RC4

-- ARC4
-- By KillaVanilla
-- Hey, go figure! I actually *was* able to write this on my own!

-- Best practice with RC4 (and stream ciphers in general, IIRC) is to combine your key and nonce (concentation works), then hash the combined result, and use that as the key.

local rc4_object = {
    s = {},
    i = 0,
    j = 0,
    key_schedule = function(self, key)
        for i=0, 255 do
            self.s[i] = i
        end
        self.j = 0
        for i=0, 255 do
            self.j = (self.j + self.s[i] + key[(i % #key)+1]) % 256
            local temp = self.s[i]
            self.s[i] = self.s[self.j]
            self.s[self.j] = temp
        end
    end,
    set_key = function(self, key)
        if type(key) == "string" then
            if #key == 0 then
                error("rc4-set_key: Invalid key length!", 2)
            end
            if #key > 256 then
                self:key_schedule({string.byte(key, 1, 256)})
            else
                self:key_schedule({string.byte(key, 1, #key)})
            end
        elseif type(key) == "table" then
            local filteredKey = {}
            for i=1, #key do
                if (type(key[i]) == "number") and ((key[i] >= 0) and (key[i] <= 255)) then
                    table.insert(filteredKey, key[i])
                    if #filteredKey >= 256 then
                        break
                    end
                end
            end
            if #filteredKey == 0 then
                error("rc4-set_key: Invalid key length!", 2)
            end
            self:key_schedule(filteredKey)
        end
    end,
    generate_byte = function(self)
        self.i = (self.i + 1) % 256
        self.j = (self.j+self.s[self.i]) % 256
        local temp = self.s[self.i]
        self.s[self.i] = self.s[self.j]
        self.s[self.j] = temp
        local k = self.s[(self.s[self.i] + self.s[self.j]) % 256]
        return k
    end,
    discard_bytes = function(self, n)
        for i=1, n do
            self:generate_byte()
        end
    end,
    crypt_array = function(self, iArray)
        local oArray = {}
        local lastPause = os.clock()
        for ctr=1, #iArray do
            local keystream = self:generate_byte()
            oArray[ctr] = bit.bxor(keystream, iArray[ctr])
            if (os.clock() - lastPause) >= 2.90 then
                os.queueEvent("")
                os.pullEvent("")
                lastPause = os.clock()
            end
        end
        return oArray
    end,
    crypt_string = function(self, iStr)
        local oStr = ""
        local oArray = {}
        local lastPause = os.clock()
        for ctr=1, #iStr do
            local keystream = self:generate_byte()
            oArray[ctr] = bit.bxor(keystream, string.byte(iStr, ctr, ctr))
            oStr = oStr..string.char(oArray[ctr])
            if (os.clock() - lastPause) >= 2.90 then
                os.queueEvent("")
                os.pullEvent("")
                lastPause = os.clock()
            end
        end
        return oStr, oArray
    end,
}

function new_rc4_object(key)
    local cxt = {}
    for i,v in pairs(rc4_object) do
        cxt[i] = v
    end
    cxt:set_key(key)
    cxt:discard_bytes(3072)
    return cxt
end

function encrypt(data, key)
    local object = new_rc4_object(key)
    if type(data) == "string" then
        return object:crypt_string(data)
    elseif type(data) == "table" then
        return object:crypt_array(data)
    end
end