challenge info  
Name: The Indecipherable Cipher
Category: Cryptography
Difficulty: Medium
Points: 300



Our crypto specialist Mr.Kasiski is currently unavailable, so help us decode this string➡️j3qrh4kgz3iptmyqxcw0zkm8i5xugs5lwl0lrwvirwktlqinexcw0zkmq5nqvpebpor5wqipqhw2ikzm4ipktzlr


The first step I took was to identify the encryption type. To achieve this, I used Cipher Identifier which provided me with a list of possible encryptions

list of possible ciphers


At first, I attempted to use base32, but it didn’t work. So, I tried the second-best option, the Vigenere cipher. Initially, it didn’t give me the flag, but then I noticed that the ciphertext also included numbers. So I decided to include numbers in the alphabet set.

alphabet parameter


which resulted in this



I thought that the text was still random, but the word “Friedrich” caught my attention. Therefore, I proceeded to separate each word in order to understand it.

“Friedrich Wilhelm Kasiski was the person who designed the AAA Kasiski Examination to decode Vigenere cipher.”

However, the flag that was accepted was actually the original plaintext.


Flag: VishwaCTF{friedrichwilhelmkasiskiwastheonewhodesignedtheaaakasiskiexaminationtodecodevignerecipher}