Description

This CTF reverse engineering challenge template creates challenges if the following format:

$ ./challenge
usage: ./challenge guess
$ ./challenge asdf
Incorrect
$ ./challenge <correct flag>
Correct: <correct flag>

The flag is encoded using the encode(...) function in ./src/encode.c. The flag is stored pre-encoded in the challenge binary. (So, the plain-text flag doesn't exist in the binary) When running ./challenge, the user input is encoded in the same way (using encode(...)). The encoded user input (guess) is then compared against the encoded flag. If these are equal, the Correct: message is printed, otherwise Incorrect. NOTE: You have to ensure that the only user input that can be encoded to the encoded flag, is the flag itself.

Creating a Challenge

STEP 1

In file: ./src/encode.c Change the line: unsigned int encoded_guess_len = guess_len; /* CHANGE THIS */ to calculate the length of your encoded guess. It's also fine to keep to keep this line as it is

STEP 2

Now, you need to perform the encoding. The default loop:

for (i = 0; i < guess_len; i++) {
    encoded_guess[i] = guess[i];
}

does not encode the guess. It populates the bytes of encoded_guess with the bytes of guess. Afterwards, encoded_guess will be the same as guess. (I.e. it copies guess to encoded_guess)

You should change the loop such that every byte encoded_guess[i] is a transformation of one (or more) bytes of guess. See the ./examples/ for insipiration.

STEP 3

Create the challenge! Run bash ./create_challenge.sh <flag> (Replace <flag> with your flag) First, this will compile the program ./driver_encode. This is a separate program that will pre-encode your flag. Second, it will compile your ./challenge program with your encode(...) function while adding the pre-encoded flag.

STEP 4

The ./challenge binary contains your challenge. Send it to someone!

Exercise:

Update ./exercise/encode_exercise.c to implement ROT13. Any alphabet character should be offset with 13:

A->N, B->O, ..., Z->M
a->n, b->o, ..., z->m

Any non-alphabet character should not be changed:

1->1, 2->2,
.->., _->_

Then, build the exercise: bash create_exercise.sh This will compile the binary ./check_exercise with your modified ./exercise/encode_exercise.c You can run ./check_exercise to see if you implemented it correctly.