64addeb006c13a0"],
110 ["00ff000000000000000000000000000000000000000000000000000000000000", [0, 0], 2,
111 "72d54dfbf12ec44b362692df94137f328fea8da73990265ec1bbbea1ae9af0ca"
112 "13b25aa26cb4a648cb9b9d1be65b2c0924a66c54d545ec1b7374f4872e99f096"],
113 ["0000000000000000000000000000000000000000000000000000000000000000", [0, 0x200000000000000], 0,
114 "c2c64d378cd536374ae204b9ef933fcd1a8b2288b3dfa49672ab765b54ee27c7"
115 "8a970e0e955c14f3a88e741b97c286f75f8fc299e8148362fa198a39531bed6d"],
116 ["000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f", [0, 0x4a000000], 1,
117 "224f51f3401bd9e12fde276fb8631ded8c131f823d2c06e27e4fcaec9ef3cf78"
118 "8a3b0aa372600a92b57974cded2b9334794cba40c63e34cdea212c4cf07d41b7"],
119 ["0000000000000000000000000000000000000000000000000000000000000001", [0, 0], 0,
120 "4540f05a9f1fb296d7736e7b208e3c96eb4fe1834688d2604f450952ed432d41"
121 "bbe2a0b6ea7566d2a5d1e7e20d42af2c53d792b1c43fea817e9ad275ae546963"],
122 ["0000000000000000000000000000000000000000000000000000000000000000", [0, 1], 0,
123 "ef3fdfd6c61578fbf5cf35bd3dd33b8009631634d21e42ac33960bd138e50d32"
124 "111e4caf237ee53ca8ad6426194a88545ddc497a0b466e7d6bbdb0041b2f586b"],
125 ["000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f", [0, 0x0706050403020100], 0,
126 "f798a189f195e66982105ffb640bb7757f579da31602fc93ec01ac56f85ac3c1"
127 "34a4547b733b46413042c9440049176905d3be59ea1c53f15916155c2be8241a"],
128 ]
129
130 FSCHACHA20_TESTS = [
131 ["000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f",
132 "0000000000000000000000000000000000000000000000000000000000000000", 256,
133 "a93df4ef03011f3db95f60d996e1785df5de38fc39bfcb663a47bb5561928349"],
134 ["01", "000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f", 5, "ea"],
135 ["e93fdb5c762804b9a706816aca31e35b11d2aa3080108ef46a5b1f1508819c0a",
136 "8ec4c3ccdaea336bdeb245636970be01266509b33f3d2642504eaf412206207a", 4096,
137 "8bfaa4eacff308fdb4a94a5ff25bd9d0c1f84b77f81239f67ff39d6e1ac280c9"],
138 ]
139
140
141 class TestFrameworkChacha(unittest.TestCase):
142 def test_chacha20(self):
143 """ChaCha20 test vectors."""
144 for test_vector in CHACHA20_TESTS:
145 hex_key, nonce, counter, hex_output = test_vector
146 key = bytes.fromhex(hex_key)
147 nonce_bytes = nonce[0].to_bytes(4, 'little') + nonce[1].to_bytes(8, 'little')
148 keystream = chacha20_block(key, nonce_bytes, counter)
149 self.assertEqual(hex_output, keystream.hex())
150
151 def test_fschacha20(self):
152 """FSChaCha20 test vectors."""
153 for test_vector in FSCHACHA20_TESTS:
154 hex_plaintext, hex_key, rekey_interval, hex_ciphertext_after_rotation = test_vector
155 plaintext = bytes.fromhex(hex_plaintext)
156 key = bytes.fromhex(hex_key)
157 fsc20 = FSChaCha20(key, rekey_interval)
158 for _ in range(rekey_interval):
159 fsc20.crypt(plaintext)
160
161 ciphertext = fsc20.crypt(plaintext)
162 self.assertEqual(hex_ciphertext_after_rotation, ciphertext.hex())
163