Running key cipher

In classical cryptography, the running key cipher is a type of polyalphabetic substitution cipher in which a text, typically from a book, is used to provide a very long keystream. Usually, the book to be used would be agreed ahead of time, while the passage to be used would be chosen randomly for each message and secretly indicated somewhere in the message.

The text used is The C Programming Language (1978 edition), and the tabula recta is the tableau. The plaintext is "Flee at once".

Page 63, line 1 is selected as the running key:

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The running key is then written under the plaintext:

The message is then sent as "JCVSR LQNPS". However, unlike a Vigenère cipher, the message is extended, the key is not repeated; the key text itself is used as the key. If the message is extended, such as, "Flee at once. We are discovered", then the running key continues as before:

To determine where to find the running key, a fake block of five ciphertext characters is subsequently added, with three denoting the page number, and two the line number, using A=0, B=1 etc. to encode digits. Such a block is called an indicator block. The indicator block will be inserted as the second last of each message. (It should be noted that many other schemes are possible for hiding indicator blocks.) Thus page 63, line 1 encodes as "AGDAB" (06301).

This yields a final message of "JCVSR LQNPS YGUIM QAWXS AGDAB MECTO".

Modern variants of the running key cipher often replace the traditional tabula recta with bitwise exclusive or, operate on whole bytes rather than alphabetic letters, and derive their running keys from large files. Apart from possibly greater entropy density of the files, and the ease of automation, there is little practical difference between such variants and traditional methods.

A more compact running key can be used if one combinatorially generates text using several start pointers (or combination rules). For example, rather than start at one place (a single pointer), one could use several start pointers and xor together the streams to form a new running key, similarly skip rules can be used. What is exchanged then is a series of pointers to the running key book and/or a series of rules for generating the new permuted running key from the initial key text. (These may be exchanged via public key encryption or in person. They may also be changed frequently without changing the running key book).

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