Thursday, July 7, 2011

Using Key Word Cipher and the Shift Cipher

A keyword cipher is a form of monoalphabetic substitution, being used as the key. The keyword cipher determines the letter matchings of the cipher alphabet to the plain alphabet. Repeats of letters in the word are removed, the cipher alphabet being then generated with the keyword matching to A,B,C etc. until the keyword is used up, whereupon the rest of the ciphertext letters are used in alphabetical order, excluding those already used in the key.

Plaintext: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Encrypted: K R Y P T O S A B C D E F G H I J L M N Q U V W X Z

With KRYPTOS as the keyword, all As become Ks, all Bs become Rs and so on. Encrypting the message "cryptography is cool" using the keyword "kryptos":

Plaintext: K N O W L E D G E I S P O W E R
Encoded: D G H V E T P S T B M I H V T L

Only one alphabet is used here, so the cipher is monoalphabetic.

The best ways to attack a keyword cipher without knowing the keyword are through known-plaintext attack, frequency analysis and discovery of the keyword (often a cryptanalist will combine all three techniques). Keyword discovery allows immediate decryption since the table can be made immediately.
In cryptography, a Caesar cipher, also known as a Caesar's cipher, the shift cipher, Caesar's code or Caesar shift, is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet. For example, with a shift of 3, A would be replaced by D, B would become E, and so on. The method is named after Julius Caesar, who used it to communicate with his generals. The encryption step performed by a Caesar cipher is often incorporated as part of more complex schemes, such as the Vigenère cipher, and still has modern application in the ROT13 system. As with all single alphabet substitution ciphers, the Caesar cipher is easily broken and in practice offers essentially no communication security.
We have learned at this course that Leonardo da Vinci has used the mirror language. Teachers and students have to retain that in transposition codes, all the letters remain the same but in a different order.

Here is our trainer's example:


What we know?

* It's a code in Modern English Language.
* 'Z' is dominant.
* There are different words disposed in this way:
- 2 letter words;
- 3 letter words;
- 4 letter words;
- 8 letter words.

What we need to know?

* Is there a key word? - not relevant
* Is it a sentence? - yes
* Word frequencies in the English language:
1) The most common letter - e
2) A single letter word: - a, I
3) Most common 2 letter words - of, to in
4) Most common 3 letter words - the, and
5) Most common 4 letter words - that
6) 'Q' is followed by 'u'.

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