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22 - Santa's UNO flag decrypt0r

Description

Level: Leet
Author: explo1t

The elves made Santa a fancy present for this Christmas season. He received a fancy new Arduino where his elves encoded a little secret for him. However, Santa is super stressed out at the moment, as the children's presents have to be sent out soon. Hence, he forgot the login, the elves told him earlier. Can you help Santa recover the login and retrieve the secret the elves sent him?

Solution

For this challenge we are given an AVR binary unoflagdecryptor.elf and from the challenge name we can guess that it's made for an Arduino Uno. I opened the binary in Ghidra to identify the interesting parts:

correct = 0;

do {
  // Some black magic
  ...

  R20 = R20 ^ R25R24;

  if (R20 == R5R4) {
    correct++;
  }

} while (...);

if (correct == 0x21) {
  // Successful login
  ...
}

We can see that the password is checked character by character in a loop. If all 0x21 characters are correct, then the flag gets decrypted and printed. All we have to do now, is to open the binary in a debugger and check the comparison of R20 and R5R4.

To debug I use qemu and avr-gdb which I run with these two commands:

qemu-system-avr -S -s -nographic -machine arduino-uno -bios main.elf
avr-gdb -ex 'target remote :1234' main.elf

I set a breakpoint at the relevant instructioins:

 0x00000648 <+428>:   eor     r20, r24
 0x0000064a <+430>:   cpse    r20, r5
 0x0000064c <+432>:   rjmp    .+2             ;  0x650 <main+436>

The first instruction is the XOR and after that the comparison happens. I set a breakpoint at 0x00000648 and check the content of r20 and r24:

(gdb) info registers r20
r20            0x73                115
(gdb) info registers r24
r24            0xf3                243

The first one contains our input (the ascii character 's'). If we step one instruction further and inspect r5, we see the value that it's compared to:

(gdb) info registers r5
r5             0x80                128

It turns out that the content of r5 can be found directly in the .data segment: 80d7463c677b95516e676690359bd75a2c6571986b665773875997cc0969277bd5. At this point, I simply stepped through the loop and extracted the values of r24 during the xor to obtain f3b628480641fc0e020810f56af3b628480641fc0e020810f56af3b628480641fc. Then I simply xored the two together to get the password santa:i_love_hardc0ded_cr3dz!!!:). Entering this gave the flag HV22{n1c3_r3v3r51n6_5k1llz_u_g07}.