Editing Running key cipher (section)

== Example ==

The key text used is a portion of ''[[The C Programming Language]]'' (1978 edition), and the ''[[tabula recta]]'' is the tableau. The plaintext here is "Flee at once".

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

<blockquote>errors can occur in several places. A label has...</blockquote>

The running key is then written under the plaintext:

{| class="wikitable" style="text-align:center"
|-
! align="left" | Plaintext
| f || l || e || e || a || t || o || n || c || e
|-
! align="left" | Running key
| E || R || R || O || R || S || C || A || N || O
|-
! align="left" | Ciphertext
| J || C || V || S || R || L || Q || N || P || S
|}

The message is then sent as "JCVSR LQNPS". However, unlike a [[Vigenère cipher]], if the message is extended, the key is not repeated; the key text itself (the text from the ''[[The C Programming Language]]'') is used as the key and can be extended for any arbitrary length. If the message is extended, such as, "Flee at once. We are discovered", then the running key continues as before:

{| class="wikitable" style="text-align:center"
|-
! align="left" | 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
|-
! align="left" | 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
|-
! align="left" | 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
|}

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. (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".