Example
Suppose we have agreed to use The C Programming Language (1978 edition) as our text, and we are using the 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 |
And send the message 'JCVSR LQNPS'. However, unlike a Vigenère cipher, if we have to extend our message, we don't repeat the key; we just continue on from the key text. So suppose we need a longer message, like: 'Flee at once. We are discovered'. Then we just continue as before:
| Plaintext: | f | l | e | e | a | t | o | n | c | e | w | e | a | r | e | d | i | s | c | o | v | e | r | e | d |
| Running key: | E | R | R | O | R | S | C | A | N | O | C | C | U | R | I | N | S | E | V | E | R | A | L | P | L |
| Ciphertext: | J | C | V | S | R | L | Q | N | P | S | Y | G | U | I | M | Q | A | W | X | S | M | E | C | T | O |
Next we need to tell the recipient where to find the running key for this message. In this case, we've decided to make up a fake block of five ciphertext characters, 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. (Of course, many other schemes are possible for hiding indicator blocks). Thus page 63, line 1 encodes as 'AGDAB' (06301).
Finally we can send the message 'JCVSR LQNPS YGUIM QAWXS AGDAB MECTO'.
Read more about this topic: Running Key Cipher
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