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simplest-validation (Reverse Engineering)

Introduction​

This is the first app I've ever cracked when I started learning reverse engineering.

The simple app is written by myself to simulate a super simple serial validation process.

  1. The script for validation is here.
  2. Run make to build the app.
  3. A validate executable should be generated.
  4. It requires no argument, and the is_validate function will always return false.
  5. Our goal is to make the program print "Activated".

Live Example​

Solution (amd64)​

Open the executable with hopper

image-20220410024636219

image-20220410024917528

EAX and RAX are similar. RAX is the 64-bit version of EAX. They are the Accumulator Register.

EAX is the Lower 32-bit of RAX, so changing the rightmost bit of RAX also changes EAX from 0x0 to 0x1.

This process locates a position where I can crack the app. I just need to set EAX to 0x1 right before ret.

image-20220410030213774

After the comparison cmp the code reaches the assembly mov eax, 0x0, let's change it to mov eat, 0x1 using Modify->Assemble Instruction.

Then produce a new executable with File->Produce New Executable...

Make the generated file executable with chmod and run it. "activated should be printed."