An approach in improving transposition cipher system

Faculty of Electrical and Computer Engineering, University of Tabriz, Tabriz, Iran
Indian Journal of Science and Technology 09/2009; 2.


Transposition ciphers are stronger than simple substitution ciphers. However, if the key is short and the message is long, then various cryptanalysis techniques can be applied to break such ciphers. By adding 8 bits (one byte) for each byte using a function and another mathematical function to position the bits in a binary tree and using its in-order tour, this cipher can be made protected. Using an in-order tour of binary tree can diffuse the eight bits (includes 7 bits produced by the function and 1 random bit) and eight bits of the plaintext. This can highly protect the cipher. However, if the key management processes are not secured the strongest ciphers can easily be broken. transposition cipher. Introduction With the growth of Internet, hackers and spies make big issues for army forces, organizations and companies. Transposition ciphers are still the most important kernel techniques in the construction of modern symmetric encryption algorithms. We will clearly see combinations of substitution and transposition ciphers in two important modern symmetric encryption algorithms: DES and AES (Mao Hewlett W-Packard, 2003). DES is not very secure because of the limitation of the space of the key is 56 bits. On the other hand AES is a new cipher. The benefit of these 2 ciphers is that they have two factors of cryptology and security, diffusion and confusion (Schinier, 1996). In part 3 we will completely clarify these two factors. In this paper we present a new algorithm that mostly uses transposition cipher, diffusion and modification of block cipher. Transposition ciphers Transposition ciphers are an important family of classical ciphers, in additional substitution ciphers, which are widely used in the constructions of modern block ciphers (Mao Hewlett W-Packard, 2003). In a transposition cipher the plaintext remains the same, but the order of characters is shuffled around. In a simple columnar transposition cipher, the plaintext is written horizontally onto a piece of graph paper of fixed width and the cipher text is read off vertically. Decryption is a matter of writing the cipher text vertically onto a piece of graph paper of identical width and then reading the plaintext off horizontally (Schinier, 1996). Cryptanalysis of these ciphers is discussed in (Sinkov, 1966; Gaines, 1956). Since the letters of the cipher text are the same as those of the plaintext, a frequency analysis on the cipher text would reveal that each letter has approximately the same likelihood as in English. This gives a better clue to a cryptanalyst, who can apply a variety of techniques to determine the right ordering of the letters to obtain the plaintext (Schinier, 1996) Putting the cipher text through a second transposition, cipher greatly enhances security. There are even more complicated transposition ciphers, but computers can break almost all of them. The German ADFGVX cipher, used during World War I, is a transposition cipher combined with a simple substitution. It was a very complex algorithm for its day but was broken by Georges Painvin, a French cryptanalyst (Kahn, 1967; Schinier, 1996). We can give an example for transposition cipher. In our example the key is a small number for example 5. In order to encrypt a message using this key, we write the key in rows of 5 letters and encrypt by writing the letters of the first column first, then the second column, etc. If the length of the plain text is not multiple of 5 then we add the appropriate number of "z"s at the end before we encrypt (Table 1, 2). Transpositions of this type are easy to break. Since the key must be a advisor of the cryptogram length, an attacker has only to count the length of the cryptogram and try each divisor in turn (Piper & Murphy, 2002).

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