Prompt

Our analysts have obtained an encrypted message. We know that the key, qizkwcgqbs was used. See if you can crack them.

Walk-Through

This challenge will give you experience decoding polyalphabetic substitution ciphers. This message is encrypted using the Vigenère cipher.

To start this challenge, take a look at the ciphertext. There is clear formatting for something resembling the flag format, however, it does not start with ‘SKY’. This is a clue that the encryption method does not move the letters like the shift or transposition ciphers used in previous challenges. There is also a key: qizkwcgqbs. A Vigenère cipher, named for cryptographer Blaise de Vigenère, uses a key and encrypts letters in place as observed.

A table, like the one from dcode.com below, is used to encrypt and decrypt the message in conjunction with the key.

To decrypt, find the row on the left side with the first letter of the key. This challenge uses ‘Q’ as the first letter in the key. In row ‘Q’, scan across the table until you find the first letter of the ciphertext ‘Y’. Letter ‘Y’ is in the column of letter ‘I’ so the first letter of the plaintext message starts with ‘I’. Use the next letter of the key to find the next plaintext letter of the message.

The key has fewer letters than the message. Once all of the letters of the key have been used, keep using the key over and over until all of the ciphertext letters have been decrypted.

You can also use an online tools to help you decode a Vigenère cipher.

Questions

Y ln xkv lubj swlzqvkht, A vmzb pjk bbua we ddgs ILQ-GQYU-8026

This message can be decoded by hand or through an online tool such as dCode or CyberChef.

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