One_Bite

One Bite

Rev

Challenge

Whenever I have friends over, I love to brag about things that I can eat in a single bite. Can you give this program a tasty flag that fits the bill?

/problems/2019/one_bite

Author: SirIan

Solution

Decompile in Hopper

int main(int arg0, int arg1) {
    puts("Give me a flag to eat: ");
    fgets(&var_40, 0x22, *__TMC_END__);
    var_4C = 0x0;
    while (sign_extend_64(var_4C) < strlen(&var_40)) {
            *(int8_t *)(rbp + sign_extend_32(var_4C) + 0xffffffffffffffc0) = *(int8_t *)(rbp + sign_extend_32(var_4C) + 0xffffffffffffffc0) & 0xff ^ 0x3c;
            var_4C = var_4C + 0x1;
    }
    if (strcmp(&var_40, "]_HZGUcHTURWcUQc[SUR[cHSc^YcOU_WA") == 0x0) {
            puts("Yum, that was a tasty flag.");
    }
    else {
            puts("That didn't taste so good :(");
    }
    rax = 0x0;
    rcx = *0x28 ^ *0x28;
    if (rcx != 0x0) {
            rax = __stack_chk_fail();
    }
    return rax;
}

From the code,

Flag goes through XOR cipher of 0x3c and then gets compared to "]_HZGUcHTURWcUQc[SUR[cHSc^YcOU_WA"

Decrypt it

$ python3

>>> enc = b"]_HZGUcHTURWcUQc[SUR[cHSc^YcOU_WA"

>>> dec = [chr(my_byte ^ 0x3c) for my_byte in enc]

>>> ''.join(dec)
'actf{i_think_im_going_to_be_sick}'

Flag

actf{i_think_im_going_to_be_sick}