3DSCTF 2016 : crypto100-cryptomaster

John says that he is the master of his personal server. He created a script that talks to him as if it was his disciple. The problem is that in order to access the server, one needs to know the logic used by the script. Access the server and get the flag.



Connected to server through nc 8002

.. he is asking me to prove that I am his master, typed "a" and he kickd me out..

Ok, saw that there is an logic for letter substitution, but let's solve this step by the quickest way.. bruteforce!


The 1st answer is Q, Q=I, now let's decrypt the cryptd sentence: QYMFRUMYGFUHKTWQJRUHG

What we know?

Create a alphadigits table like this and complete by substitution until figure out the sentence: