Autokey Cipher Ideally want a key as long as the message
{"Image DocumentName": "Autokey Cipher Ideally want a key as long as the message", "Image ImageDescription": "Vigen\u00e8re proposed the autokey cipher. with keyword is prefixed to message as key. knowing keyword can recover the first few letters. use these in turn on the rest of the message. but still have frequency characteristics to attack. eg. given key deceptive. key: deceptivewearediscoveredsav. plaintext: wearediscoveredsaveyourself. ciphertext:ZICVTWQNGKZEIIGASXSTSLVVWLA. Taking the polyalphabetic idea to the extreme, want as many different translation alphabets as letters in the message being sent. One way of doing this with a smallish key, is to use the Autokey cipher. The example uses the keyword DECEPTIVE prefixed to as much of the message WEAREDISCOVEREDSAV as is needed. When deciphering, recover the first 9 letters using the keyword DECEPTIVE . Then instead of repeating the keyword, start using the recovered letters from the message WEAREDISC . As recover more letters, have more of key to recover later letters. Problem is that the same language characteristics are used by the key as the message. ie. a key of E will be used more often than a T etc hence an E encrypted with a key of E occurs with probability (0.1275)2 = , about twice as often as a T encrypted with a key of T have to use a larger frequency table, but it exists given sufficient ciphertext this can be broken.", "Image XResolution": "37", "Image YResolution": "37", "Image ResolutionUnit": "Pixels/Centimeter", "Image Software": "SlidePlayer", "Image YCbCrPositioning": "Centered"}