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@max1220
Created March 5, 2021 16:55
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Simple RC4 Stream Cipher implementation in Lua
-- small Lua RC4 library.
-- Tested in luajit and lua5.3.
-- Use for testing purposes only!
-- RC4 is broken, and this is not a well-tested implementation.
local ok,bit = pcall(require, "bit")
if not ok then
bit = require("bit32")
end
assert(bit and (bit.bxor(0xFF,0xF0)==0x0F)) -- test bit library xor
local rc4 = {}
-- Swap two table values based on their index.
local function swap(t,ia,ib)
local a = t[ia]
local b = t[ib]
t[ia] = b
t[ib] = a
end
-- Dump a state(S) in human-readable form as a string(also valid lua table)
function rc4.dump_S(S)
local str = {}
for i=0, 255 do
str[i+1] = ("%.2X"):format(S[i])
end
return ("{\n\ti = %d,\n\tj = %d,\n\tdata = %q\n}"):format(S.i, S.j, table.concat(str))
end
-- Key-scheduling algorithm.
-- Based on the key, generate the initial state(S)
-- Returns new RC4 state(S)
function rc4.ksa(key)
local S = { i = 0, j = 0}
for i=0, 255 do
S[i] = i
end
local j = 0
local key_len = #key
for i=0, 255 do
j = (j + S[i] + string.byte(key, (i%key_len)+1)) % 256
swap(S, i,j)
end
return S
end
-- Pseudo-random number generator.
-- Based on the state(S) return the next pseudo-random number(0-255), and iterate the state.
-- This sequence of numbers is used as the keystream for encryption/decryption.
function rc4.prng(S)
S.i = (S.i+1) % 256
S.j = (S.j+S[S.i]) % 256
S.i, S.j = S.j, S.i
swap(S, "i","j")
return S[(S[S.i]+S[S.j]) % 256]
end
-- Generate the initial keystate using the ksa,
-- then iterate over every byte in the plaintext,
-- and xor it with the keystream byte returned by the prng iteration.
function rc4:encrypt(key, plaintext)
assert(type(key)=="string")
assert(type(plaintext)=="string")
assert(#key>0)
assert(#key<=16)
local cyphertext = {}
local S = self.ksa(key) -- generate the initial state based on the key
for i=1, #plaintext do
local prn = self.prng(S) -- get a random number by iterating the prng state
cyphertext[i] = string.char(bit.bxor(prn, string.byte(plaintext, i)))
end
return table.concat(cyphertext)
end
-- Decrypt works the same as encrypt:
-- plaintext = rc4:decrypt(key, cyphertext)
rc4.decrypt = rc4.encrypt
return rc4
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