CRYPTOHACK

RSA or HMAC?

Help

This page offers a convenient way for you to interact with the "RSA or HMAC?" challenge functions. You can also use GET requests to send and receive data directly from the listed routes/endpoints if you wish. For more information see the FAQ.

Your aim is to recover the FLAG value. Once you have have it, submit it on the Crypto On The Web challenge page.

Source

import jwt # note this is the PyJWT module (version 1.5.0), not python-jwt


# Key generated using: openssl genrsa -out rsa-or-hmac-private.pem 2048
with open('challenge_files/rsa-or-hmac-private.pem', 'rb') as f:
   PRIVATE_KEY = f.read()
with open('challenge_files/rsa-or-hmac-public.pem', 'rb') as f:
   PUBLIC_KEY = f.read()

FLAG = ?


@chal.route('/rsa-or-hmac/authorise/<token>/')
def authorise(token):
    try:
        decoded = jwt.decode(token, PUBLIC_KEY)
    except Exception as e:
        return {"error": str(e)}

    if "admin" in decoded and decoded["admin"]:
        return {"response": f"Welcome admin, here is your flag: {FLAG}"}
    elif "username" in decoded:
        return {"response": f"Welcome {decoded['username']}"}
    else:
        return {"error": "There is something wrong with your session, goodbye"}


@chal.route('/rsa-or-hmac/create_session/<username>/')
def create_session(username):
    encoded = jwt.encode({'username': username, 'admin': False}, PRIVATE_KEY, algorithm='RS256')
    return {"session": encoded.decode()}


@chal.route('/rsa-or-hmac/get_pubkey/')
def get_pubkey():
    return {"pubkey": PUBLIC_KEY}


Interact

authorise(token)
String Input Only
create_session(username)
String Input Only
get_pubkey()
Output