Problem writing inverse for XOR encoding loop

Question

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
00854130CE21C60000BD7F26062B09315C26E24D80378DD9FB568AF57C765D9A2C90A6B0124EA36014881A48A185B44FC9A6D922D03E1F91FB0468819721E639E21AC617D44D3A7E952C2211EDB36ABCFC81B51E8AC205DCC750D7EA0C18F49CE8A119A8DA67591C97B5B7D6C9EF61F7F25E6EBDC10EA0BAA1F3883321019841A66B1EF1437E3ADE204E95C329FC1CCC772056AA97F71CBE07313C735FBB9A66FE06EDCD5493418F271FE8C3EBE86CEC41A6A4B69F9EADDFAny insight appreciated,
Cheers.

Answer 1

bensum wrote:
Sorry, I didn't realise posting in this forum and any other forum were mutually exclusive events. Oh, you've posted in both? So people can waste their time repeating what others have already said. Nice.
I haven't posted my code because it's Ruby like you say and I wouldn't expect people here to know or correct Ruby.Then why post here at all? If your code doesn't work, it's a bug in your code. It's rather hard to correct it without seeing the code. If you want help here, you should try to write the code in Java. If it works, great, translate it to Ruby, and if that doesn't work, post in the Ruby forum. If you can't get the Java version to work, post here with details.
It seems to me that the problem is more about a property of this algorithm that I don't know about than my particular code (because it converges far down to the correct values). I don't see how it could converge far down to the correct values, without being correct at the beginning, that's the problem. Anybody any ideas?I have an idea that there's a bug in your code and that the best way to diagnose it is to see the code. :-)