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Hi, regarding 'XOR'

 
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PostPosted: Thu May 12, 2016 12:47 pm    Post subject: Hi, regarding 'XOR' Reply with quote
Hello, I'm having some problems understanding the xor instructions. I heard it can encrypt values, is that right? I can't seem to add on what it actually does. Let's say the game uses 50 as health, there's a xor instruction under it. Would I then not be able to search for it? How does this all work..? Thanks! Razz
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ParkourPenguin
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PostPosted: Thu May 12, 2016 2:58 pm    Post subject: Reply with quote
From wikipedia:
Quote:
A bitwise XOR takes two bit patterns of equal length and performs the logical exclusive OR operation on each pair of corresponding bits. The result in each position is 1 if only the first bit is 1 or only the second bit is 1, but will be 0 if both are 0 or both are 1. In this we perform the comparison of two bits, being 1 if the two bits are different, and 0 if they are the same. For example:
Code:
    0101 (decimal 5)
XOR 0011 (decimal 3)
  = 0110 (decimal 6)


This is sometimes used to encrypt values by using xor with a value and a constant number. For example, let's say your health is at 50 and you take 10 damage:
Code:
//get the value:
  mov edx,[esi+1C]    // edx == 3,007,978,837 (dec)
  xor edx,B34A1D67    // edx == 50 (dec)

//do something with it:
  sub edx,A          // edx == 40 (dec)

//store the value:
  xor edx,B34A1D67    // edx == 3,007,978,831 (dec)
  mov [esi+1C],edx

The value as it is stored in memory does not even closely resemble the value as it is treated by the game, and changes to said value in memory don't always reflect the changes the game actually makes to it. While this makes directly finding it more difficult, even beginners would still be able to easily figure it out if this is the only thing you do to encrypt a value.
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