# Running-key cipher · Concept

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

Source: https://mysteriesdept.com/concepts/CPT0DECW006275C/running-key-cipher  
Publisher: Mysteries Department  
Licence: CC BY 4.0 (credit Mysteries Department and link the page)  

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[Concepts](https://mysteriesdept.com/concepts)Running-key cipher

Concept · cryptography · in 1 mystery

# Running-key cipher

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

[Beale ciphers](https://mysteriesdept.com/mysteries/beale-ciphers)

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](https://mysteriesdept.com/mysteries/beale-ciphers)**

[Ciphers 1 and 3 are book ciphers on real key texts and encode English.](https://mysteriesdept.com/mysteries/beale-ciphers/answers/ANS0CR7J1BC7GG3/ciphers-1-and-3-are-book-ciphers-on-real-key-texts-and)[Cipher 2 is a real book cipher and decodes cleanly, which shows the author could make one.](https://mysteriesdept.com/mysteries/beale-ciphers/evidence/EVD0CR7J1K673XD/cipher-2-is-a-real-book-cipher-and-decodes-cleanly-which)

## Cite it

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

Key

**CPT** 0DECW006275C

Record

Concept

Field

Cryptography

Related

[Vigenère cipher](https://mysteriesdept.com/concepts/CPT0DECW00CABZA/vigenere-cipher) · [Gromark cipher](https://mysteriesdept.com/concepts/CPT0DECW00HS0D8/gromark-cipher)

Version

2

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**How this page is made.** The definition is the site's, in its own words. “Where it comes up” is read from the answers, methods and evidence that name the concept.

Related concepts

[Vigenère cipher](https://mysteriesdept.com/concepts/CPT0DECW00CABZA/vigenere-cipher)[Gromark cipher](https://mysteriesdept.com/concepts/CPT0DECW00HS0D8/gromark-cipher)

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