Porting a substitution cipher in Lua to Python

Question

I am porting a substitution cipher function which unscrambles (decodes) a given string in Lua.

function unscramble(str)
  local res = ""
  local dtable = "7,Jsg(E<\fIvBT@3_{|k\005Ww0#P\000\015\031rmG]~\030}\"xut\017X\004\016\006`+\t\001)l*\aq%ULh.6 \b\025;OQ\003\\\002\029ZN\0235\014[$e1K\027d\v4Y!^\rVi8fMc'>b:RjHA-CznS\021\028a\026\022F9o\n\018\019?yp\020=/&D2\024"
  for i = 1, #str do
    local b = str:byte(i)
    if b > 0 and b <= 127 then
      res = res .. string.char(dtable:byte(b))
    else
      res = res .. string.char(b)
    end
  end
  return res
end

The idea seems pretty straightforward - to replace all ASCII symbols with their positional counterparts in dtable variable.

I used https://www.tutorialspoint.com/execute_lua_online.php to verify

print(unscramble("dM22r"))

Hello

My best shot at it in Python is

def unscramble(str):
    res = ''
    dtable = b"7,Jsg(E<\fIvBT@3_{|k\005Ww0#P\000\015\031rmG]~\030}\"xut\017X\004\016\006`+\t\001)l*\aq%ULh.6 \b\025;OQ\003\\\002\029ZN\0235\014[$e1K\027d\v4Y!^\rVi8fMc'>b:RjHA-CznS\021\028a\026\022F9o\n\018\019?yp\020=/&D2\024"
    for b in str:
        res += chr(dtable[b-1]) if 0 < b < 128 else chr(b)
    return res

and the result

print(unscramble(b"dM22r"))

j$llF

Well, it does not work. The problem is that I don't know Lua at all and am just making educated guesses below based on general knowledge. In this specific case the interpretation of bytestrings seems to be off.

Could somebody point me in the right direction?

Answer 1

The string literal "\123" is not parsed the same way by both languages. Python interprets the escape sequence as the character with an octal value of 123 (decimal 83), but Lua treats it as the character with decimal value 123. You need to properly encode the bytes:

bytes([
    0x37, 0x2c, 0x4a, 0x73, 0x67, 0x28, 0x45, 0x3c, 0x0c, 0x49, 0x76, 0x42, 0x54, 0x40, 0x33, 0x5f,
    0x7b, 0x7c, 0x6b, 0x05, 0x57, 0x77, 0x30, 0x23, 0x50, 0x00, 0x0f, 0x1f, 0x72, 0x6d, 0x47, 0x5d,
    0x7e, 0x1e, 0x7d, 0x22, 0x78, 0x75, 0x74, 0x11, 0x58, 0x04, 0x10, 0x06, 0x60, 0x2b, 0x09, 0x01,
    0x29, 0x6c, 0x2a, 0x07, 0x71, 0x25, 0x55, 0x4c, 0x68, 0x2e, 0x36, 0x20, 0x08, 0x19, 0x3b, 0x4f,
    0x51, 0x03, 0x5c, 0x02, 0x1d, 0x5a, 0x4e, 0x17, 0x35, 0x0e, 0x5b, 0x24, 0x65, 0x31, 0x4b, 0x1b,
    0x64, 0x0b, 0x34, 0x59, 0x21, 0x5e, 0x0d, 0x56, 0x69, 0x38, 0x66, 0x4d, 0x63, 0x27, 0x3e, 0x62,
    0x3a, 0x52, 0x6a, 0x48, 0x41, 0x2d, 0x43, 0x7a, 0x6e, 0x53, 0x15, 0x1c, 0x61, 0x1a, 0x16, 0x46,
    0x39, 0x6f, 0x0a, 0x12, 0x13, 0x3f, 0x79, 0x70, 0x14, 0x3d, 0x2f, 0x26, 0x44, 0x32, 0x18
])

Or:

b'7,Jsg(E<\x0cIvBT@3_{|k\x05Ww0#P\x00\x0f\x1frmG]~\x1e}"xut\x11X\x04\x10\x06`+\t\x01)l*\x07q%ULh.6 \x08\x19;OQ\x03\\\x02\x1dZN\x175\x0e[$e1K\x1bd\x0b4Y!^\rVi8fMc\'>b:RjHA-CznS\x15\x1ca\x1a\x16F9o\n\x12\x13?yp\x14=/&D2\x18'