Running Key Cipher

Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. 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 use would be chosen randomly for each message and secretly indicated somewhere in the message. Suppose we have agreed to use The C Programming Language (1978 edition) as our text, and we are using the ordinary tabula recta as our tableau. We need to send the message 'Flee at once'. First, we choose a starting point. Let us choose page 63, line 1: errors can occur in several places. A label has... We write out the running key under our plaintext: Plaintext: f l e e a t o n c e Running key: E R R O R S C A N O Ciphertext: J C V S R L Q N P S