Question

I am currently trying to write a decryption routine, and one of the steps in the encryption routine is this:

xor eax,edx  // xors eax(current character) and edx (the key)

Obviously I will know the key so any ideas how I can reverse this operation?

Answer 1

XOR is its own inverse. (Bitwise: XOR’ing with 0 will not change a bit. XOR’ing with 1 will flip it, and doing it again will flip it back.)

Just XOR the “encrypted” value and the key again:

encrypted = plain XOR key
encrypted XOR key = (plain XOR key) XOR key
encrypted XOR key = plain XOR (key XOR key)
encrypted XOR key = plain XOR (000...0)
encrypted XOR key = plain