Design and Implement of a Homogeneous Multiple Block Cipher Algorithm

Prof, Assist.; O. Ibadi Ph.D, Abdulkareem

	Design and Implement of a Homogeneous Multiple Block Cipher Algorithm
Baghdad College of Economic sciences University
Article 1, Volume 20140429, Issue 0, 2013, Pages 489-501
Authors
Assist. Prof; Abdulkareem O. Ibadi Ph.D
Abstract
Cryptography (or cryptology) is a discipline of mathematics and computer science concerned with information security and related issues, particularly encryption and authentication and such applications as access control [6]. Prior to the early 20th century, cryptography was chiefly concerned with linguistic patterns. While it now makes extensive use of mathematics, including topics from information theory, computational complexity, statistics, combinatory, and especially number theory. The history of cryptography can be divided into three stages they are: • In the first stage, algorithms had to be implemental with paper and ink such as substitution ciphers and transposition ciphers. The encryption process can be defined as E(M), where M is the plaintext Decryption is done using the map , where C is the ciphertext. Such schemes are not very secure. The system can be strengthened in various ways, but none too effective. • The second stage of cryptography was that of cryptographic engines. This is associated to the period of the World War II, and the most famous crypto engine was the German Enigma machine. • The last stage is modern cryptography. Its central feature is the reliance on mathematics and electronic computers. Computers enabled the use of much more sophisticated encryption algorithms, and mathematics told us how to design them. It is during this most recent stage that cryptography becomes much more a science. The modern stage of cryptography can be broken down into two areas of study: Symmetric-key and Public-key. Symmetric-key cryptography refers to encryption methods in which both the sender and receiver share the same key. The modern study of symmetric-key ciphers relates mainly to the study of block ciphers and stream ciphers and to their applications. A block cipher take as input a block of plaintext data and a key, and output a block of ciphertext data of the same size. The main drawback of symmetric ciphers is that the two communicating parties must share a secret key: it is not easy to ensure that the secret is not compromised during transport. This drawback leads to the invention of Public-key cryptographic systems in 1976 by Whitfield Diffie and Martin Hellman in which two different but related keys are used: one for encryption and one for decryption (public-key cryptography is also called asymmetric-key cryptography because of the difference between the keys). Good cryptographic systems should always be designed so that they are as difficult to break as possible. It is possible to build systems that cannot be broken in practice (though this cannot usually be proved). There is no excuse for a system designer to leave the system breakable. Any mechanisms that can be used to circumvent security must be made explicit, documented, and brought into the attention of the end users. In theory, any cryptographic method with a key can be broken by trying all possible keys in sequence. If using brute force to try all keys is the only option, the required computing power increases exponentially with the length of the key. A system with 56-bit keys, such as DES, requires a substantial effort, but using massive distributed systems requires only hours of computing. It is currently believed that keys with at least 128 bits (as in AES, for example) will be sufficient against brute-force attacks into the foreseeable future. However, key length is not the only relevant issue. Many ciphers can be broken without trying all possible keys. In general, it is very difficult to design ciphers that could not be broken more effectively using other methods. The strength of the algorithm must be depends on the secrecy of the key rather than the secrecy of the algorithm. Generally, no algorithm that depends on the secrecy of the algorithm is secure. For professionals, it is easy to disassemble and reverse-engineer the algorithm. The keys used in public-key algorithms are usually much longer than those used in symmetric algorithms. There the problem is not that of guessing the right key, but deriving the matching private key from the public key. It should be emphasized that the strength of a cryptographic system is usually equal to its weakest link. No aspect of the system design should be overlooked, from the choice of algorithms to the key distribution and usage policies. There are 2n possible different plaintext blocks (where n is the block length) and, for the encryption to be reversible (i.e., for decryption to be possible), each must produce a unique ciphertext block. Such a transformation is called reversible, or nonsingular. Figure 1 illustrates the logic of a general substitution cipher for n = 4. A 4-bit input produces one of 16 possible input states, which is mapped by the substitution cipher into a unique one of 16 possible output states, each of which is represented by 4 ciphertext bits. The encryption and decryption mappings can be defined by a tabulation. This is the most general form of block cipher and can be used to define any reversible mapping between plaintext and ciphertext. Feistel refers to this as the ideal block cipher, because it allows for the maximum number of possible encryption mappings from the plaintext block [1]. If a small block size is used then the system is more equivalent to a classical substitution cipher and is vulnerable to a statistical analysis of the plaintext. If block size is sufficiently large then the statistical characteristics cryptanalysis is infeasible. The ideal block cipher for a large block size is not practical because the mapping itself constitutes the key[1]. Feistel proposed that product cipher can utilize the concept of ideal block. Product cipher can be defined as the execution of two or more simple ciphers in sequence in such a way that the final result or product is cryptographically stronger than any of the component ciphers. The product cipher uses a block cipher with a key length of k bits and a block length of n bits, with 2k possible transformations, rather than the 2n transformations available with the ideal block cipher. Feistel proposed using substitutions and permutations in a product cipher.

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