October 2018
Beginner to intermediate
436 pages
9h 36m
English
Content preview from Mastering Reverse Engineering
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Loop codes
Decryption can easily be identified by inspecting code that runs in a loop:
mov ecx, 0x10 mov esi, 0x00402000loc_00401000: mov al, [esi] sub al, 0x20 mov [esi], al inc esi dec ecx jnz loc_00401000
This loop code is controlled by a conditional jump. To identify a decryption or an encryption code, it should have a source and a destination. In this code, the source starts at address 0x00402000, with the destination also at the same address. Each byte in the data is modified by an algorithm. In this case, the algorithm is a simple subtraction of 0x20 from the byte being changed. The loop ends only when 0x10 bytes of data have been modified. 0x20 is identified as the encryption/decryption key.
The algorithm can vary, using standard ...
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