Autokey Ciphers

1
Autokey Ciphers
2
History

• The Autokey Cipher is almost identical to the
  Vigenère Cipher only its more secure
• It was first invented by Girolamo Cardano and was
  made more secure by Vigenère

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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 MEETMEFORL
UNCH
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Encryption II

• After you assign a keyword to all of the plain
  text, you are going to need a tabula recta.

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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

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Example

• Key  RESTAURANTMEET
• Plaintext MEETMEFORLUNCH

Cipher letter D Continue this for the rest of
the message.
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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?

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Decryption

• Assuming the buddy who sent you the secret
  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

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Example, Cont.

• Key  R ESTAURANTMEET
• Ciphertext D IWMMYWOEEGRGA

Plaintext M Repeat!
The secret message Meet me for lunch.
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Cryptanalysis

• Now, say your buddy sent you the secret message,
  but didnt give you the secret password - you now
  have a wonderful problem!

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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

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Example
Promising!

• 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

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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

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Example Cont.
Plain text looks good!

• 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.
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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

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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
  .

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The secret message?

• Meet at the fountain.
• and no, Mr. Evans, that wasnt relatively
  pain-free.

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Sources

• http//realmoftwelve.kryptos.info/toolbox/autokey.
  html
• http//en.wikipedia.org/wiki/Autokey_cipher