autokey ciphers. history the autokey cipher is almost identical to the vigenère cipher – only...
TRANSCRIPT
![Page 1: Autokey Ciphers. History The Autokey Cipher is almost identical to the Vigenère Cipher – only it’s more secure It was first invented by Girolamo Cardano](https://reader033.vdocuments.mx/reader033/viewer/2022061609/56649dd05503460f94ac5178/html5/thumbnails/1.jpg)
Autokey Ciphers
![Page 2: Autokey Ciphers. History The Autokey Cipher is almost identical to the Vigenère Cipher – only it’s more secure It was first invented by Girolamo Cardano](https://reader033.vdocuments.mx/reader033/viewer/2022061609/56649dd05503460f94ac5178/html5/thumbnails/2.jpg)
History
• The Autokey Cipher is almost identical to the Vigenère Cipher – only it’s more secure
• It was first invented by Girolamo Cardano and was made more secure by Vigenère
![Page 3: Autokey Ciphers. History The Autokey Cipher is almost identical to the Vigenère Cipher – only it’s more secure It was first invented by Girolamo Cardano](https://reader033.vdocuments.mx/reader033/viewer/2022061609/56649dd05503460f94ac5178/html5/thumbnails/3.jpg)
Encryption
• Start by choosing plain text and a keyword of any length
• Write the keyword over the plain text, and then immediately after the keyword, begin writing the plain text
Key: RESTAURANTMEET
Plaintext: MEETMEFORLUNCH
![Page 4: Autokey Ciphers. History The Autokey Cipher is almost identical to the Vigenère Cipher – only it’s more secure It was first invented by Girolamo Cardano](https://reader033.vdocuments.mx/reader033/viewer/2022061609/56649dd05503460f94ac5178/html5/thumbnails/4.jpg)
Encryption II
• After you assign a keyword to all of the plain text, you are going to need a tabula recta.
![Page 5: Autokey Ciphers. History The Autokey Cipher is almost identical to the Vigenère Cipher – only it’s more secure It was first invented by Girolamo Cardano](https://reader033.vdocuments.mx/reader033/viewer/2022061609/56649dd05503460f94ac5178/html5/thumbnails/5.jpg)
Encryption III
• In order to encrypt plain text, you locate the row with the first letter to be encrypted, and the column with the first letter of the key. The letter where the line and column cross is the ciphertext letter
![Page 6: Autokey Ciphers. History The Autokey Cipher is almost identical to the Vigenère Cipher – only it’s more secure It was first invented by Girolamo Cardano](https://reader033.vdocuments.mx/reader033/viewer/2022061609/56649dd05503460f94ac5178/html5/thumbnails/6.jpg)
Example
• Key: RESTAURANTMEET• Plaintext: MEETMEFORLUNCH
Cipher letter = D
Continue this for the rest of the message.
![Page 7: Autokey Ciphers. History The Autokey Cipher is almost identical to the Vigenère Cipher – only it’s more secure It was first invented by Girolamo Cardano](https://reader033.vdocuments.mx/reader033/viewer/2022061609/56649dd05503460f94ac5178/html5/thumbnails/7.jpg)
Example, cont.
• You should now have this encrypted text: Key: RESTAURANTMEET
Plaintext: MEETMEFORLUNCH
Ciphertext: DIWMMYWOEEGRGA
• Now, if someone gave you the message “DIWMMYWOEEGRGA”, would you know how to decrypt it?
![Page 8: Autokey Ciphers. History The Autokey Cipher is almost identical to the Vigenère Cipher – only it’s more secure It was first invented by Girolamo Cardano](https://reader033.vdocuments.mx/reader033/viewer/2022061609/56649dd05503460f94ac5178/html5/thumbnails/8.jpg)
Decryption
• Assuming the buddy who sent you the secret messagesecret message likes you, he probably sent you the keyword, too.
• With a key of RESTAURANT and ciphertext DIWMMYWOEEGRGA, find the key characters in the top-column header and the ciphertext characters in the table
![Page 9: Autokey Ciphers. History The Autokey Cipher is almost identical to the Vigenère Cipher – only it’s more secure It was first invented by Girolamo Cardano](https://reader033.vdocuments.mx/reader033/viewer/2022061609/56649dd05503460f94ac5178/html5/thumbnails/9.jpg)
Example, Cont.
Key: R ESTAURANTMEET
Ciphertext: D IWMMYWOEEGRGA
Plaintext = M
Repeat!
The secret message:
“Meet me for lunch”.
![Page 10: Autokey Ciphers. History The Autokey Cipher is almost identical to the Vigenère Cipher – only it’s more secure It was first invented by Girolamo Cardano](https://reader033.vdocuments.mx/reader033/viewer/2022061609/56649dd05503460f94ac5178/html5/thumbnails/10.jpg)
Cryptanalysis
• Now, say your buddy sent you the secret secret messagemessage, but didn’t give you the secret secret password password - you now have a wonderful problem!
