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())