Cryptanalysis of the Autokey Cipher involves trying common words, letters, and letter combinations in the plaintext and using the relationship between the plaintext and key to reveal more of both. By guessing fragments of the plaintext shifted in the key, if the guesses are correct they will also appear in the plaintext shifted the other way. This feedback allows the cryptanalyst to quickly fill in more of the plaintext and key to decrypt the message.

1. Cryptanalysis of
Autokey Cipher

2. Agenda
1 What is Autokey Cipher?
2 An Example
3 What is Cryptanalysis?
4 Cryptanalysis of Autokey Cipher
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3. 1. What is Autokey Cipher?
• An autokey cipher (also known as the autoclave cipher) is a cipher which incorporates the
message (the plaintext) into the key.
• There are two forms of autokey cipher:
1. key autokey cipher,
2. text autokey ciphers
• A key-autokey cipher uses previous members of the keystream to determine the next element in
the keystream.
• A text-autokey uses the previous message text to determine the next element in the keystream.
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4. 2. An Example
• message: meet at the fountain
• keyword: KILT
Hence,
plaintext: MEETATTHEFOUNTAIN
key: KILTMEETATTHEFOUN
ciphertext: WMPMMXXAEYHBRYOCA
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5. 3. What is Cryptanalysis?
• Cryptanalysis is the study of analyzing information systems in order to study the
hidden aspects of the systems.
• Cryptanalysis is used to breach cryptographic security systems and gain access to
the contents of encrypted messages, even if the cryptographic key is unknown.
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6. 4. Cryptanalysis of Autokey Cipher?
• We will use previously ciphered text for cryptanalysis.
• We try common words, bigrams, trigrams etc. in all possible positions in the key.
For example, THE:
Ciphertext Key plaintext
WMP MMX XAE YHB RYO CA
W MPM MXX AEY HBR YOC A
WM PMM XXA EYH BRY OCA
THE THE THE THE THE ..
. THE THE THE THE THE .
.. THE THE THE THE THE
DFL TFT ETA FAX YRK ..
. TII TQT HXU OUN FHY .
.. WFI EQW LRD IKU VVW
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7. Cryptanalysis of Autokey Cipher (cont.)
• We sort the plaintext fragments in order of likelihood:
unlikely <------------------> promising
EQW DFL TFT ... ... ... ... ETA OUN FAX
• We know that a correct plaintext fragment will also appear in the key, shifted right by the length
of the keyword.
• Similarly our guessed key fragment ("THE") will also appear in the plaintext shifted left.
• So by guessing keyword lengths (probably between 3 and 12) we can reveal more plaintext and
key
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8. Cryptanalysis of Autokey Cipher (cont.)
• Trying this with "OUN" (possibly after wasting some time with the others):
shift by 4:
ciphertext:
WMPMMXXAEYHBRYOCA
key:
......ETA.THE.OUN
plaintext:
......THE.OUN.AIN
by 5:
ciphertext:
WMPMMXXAEYHBRYOCA
key:
.....EQW..THE..OU
plaintext:
.....THE..OUN..OG
by 6:
ciphertext:
WMPMMXXAEYHBRYOCA
key:
....TQT...THE...O
plaintext:
....THE...OUN...M
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9. Cryptanalysis of Autokey Cipher (cont.)
• We see that a shift of 4 looks good (both of the others have unlikely Qs), so we shift
the revealed "ETA" back by 4 into the plaintext:
ciphertext: WMPMMXXAEYHBRYOCA
key: ..LTM.ETA.THE.OUN
plaintext: ..ETA.THE.OUN.AIN
• We have a lot to work with now. The keyword is probably 4 characters long ("..LT"),
and we have some of the message:
M.ETA.THE.OUN.AIN
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10. Cryptanalysis of Autokey Cipher (cont.)
• Because our plaintext guesses have an effect on the key 4 characters to the left, we
get feedback on correct/incorrect guesses, so we can quickly fill in the gaps:
MEETATTHEFOUNTAIN
• The ease of cryptanalysis is thanks to the feedback from the relationship between
plaintext and key.
• A 3-character guess reveals 6 more characters, which then reveal further characters,
creating a cascade effect, allowing us to rule out incorrect guesses quickly.
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11. Thank you