![Page 11: Autokey Ciphers. History The Autokey Cipher is almost identical to the Vigenère Cipher – only it’s more secure It was first invented by Girolamo Cardano](https://reader033.vdocuments.mx/reader033/viewer/2022061609/56649dd05503460f94ac5178/html5/thumbnails/11.jpg)
How to solve it
• Choose common trigrams or bigrams in all possible positions for the key.
• Here, we use “the”.
• Try it in all possible positions for the key
![Page 12: Autokey Ciphers. History The Autokey Cipher is almost identical to the Vigenère Cipher – only it’s more secure It was first invented by Girolamo Cardano](https://reader033.vdocuments.mx/reader033/viewer/2022061609/56649dd05503460f94ac5178/html5/thumbnails/12.jpg)
Example Ciphertext: WMP MMX XAE YHB RYO CA
Key: THE THE THE THE THE ..
Plaintext: DFL TFT ETA FAX YRK ..
Ciphertext: W MPM MXX AEY HBR YOC A
Key: . THE THE THE THE THE .
Plaintext: . TII TQT HXU OUN FHY .
Ciphertext: WM PMM XXA EYH BRY OCA
Key: .. THE THE THE THE THE
Plaintext: .. WFI EQW LRD IKU VVW
Promising!
![Page 13: Autokey Ciphers. History The Autokey Cipher is almost identical to the Vigenère Cipher – only it’s more secure It was first invented by Girolamo Cardano](https://reader033.vdocuments.mx/reader033/viewer/2022061609/56649dd05503460f94ac5178/html5/thumbnails/13.jpg)
Example Cont.
• We also know that some of the plain text is also in the key, to the right of the keyword
• In the previous slide, we marked the possible plain text fragments “ETA”, “OUN” and “FAX” as promising
• Now we will use these, plus “THE” to try and guess the keyword length
![Page 14: Autokey Ciphers. History The Autokey Cipher is almost identical to the Vigenère Cipher – only it’s more secure It was first invented by Girolamo Cardano](https://reader033.vdocuments.mx/reader033/viewer/2022061609/56649dd05503460f94ac5178/html5/thumbnails/14.jpg)
Example Cont.
Shift by 4:
Ciphertext: WMPM M XXA E YHB R YOC A
Key: . . . . . . ETA . THE . OUN .
Plaintext: . . . . . .THE . OUN . AIN .
Shift by 5:
Ciphertext: WMPMMXXAEYHBRYOCA
Key: .....EQW..THE..OU
Plaintext: .....THE..OUN..OG
Unlikely to have a “Q” in the keyword.
Plain text looks good!
![Page 15: Autokey Ciphers. History The Autokey Cipher is almost identical to the Vigenère Cipher – only it’s more secure It was first invented by Girolamo Cardano](https://reader033.vdocuments.mx/reader033/viewer/2022061609/56649dd05503460f94ac5178/html5/thumbnails/15.jpg)
Example Cont.
• Now we have to use our brains
• The four letter shift looks good, so we can guess that the keyword is probably about 4 characters long
• Through the “guess-and-check” method, we can fill in some blanks
![Page 16: Autokey Ciphers. History The Autokey Cipher is almost identical to the Vigenère Cipher – only it’s more secure It was first invented by Girolamo Cardano](https://reader033.vdocuments.mx/reader033/viewer/2022061609/56649dd05503460f94ac5178/html5/thumbnails/16.jpg)
Almost done!
• You should now have something like this:
ciphertext: WMPMM X XAE YHB R YOC A
key: . . LTM . ETA . THE . OUN .
Plaintext: . .ETA . THE . OUN . AIN .
![Page 17: Autokey Ciphers. History The Autokey Cipher is almost identical to the Vigenère Cipher – only it’s more secure It was first invented by Girolamo Cardano](https://reader033.vdocuments.mx/reader033/viewer/2022061609/56649dd05503460f94ac5178/html5/thumbnails/17.jpg)
The secret message?
• “Meet at the fountain”.
• …and no, Mr. Evans, that wasn’t “relatively pain-free.”
![Page 18: Autokey Ciphers. History The Autokey Cipher is almost identical to the Vigenère Cipher – only it’s more secure It was first invented by Girolamo Cardano](https://reader033.vdocuments.mx/reader033/viewer/2022061609/56649dd05503460f94ac5178/html5/thumbnails/18.jpg)
Sources
• http://realmoftwelve.kryptos.info/toolbox/autokey.html
• http://en.wikipedia.org/wiki/Autokey_cipher