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| import struct
from typing import Generator, Tuple
class ChaCha20:
def __init__(self, key: bytes, nonce: bytes, counter: int = 0):
self.state = [
0x61707865, 0x3320646e, 0x79622d32, 0x6b206574,
*struct.unpack('<8I', key),
counter,
*struct.unpack('<3I', nonce)
]
@staticmethod
def _quarter_round(a: int, b: int, c: int, d: int) -> Tuple[int, int, int, int]:
a = (a + b) & 0xFFFFFFFF
d = ((d ^ a) << 16 | (d ^ a) >> 16) & 0xFFFFFFFF
c = (c + d) & 0xFFFFFFFF
b = ((b ^ c) << 12 | (b ^ c) >> 20) & 0xFFFFFFFF
a = (a + b) & 0xFFFFFFFF
d = ((d ^ a) << 8 | (d ^ a) >> 24) & 0xFFFFFFFF
c = (c + d) & 0xFFFFFFFF
b = ((b ^ c) << 7 | (b ^ c) >> 25) & 0xFFFFFFFF
return a, b, c, d
def _generate_block(self) -> bytes:
state = self.state.copy()
for _ in range(10):
state[0], state[4], state[8], state[12] = self._quarter_round(state[0], state[4], state[8], state[12])
state[1], state[5], state[9], state[13] = self._quarter_round(state[1], state[5], state[9], state[13])
state[2], state[6], state[10], state[14] = self._quarter_round(state[2], state[6], state[10], state[14])
state[3], state[7], state[11], state[15] = self._quarter_round(state[3], state[7], state[11], state[15])
state[0], state[5], state[10], state[15] = self._quarter_round(state[0], state[5], state[10], state[15])
state[1], state[6], state[11], state[12] = self._quarter_round(state[1], state[6], state[11], state[12])
state[2], state[7], state[8], state[13] = self._quarter_round(state[2], state[7], state[8], state[13])
state[3], state[4], state[9], state[14] = self._quarter_round(state[3], state[4], state[9], state[14])
block = bytearray()
for i in range(16):
block.extend(struct.pack('<I', (state[i] + self.state[i]) & 0xFFFFFFFF))
self.state[12] = (self.state[12] + 1) & 0xFFFFFFFF
return bytes(block)
def keystream(self) -> Generator[bytes, None, None]:
while True:
yield self._generate_block()
def decrypt(ciphertext: bytes, key: str) -> bytes:
key_bytes = key.ljust(32, '\0').encode()[:32]
chacha = ChaCha20(key_bytes, b'\x00'*12)
plain = bytearray()
for i, (c, k) in enumerate(zip(ciphertext, (b for block in chacha.keystream() for b in block))):
plain.append(((c - 1) % 256) ^ k)
return bytes(plain)
ciphertext = bytes.fromhex("""12 1D 58 7A 7F 60 FE 96 74 D6 7B 56 67 CE 72 3F F2 D2 CE 47 19 59 9B 09 42 07 2C 55 B7 44 10 DE 46 E5 38 B7 74 95 78 42 95 BD 7D B5 64 55 CD 52 8D D2 69 C6 10 5F 82 29 29 9D 30 F6 B4""".replace(" ", "").replace("\n", ""))
print(decrypt(ciphertext, "qewuri").decode('utf-8'))
|
