TheImpostor.md

rhme-2016 write-up The Impostor

The Impostor (Reverse - 300 pts)

This binary ask for a 16 bytes password.
First, the FLAG string is load in the function @ 0x1300.
Starting the reverse from 0x1300:
    -> 0xed0 (nothing interesting)
    -> 0xeb2 (nothing interesting)
    -> 0xa32 (!)
The jump from 0xa32 is part of a big jump table which is called from an address incremented by 2 at the begining of the function.
Reversing this function, we can see that the base address is 0x0068.
Looking at this address, the code doesn't look like an AVR code (until 0x05d4).
With the help of radare2 and rasm2, we tried to desassemble this block with differents architectures until found something interesting (arm thumb).
With the help of the Hopper decompiler, we discovered a XTEA algorithm used in order to encrypt 16 bytes.
We just need to reverse this function with the following C code:

#include <stdio.h>
#include <stdint.h>

void tea_decrypt (uint32_t* buff, uint32_t* keys)
{
	uint32_t delta =  0xAB64E218;
	uint32_t sum = delta * 32;

	for (int i = 0; i < 32; i++) {
		buff[1] -= (((buff[0] << 4)^(buff[0] >> 5)) + buff[0]) ^ (sum + keys[(sum >> 11) & 3]);
		sum -= delta;
		buff[0] -= (((buff[1] << 4)^(buff[1] >> 5)) + buff[1]) ^ (sum + keys[sum & 3]);
	}
}

void main()
{
	uint32_t key[]  = {0x373D3943, 0x49A1C621, 0x80C6B0, 0x3C93C7B};
	uint32_t buff[] = {0xFC791D6B, 0x924E6C8F, 0x795F34A2, 0xEDAE901, 0};

	tea_decrypt (buff, key);
	tea_decrypt (&buff[2], key);
	printf("%s\n", buff);
}

The code was: 4rM_c0rT3xM0_4vR