PicoCTF 2022 — Reverse Engineering: Keygenme

Description

Can you get the flag?

Solve

Running through the code in ghidra I was able to locate the comparison its doing with the provided user key and the expected characteristics about the key comparison.

The key is expected to be 36 chars long

The key starts with picoCTF{br1ng_y0ur_0wn_k3y_ and ends with } leaving 8 bytes to work out

We can target the strlen function to get close to our comparison by opening the binary with GDB and setting a breakpoint for strlen

break strlen

Running the application now with a dummy key filling the blank space with AAAAAAAA will help us to process as the hex value of A is 0x41.

r <<<picoCTF{br1ng_y0ur_0wn_k3y_AAAAAAAA}

Continuing forward (c) until the provided key is present in a register will jump over the parts before the section we care about.

Once the dummy key is in a register we are in the function that checks the user provided key. There will be 3 calls to this function so continuing (c) another 2 times will jump over the non important ones used for the MD5 calculations.

Once at the call to strlen we care about we can call step (s) to step over the strlen function as we dont want to go into it. From this point we are going to use only "step into" (si) to get the remainder of the key.

Stepping in continually, we can see that the for loop is taking each char and moving it to register rdx for comparison.

Continuing to step in we see that the expected key is loaded into register rax before a comparison is completed.

Following this process up until out section of 8 bytes we then pause at the cmp function and see that the values of rax and rdx are different. This is because our key at rdx is incorrect.

We are able to change the value of rdx to the value expected and document the value of rax somewhere for later use.

set $rdx=$rax

Then, stepping in again (si) shows that the jump condition is true as a result of rax and rdx being equal.

This process is repeated for the remaining 7 unknown bytes until the remainder of the key is resolved.

With the string of saved hex values we can produce the working key

key = '3362373063613165'
key = pwn.unhex(key).decode()
key = 'picoCTF{br1ng_y0ur_0wn_k3y_' + key + '}'
print(key)

This key can then be passed to the binary to confirm it works.

$ ./keygenme 
Enter your license key: picoCTF{br1ng_y0ur_0wn_k3y_3b70ca1e}
That key is valid.

Flag

The key is the flag: picoCTF{br1ng_y0ur_0wn_k3y_3b70ca1e}