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Polyalphabetic Ciphers| Category | | | Summary | | | Body | Polyalphabetic Ciphers
Caesar ciphers or rotation ciphers (also called ROT - n ciphers) are relatively easy to break. Same goes for mono alphabetic substitution ciphers. The Playfair Cipher is a slight improvement, but still retains some structural sense making it easy to break. A poly alphabetic substitution cipher is a vast improvement over these techniques. The basic principle in all poly alphabetic ciphers is the use of multiple but different mono alphabetic substitution techniques. I'll explain this particular concept using the example of the most widely known poly alphabetic substitution cipher, the Vigenere Cipher.
I'm assuming that the reader knows the basic ROT-n cipher, also known as the Caesar cipher. Let us suppose that each letter from 'a' to 'z' denotes a number. Suppose 'a' stands for 0, 'b' for 1, 'c' for 2 and so on. So, in the Vigenere cipher, each letter key value will denote the shift to be done for that particular letter, i.e. the 'n' in the ROT-n cipher. The concept will become more clear with the use of an example.
Let key = abc
Let plaintext = xyz (The message to be encrypted)
Thus cipher text = xzb (Shifts of 0,1,2 respectively)
Lets take another example,
Key = ad
Plaintext = abcd
Thus cipher text = aecg (Shifts of 0,3,0,3)
The second example shows us an important technique. If the key is of a lesser length than the plaintext, then use the key repeatedly till you traverse the whole length of the plaintext. Decryption is simple, just reverse the shift process. The advantage of this cipher is that it is easy to implement and any letter frequency information is lost. But this does not mean that this algorithm is very difficult to break, even without the key information. To make the cipher still more secure against cryptanalysis, an AT&T engineer Gilbert Vernam suggested a key construction system in 1918. The basic concept of this method is that the key should be as long as the plaintext and should have no statistical relationship to it. This is the basic concept used in One-Time Pads. The system suggested by Vernam works on the basic principle of bit wise XOR operation.
c(i) = p(i) XOR k(i) Encryption
p(i) = c(i) XOR k(i) Decryption
References :-
Cryptography and Network Security, by William Stallings
Applied Cryptography, by Bruce Schiener |
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This article was imported from the CyberArmy University site. (original author: rae)
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