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[PicoCTF 2018] - crypto - Blaise's Cipher

This is one of my writeups for PicoCTF 2018


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 59396.

Hints :

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


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}