Created
June 25, 2021 19:58
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This is an alternative way to solve the X MAS CTF 2019 challenge Discount VMProtect by instrumenting the binary with the help of intel pintools
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| import os | |
| import string | |
| START_CHAR = "|" | |
| POSSIBLE_CHARS = "|" + string.digits + "}{_-" + string.ascii_uppercase + string.ascii_lowercase | |
| def get_count(cmd): | |
| pipe = os.popen(cmd) | |
| pipe.readline() | |
| count = pipe.readline().split()[1] | |
| return int(count, 0) | |
| # Uncomment for Debug statements | |
| def main(): | |
| cmd = f'echo "{START_CHAR}" | ../../../pin -t obj-intel64/inscount0.so -- ~/VM' | |
| count_prev = get_count(cmd) | |
| flag = "" | |
| j = 0 | |
| while True: | |
| if "X-MAS{" in flag and "}" in flag: | |
| break | |
| i = POSSIBLE_CHARS[j] | |
| j+=1 | |
| cmd = f'echo "{flag}{i}" | ../../../pin -t obj-intel64/inscount0.so -- ~/VM' | |
| if i == "|": | |
| count_prev = get_count(cmd) | |
| #print(f"[*] Trying char: {flag}{i} | Instruction Count: {count_prev}") | |
| continue | |
| count = get_count(cmd) | |
| #print(f"[*] Trying char: {flag}{i} | Instruction Count: {count}") | |
| if count == count_prev: | |
| continue | |
| elif count != count_prev and 50 < abs(count-count_prev): | |
| flag += i | |
| #print(f"[!] Flag character found!") | |
| print(f"[!] Flag: {flag}\n") | |
| j = 0 | |
| print(f"[+] Final Flag: {flag}") | |
| if __name__ == '__main__': | |
| main() |
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