Keygen

Reverse Engineering - Points: 600

Can you figure out what the key to this program is?

keygen-1

When going though the binary with Ghidra, we can see that the key is 15 characters long and has the flag format, so the first bytes and the last one are fixed. We need to find the 8 bytes between the brackets and since there is a function that checks whether a byte is digit, upper- or lowercase we can assume that these are the possible characters. The following angr script tries to find the correct key with consideration of the restrictions by searching for a path to the address 0x0804936f where Key valid! is printed out and avoiding all the addresses of error cases.

import angr
import claripy
import logging


logging.getLogger('angr.sim_manager').setLevel(logging.DEBUG)

proj = angr.Project("./keygen-1")

argv1 = claripy.BVS('argv1', 0xf*8)

state = proj.factory.entry_state(args=["./keygen-1", argv1])

state.solver.add(argv1.get_byte(0) == 0x6e)
state.solver.add(argv1.get_byte(1) == 0x61)
state.solver.add(argv1.get_byte(2) == 0x63)
state.solver.add(argv1.get_byte(3) == 0x74)
state.solver.add(argv1.get_byte(4) == 0x66)
state.solver.add(argv1.get_byte(5) == 0x7b)
state.solver.add(argv1.get_byte(14) == 0x7d)

for i in range(6,14):
    digit = state.solver.And(argv1.get_byte(i) >= 0x30, argv1.get_byte(i) <= 0x39)
    lower = state.solver.And(argv1.get_byte(i) >= 0x41, argv1.get_byte(i) <= 0x5a)
    upper = state.solver.And(argv1.get_byte(i) >= 0x61, argv1.get_byte(i) <= 0x7a)
    state.solver.add(state.solver.Or(digit, lower, upper))

sm = proj.factory.simulation_manager(state)
sm.explore(find=(0x0804936f), avoid=(0x0804938, 0x080492d8, 0x080492c4, 0x080492a5))

found = sm.found[0]
solution = found.solver.eval(argv1, cast_to=bytes)
print(repr(solution))

flag: nactf{GEZhxWsw}