picoCTF 2022 | substitution0, substitution1, and substitution2 Write-up

Challenge description


A message has come in but it seems to be all scrambled. Luckily it seems to have the key at the beginning. Can you crack this substitution cipher? Download the message here.


A second message has come in the mail, and it seems almost identical to the first one. Maybe the same thing will work again. Download the message here.


It seems that another encrypted message has been intercepted. The encryptor seems to have learned their lesson though and now there isn’t any punctuation! Can you still crack the cipher? Download the message here.

Category: Cryptography


These 3 challenges have very similar solutions, so I decided to group them up together. For all of them, I used this site. Just paste in the ciphertext, click “Break Cipher” and let it do its magic.

The flag for substitution0


In substitution1, the flag that the tool gave me was actually slightly wrong, which I found out after trying to submit it:

Looking through the decoded result again, I decided to try changing the “J” to a “Q” so that it spells out “Frequency”. Submitting that seems to work, but I’m not sure if the problem was with the challenge or with the online tool, or if this was intentional.

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