suri-xor-non-zero.lua

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This lua script can be run standalone and verbosely on a binary file with
echo "run()" | lua -i <script name> <binary file>

Chris Wakelin
--]]

function init (args)
    local needs = {}
    needs["http.response_body"] = tostring(true)
    return needs
end

function xor0(byte, key)
   local bit = require("bit")
   if byte == key or byte == 0 then
       return byte
   end
   return bit.bxor(byte,key)
end

-- return match via table
function common(a,verbose)
    local result = {}
    local bit = require("bit")

    if #a < 1024 then 
        return 0
    end

-- Check for XOR with 0 and XOR-key bytes left alone
-- PE offset is (nearly?) always divisible by 8, so key lengths 1,2,4 will always be detected
-- Can match other key lengths in some cases where the remainder on dividing into 0x3c is 0,1,2 or 4
-- and on dividing into the PE offset is 0 or 1 (or 4 when the key length is 5)
    key = {xor0(a:byte(1), string.byte('M')), xor0(a:byte(2), string.byte('Z')), xor0(a:byte(3), 0x90), 0, xor0(a:byte(5), 0x03), 0, 0, 0}

    key_lengths = {1,3,5,6,7,8}
    for n,l in pairs(key_lengths) do
      
        koffset = 0x3c % l
        pe = xor0(a:byte(0x3c+1),key[1+koffset]) + (256*xor0(a:byte(0x3c+2),key[((1+koffset) % l) + 1]))
        if verbose==1 then print("Trying PE header at " .. pe) end

        koffset = pe % l
        if ((pe < 4096) and (pe < #a-4)) then
            if xor0(a:byte(pe+1),key[1+koffset]) == string.byte('P') and
               xor0(a:byte(pe+2),key[((1+koffset) % l) + 1]) == string.byte('E') and
               a:byte(pe+3) == 0 and
               a:byte(pe+4) == 0 then
                if verbose==1 then print("Found " .. l .. "-byte XOR-but-not-zero key " .. key[1] .. "," .. key[2] .. " - PE block at " .. pe) end
                return 1
            end
        end
    end

    return 0
end

-- return match via table
function match(args)
    local t = tostring(args["http.response_body"])
    return common(t,0)
end

function run()
  local f = io.open(arg[1])
  local t = f:read("*all")
  f:close()
  common(t,1)
end