Running key cipher
A running key cipher uses a long, nonrepeating text or sequence as the keystream for polyalphabetic substitution.
Definition
A running key cipher combines plaintext with a key sequence that is normally at least as long as the message. In a common form, letters from a book passage select successive Vigenère shifts instead of repeating a short keyword.
Running key and autokey
A running key comes from an external long source. Autokey starts with a short secret primer and extends it with plaintext or ciphertext. If a public book is used, the edition, passage, and starting position must effectively remain secret.
Security limits
Natural-language key text is not random: it has letter frequencies and correlations that combine with those of the plaintext. Specialized statistical attacks can exploit this structure, so the method does not provide modern security.
No. A one-time pad requires uniformly random, secret, nonreused key material; ordinary book text does not meet that requirement.
Reuse creates additional relationships between messages and makes cryptanalysis easier.