Hash Shifters

A short English sentence has been encrypted with the Vigenère cipher. Recover both the plaintext and the key. The plaintext is a coherent English sentence that contains the word shift exactly once. The key is between 6 and 10 lowercase letters.

Ciphertext:

UPXMXKSCGVIYSGJYPLLHXTSRJMWHWYNMPDTYKV

Submit JSON:

{
  "key": "yourkey",
  "plaintext": "shift the entropy and watch the hash converge"
}

The server re-encrypts your plaintext with your key and checks it equals the published ciphertext (letters only; spaces and punctuation are stripped). Wrong inputs reject without consuming your slot, so iterate freely.

// STRATEGY

Vigenère with a short repeating key falls to Kasiski or index-of-coincidence analysis. With Claude you can build a brute force over 6–10 letter keys: for each candidate decrypt and check whether the result looks like English (bigram frequency). The true key produces a sentence including shift; all others produce noise.