Editing Autokey cipher (section)

==Method==
The autokey cipher, as used by members of the [[American Cryptogram Association]], starts with a relatively-short keyword, the ''primer'', and appends the message to it. For example, if the keyword is {{code|QUEENLY}} and the message is {{code|attack at dawn}}, then the key would be {{code|QUEENLYATTACKATDAWN}}.<ref>{{cite web|url=http://asecuritysite.com/security/Coding/autokey?word=attackatdawn%2Cqueenly |title=Autokey Calculator |publisher=Asecuritysite.com |access-date=2012-12-26 |url-status=dead |archive-url=https://web.archive.org/web/20131202225102/http://asecuritysite.com/security/Coding/autokey?word=attackatdawn%2Cqueenly |archive-date=2013-12-02 }}</ref>

 Plaintext:  attackatdawn
 Key:        QUEENLYATTACKATDAWN
 Ciphertext: QNXEPVYTWTWP

The ciphertext message would thus be "QNXEPVYTWTWP".

To decrypt the message, the recipient would start by writing down the agreed-upon keyword.

 QNXEPVYTWTWP
 QUEENLY

The first letter of the key, Q, would then be taken, and that row would be found in a tabula recta. That column for the first letter of the ciphertext would be looked across, also Q in this case, and the letter to the top would be retrieved, A. Now, that letter would be added to the end of the key:

 QNXEPVYTWTWP
 QUEENLY'''A'''
 '''a'''

Then, since the next letter in the key is U and the next letter in the ciphertext is N, the U row is looked across to find the N to retrieve T:

 QNXEPVYTWTWP
 QUEENLY'''AT'''
 '''at'''

That continues until the entire key is reconstructed, when the primer can be removed from the start.

 QNXEPVYTWTWP
 QUEENLY'''ATTACKATDAWN'''
 '''attackatdawn'''

With Vigenère's autokey cipher, a single mistake in encryption renders the rest of the message unintelligible.<ref name="hoffstein" />