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Copy pathrc4mod.py
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53 lines (47 loc) · 1.9 KB
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from __future__ import annotations
from itertools import cycle
from refinery.lib.argformats import PythonExpression
from refinery.lib.types import Param
from refinery.units.crypto.cipher import Arg, StreamCipherUnit
class rc4mod(StreamCipherUnit):
"""
Implements a modified version of the RC4 stream cipher where several aspects can be altered.
"""
def __init__(
self, key, stateful=False, discard=0, *,
size: Param[int, Arg.Number('-t', bound=(1, None), help='Table size, {default} by default.')] = 0x100,
step: Param[int, Arg.Number('-i', bound=(1, None), help=(
'Increment the counter by N during the cipher loop. The default is {default}.'))] = 1,
body: Param[str, Arg.String('-K', help=(
'Optional expression involving the table T and the two indices A and B. This expression is used '
'to compute the actual key stream byte during each RC4 round. The default is {default}.'
))] = 'T[T[A]+T[B]]'
):
super().__init__(
key=key,
stateful=stateful,
discard=discard,
body=body,
size=size,
step=step,
)
def keystream(self):
size = self.args.size
step = self.args.step
body = PythonExpression(self.args.body, *'TAB', modulus=size)
tablerange = range(max(size, 0x100))
b, table = 0, bytearray(k & 0xFF for k in tablerange)
for a, keybyte in zip(tablerange, cycle(self.args.key)):
t = table[a]
b = (b + keybyte + t) % size
table[a] = table[b]
table[b] = t
self.log_debug(lambda: F'SBOX = {table.hex(" ").upper()}', clip=True)
b, a = 0, 0
while True:
a = (a + step) % size
t = table[a]
b = (b + t) % size
table[a] = table[b]
table[b] = t
yield body(T=table, A=a, B=b)