Breaking Enigma with Index of Coincidence on a Commodore 64 (imapenguin.com)

The article shows how to attack Enigma using only ciphertext by computing the index of coincidence (IC) on a Commodore 64. Instead of needing a crib, it filters rotor settings based on how closely the decrypted text’s letter-frequency statistics match German. It demonstrates the method on a 60-character intercepted U-boat weather message, finding the correct rotor positions among thousands of candidates by applying an IC threshold and then manually checking the readable German output.