From: Chris Hulan <chris.hulan@...>
Date: 2008-03-01T01:54:58+09:00
Subject: Re: problem writing inverse to java xor encoding function

On Feb 29, 10:56 am, Ben Summers <thequestionis...@hotmail.com> wrote:
> Hi,
> I'm trying to create an inverse to the following Java function in Ruby.
>
> public static void encXORPass(byte[] raw, final int offset, final int
> size, int key)
>   {
>     int stop = size-8;
>     int pos = 4 + offset;
>     int edx;
>     int ecx = key; // Initial xor key
>
>     while (pos < stop)
>     {
> // get 4 bytes from raw into an int
>       edx = (raw[pos] & 0xFF);
>       edx |= (raw[pos+1] & 0xFF) << 8;
>       edx |= (raw[pos+2] & 0xFF) << 16;
>       edx |=  (raw[pos+3] & 0xFF) << 24;
>
> // do things
>       ecx += edx;
>       edx ^= ecx;
>
> // put the xor'd int back into raw
>       raw[pos++] = (byte) (edx & 0xFF);
>       raw[pos++] = (byte) (edx >> 8 & 0xFF);
>       raw[pos++] = (byte) (edx >> 16 & 0xFF);
>       raw[pos++] = (byte) (edx >> 24 & 0xFF);
>     }
> // store the final key in the last 4 bytes of raw
>     raw[pos++] = (byte) (ecx & 0xFF);
>     raw[pos++] = (byte) (ecx >> 8 & 0xFF);
>     raw[pos++] = (byte) (ecx >> 16 & 0xFF);
>     raw[pos++] = (byte) (ecx >> 24 & 0xFF);
>
> }
>
> I'm basically walking through the same loop backwards, but for some
> reason my code doesn't currently completely decode the data. By
> "completely" I mean a few bytes into the loop it actually converges to
> the correct values, but not at the beginning. I've been trying to figure
> out why for weeks and I'm not making any progress so thought I'd post it
> here.
>
> Here is an example of a correct decoded sequence, followed by my
> incorrect decoded sequence (yes they are different if you go far enough
> to the right):
>
> 00854130CE21C60000BD7F26062B09315C26E24D80378DD9FB568AF57C765D9A2C90A6B0124EA36014881A48A185B44FC9A6D922D03E1F91FB0468819721E639E21AC617D44D3A7E952C2211EDB36ABCFC81B51E8AC205DCC750D7EA0C18F49CE8A119A8DA67591C97B5B7D6C9EF61F7F25E6EBDC10EA0BAA1F388D3210198B1A66B1E09437E3AA2204E95DD29FC9CC37720B6AD97F7E0BD0731C3725F3B6566FEC6F2CD5473468F2700E8436B286DACD9FE6C8B9F9EADDF
> 00854130CE21C60000BD7F26062B09315C26E24D80378DD9FB568AF57C765D9A2C90A6B0124EA36014881A48A185B44FC9A6D922D03E1F91FB0468819721E639E21AC617D44D3A7E952C2211EDB36ABCFC81B51E8AC205DCC750D7EA0C18F49CE8A119A8DA67591C97B5B7D6C9EF61F7F25E6EBDC10EA0BAA1F3883321019841A66B1EF1437E3ADE204E95C329FC1CCC772056AA97F71CBE07313C735FBB9A66FE06EDCD5493418F271FE8C3EBE86CEC41A6A4B69F9EADDF
>
> The relevant ruby code is attached,
> Any insight appreciated,
> Cheers.
>
> Attachments:http://www.ruby-forum.com/attachment/1502/xor.rb
>
> --
> Posted viahttp://www.ruby-forum.com/.

FYI
XOR(XOR(x)) == x

so to decode just run the encoded string through the xor

Cheers