Cryptography: Cesar Cipher

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Presentation transcript:

Cryptography: Cesar Cipher Computer Security Cryptography: Cesar Cipher

INTRODUCTION "RETURN TO ROME" "UHWXUA WR URPH" One of the simplest examples of a substitution cipher is the Caesar cipher, which is said to have been used by Julius Caesar to communicate with his army. Caesar decided that shifting each letter in the message would be his standard algorithm Using the Caesar Shift (3 to the right), the message : "RETURN TO ROME" would be encrypted as: "UHWXUA WR URPH"

EXAMPLE For example, if we use a shift of 19, then we get the following pair of ciphertext and plaintext alphabets: 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 Ciphertext: T U V W X Y Z A B C D E F G H I J K L M N O P Q R S Essentially, each letter of the alphabet has been shifted nineteen places ahead in the alphabet, wrapping around the end if necessary. Notice that punctuation and blanks are not enciphered but are copied over as themselves.

Breaking a Caesar Cipher (Cryptanalysis) Can a computer guess what shift was used in creating a Caesar cipher? The answer, of course, is yes. But how does it work? The unknown shift is one of 26 possible shifts. One technique might be to try each of the 26 possible shifts and check which of these resulted in readable English text. But this approach has limitations. The main problem is that the computer would need a comprehensive dictionary in order to be able to recognize the words of any given cryptogram.

Breaking a Caesar Cipher (Cryptanalysis) A better approach makes use of statistical data about English letter frequencies. It is known that in a text of 1000 letters of various English alphabet occur with about the following relative frequencies: This information can be useful in deciding the most likely shift used on a given enciphered message.

Breaking a Caesar Cipher (Cryptanalysis) Suppose the enciphered message is: K DKVO DYVN LI KX SNSYD, PEVV YP CYEXN KXN PEBI, CSQXSPISXQ XYDRSXQ. We can tally the frequencies of the letters in this enciphered message, thus : Now we can now shift the two tallies so that the large and small frequencies from each frequency distribution match up roughly.