Introduction

In this challenge, we had to write a quine in Python. A quine is a program that prints its own source code. We also got the source code of the program that will run our quine.

Investigation

After looking into the code, we can see these restrictions:

assert x == "<END>", "what did you do this time."
assert all(ord(i) < 128 for i in program), "i don't speak foreign languages."
assert all(
    i not in program for i in ['"', "'", "_"]
), "who uses strings anyway? it's not like quines require strings."
assert program != "", "scuse me, just cleaning out the garbage."
assert "pyquinejailgolf" in program, "are you sure you wrote this program yourself?"
assert len(program) == 343

Our program can’t use ", ', _ (basically strings are not allowed) and it has to be exactly 343 characters long. We also have to include the string pyquinejailgolf in our program.

We also see that the program runs our code using exec and modifies the __builtins__ dictionary to prevent us from using some built-in functions. After doing some research, we found out that we can still use the __import__ function to import modules.

Solution

The runtime saves our program in the program variable in runtime/external_run.py file. That means we can easily access the source code of our program.

with open('runtime/output.txt', 'w+') as __import__('sys').stdout:
    program = 'OUR CODE'
    safe_builtins = {}
    for i in dir(__builtins__):
        if i[0] not in __import__('string').ascii_lowercase:
            safe_builtins[i] = eval(i)
    safe_builtins['print'] = print
    new_builtins = {'__builtins__':safe_builtins}
    try:exec(program, new_builtins, new_builtins)
    except Exception as e:print(e)

We can use the following code to create a quine:

import sys;a=sys.argv[0];import io;sys.stdout.write(io.open(a).read()[83:426][::-1])#pyquinejailgolf

We’re using sys.stdout.write instead of print because print puts a newline character at the end of the string.
We’re using io.open instead of open because open is a built-in function and we can’t use it (not in the __builtins__ dictionary).
We’re using sys.argv[0] to get the name of the file that is being executed.

The code reads its own source code, removes the first 83 characters (which are not part of the program variable) and the last 426 characters (which are not part of the program variable) and then reverses the string. The last part of the code is the pyquinejailgolf string that is required by the challenge.

Since we need 434 characters, we just add a character at the end, e. g #

Our final program will look like this:

import sys;a=sys.argv[0];import io;sys.stdout.write(io.open(a).read()[83:426][::-1])#pyquinejailgolf###################################################################################################################################################################################################################################################

And that’s it! We can now paste the code to the server and get the flag.

amateursCTF{what_is_a_quine_is_that_like_a_reversed_reverse_quine?}