Running-key cipher

A polyalphabetic cipher whose key is a long text rather than a repeating word.

A Vigenère-style cipher keyed by a passage from a book or document instead of a short repeating word, so the key never cycles and the usual period tests fail. The Beale ciphers' one solved page is a book cipher of a related kind, keyed to the Declaration of Independence, and running keys are among the readings tested for Kryptos K4.

Where it comes up

Beale ciphers

Ciphers 1 and 3 are book ciphers on real key texts and encode English.Cipher 2 is a real book cipher and decodes cleanly, which shows the author could make one.

Cite it

Mysteries Department. (2026). Running-key cipher [Concept CPT0DECW006275C]. https://mysteriesdept.com/r/CPT0DECW006275C

@misc{mysteriesdept_cpt0decw006275c,
  title     = {Running-key cipher},
  author    = {{Mysteries Department}},
  year      = {2026},
  note      = {Concept, catalog key CPT0DECW006275C},
  url       = {https://mysteriesdept.com/r/CPT0DECW006275C}
}