Editing Running key cipher (section)

{{short description|Type of polyalphabetic substitution cipher}}
{{refimprove|date=March 2016}}
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]]. The earliest description of such a cipher was given in 1892 by French mathematician Arthur Joseph Hermann (better known for founding [[Éditions Hermann]]). Usually, the book to be used would be agreed ahead of time, while the passage to be used would be chosen [[random]]ly for each message and secretly indicated somewhere in the message.