One Bite Rev 60
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
A binary file is also provided.
After decompiling the code with Ghidra and cleaning it up a little:
undefined8 main(void)
{
int test;
size_t break_condition;
long in_FS_OFFSET;
int counter;
byte input[40];
long local_20;
local_20 = *(long *)(in_FS_OFFSET + 0x28);
puts("Give me a flag to eat: ");
fgets((char *)input, 0x22, stdin);
counter = 0;
while (true)
{
break_condition = strlen(input);
if ( break_condition <= counter)
break;
input[counter] = input[counter] ^ 0x3c;
counter = counter + 1;
}
test = strcmp(input, "]_HZGUcHTURWcUQc[SUR[cHSc^YcOU_WA");
if ( test == 0)
{
puts("Yum, that was a tasty flag.");
}
else
{
puts("That didn\'t taste so good :(");
}
if (local_20 != *(long *)(in_FS_OFFSET + 0x28))
{
/* WARNING: Subroutine does not return */
__stack_chk_fail();
}
return 0;
}The user input gets XORed byte-wise with 0x3c and compared against a string ]_HZGUcHTURWcUQc[SUR[cHSc^YcOU_WA.
The inverse of XOR is XOR, so using 0x3c to reverse the string should yield the flag.
This is a great utility for operations like this: https://gchq.github.io/CyberChef/
The XOR returns the flag:
Key = 3c: actf{i_think_im_going_to_be_sick}