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[PicoCTF 2018] - crypto - Blaise's Cipher

This is one of my writeups for PicoCTF 2018

Problem

My buddy Blaise told me he learned about this cool cipher invented by a guy also named Blaise! Can you figure out what it says? Connect with nc 2018shell3.picoctf.com 59396.

Hints :

  1. There are tools that make this easy.
  2. This cipher was NOT invented by Pascal

Solution

This task, like the first warmup task, involves deciphering text encrypted with the Vigenere Cipher - the cipher being indicated by the hints. But this time we don't have the key.

The Wikipedia page gives us the method : it consist of guessing they key length by observing repetitions in the ciphertext, then solving n caesar ciphers (where n is the length of the key).

I used an online tool to do that. The key is flag and the flag is in the middle of the plaintext : picoCTF{v1gn3r3_c1ph3rs_ar3n7_bad_901e13a1}