Talk:Autokey cipher
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This article has no information on how the cypher was broken. ✏ Sverdrup 02:28, 21 Nov 2004 (UTC)
So if the message is incorporated in the key, why do you need to decode the cyphertext? You've already got the message. {Answer: The key+message is just a temporary copy used by the sender. The recipient doesn't have it, only the original key and the cyphertext.}
{T}o answer your question: if you have cipher text that says WMPM MX XAF YHBRYOCA and you know the keyword of KILT, then you can decode the message as such:
CIPHERTEXT:_WMPM MX XAF YHBRYOCA
KEY:________KILT
{ Go across the top of the table to the key letter (K), search down to the cyphertext letter (W), then find the plain text letter at the left edge (M). After four iterations, you have: }
PLAINTEXT:__MEET
Now that some { = key length } of the plaintext is found, it is appended to the end of the key:
CIPHERTEXT:_WMPM MX XAF YHBRYOCA
KEY:________KILT ME ETA TTHEFOUN
PLAINTEXT:__MEET AT THE FOUNTAIN
so, the reciever of this message needs to have that original key, which a potential "eve" would not have. the reason to use this is that the methods to break the vignere cipher can't be applied beccause the key is as long as the message, so the length of the key can't be found by measuring the distance between repeated phrases.
--Wakingrufus 19:38, 10 December 2005 (UTC) {edit by gatmo for additional detail - feel free to rephrase}
"This text-autokey cipher was hailed as "le chiffre indéchiffrable", and was indeed undeciphered for over 200 years, until Charles Babbage discovered a means of breaking the cipher." isn't this what happened to the vigenere cipher? how was this cipher actually broken?--Wakingrufus 19:38, 10 December 2005 (UTC)
I've added an example of cryptanalysis. Don't know exactly how Babbage first cracked Autokey but probably along the same lines. Frankd 12:21, 12 June 2006 (UTC)
Frankd: Thanks. That example was very helpful in understanding the basic frame of mind a cryptanalyst has to be in to break a code.
I am a little bit confused: is Vigenere autokey cipher self-synchronizing? --Necago 16:38, 9 June 2007 (UTC)
