running-key cipher

The running-key cipher is a method within cryptology used for encrypting information. It's a variation of the Vigenère cipher, which uses multiple substitution alphabets to obscure plaintext patterns. Unlike simpler Vigenère ciphers that repeat a keyword, a running-key cipher uses a non-repeating text as the key. This key can be a prearranged, unpredictable sequence of characters.123

While this method eliminates the periodicity found in repeating-key ciphers, making it harder to break, it's not impregnable. Cryptanalysts can still attempt to decipher it by assuming the ciphertext and key share similar symbol frequency distributions or by using a "probable-word" method to identify likely plaintext segments. The effectiveness of cryptanalysis depends on the amount of ciphertext available and the statistical properties of the language used.12