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An Efficient Algorithm for Polyalphabetic Substitution Using Infinite Number of Alphabetical Tables

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An Efficient Algorithm for Polyalphabetic Substitution Using Infinite Number of Alphabetical Tables International Journal of Applied Engineering Research ISSN 0973-4562 Volume 13, Number 4 (2018) Spl. © Research India Publications. http://www.ripublication.com An Efficient Algorithm for Polyalphabetic Substitution Using Infinite Number of Alphabetical Tables Ms. Ranju S. Kartha1 Research Scholar, School of Computer Science, Mahatma Gandhi University, P.D. Hills, Kottayam, Kerala, India Dr. Varghese Paul2 Department of Information Technology, RSET, Cochin, Kerala, India Abstract to send messages so that no one, but the expected recipient Many cryptographic algorithms are available for providing a can read. Cryptanalysis is trying to break the security of the secure communication. These cryptographic algorithms can be communication and read those messages. classified according to their encrypting process. They are Encryption is the act of taking a message called substitution cipher and transposition cipher. Polyalphabetic plaintext and applying an operation called a cipher so that we cipher is based on substitution technique, where the plaintext can receive an unreadable message as the output called letters are encrypted depending upon their placement in the ciphertext. The reverse process taking the unreadable output text and the keyword. Vigenere cipher is one of the most and transforming back into a readable plaintext is called efficient Polyalphabetic substitution cipher. But it is decryption. There are two main components in encryption: vulnerable to attacks because of its repeating nature of the cipher and the key. Cipher means the set of rules that we are keyword. To overcome this, here we are presenting a new using to encode the message. And the key tells how to arrange cipher which uses multiple random tables (26x26) for those rules. Otherwise, the rule will be same at all the time encryption. In this proposed cipher, the keyword is repeating and anybody can decrypt the message very easily. For until it matches with the plaintext length. But here, whenever decryption, we need the cipher which we used and the key. the keyword repeats, this cipher generates a 26x26 random Usually, the attacker cracks the code by trying all possible table for encryption. Also, each table will be completely combinations of the keyword or analyzing the code by independent of the previous table. The cryptanalysis of working backward from it. If it is not possible to determine Vigenere cipher does not help the crackers to break this code. the combination of cipher and the key we can say that the So this proposed Polyalphabetic cipher is unbreakable. code is an unbreakable one. People keep coming up with new and better ciphers but it is hard to make them unbreakable. Keywords: Polyalphabetic Cipher, Vigenere Cipher, The Substitution cipher is a classical method of Vigenere Table, Kasiski Method, Index of Coincidence IC. cryptography; it replaces every plaintext letter by a corresponding ciphertext letter. One of the oldest and simplest Introduction ways of encrypting the message is Caesar cipher. In this case, Data is the heart of all organizations and the protection of this the key is just a number representing how many letters of the data is even more important. Hackers are smarter than ever alphabet we shift it. For example, in a Caesar cipher, if the and the computer programs to steal data are easier to obtain key is 3, A would become D, B would become E and so on. than ever. So data should be handled securely at all stages, The shift operation is performed modulo 26 [9]. But it is easy from transmission, computation, and persistence. Modern to crack, even if we didn't know the key, we can do 25 tries to societies, we are heavily dependent on information and get the message. Caesar cipher is one simple type of communication technologies. Thus there is a growing concern Monoalphabetic cipher. Monoalphabetic cipher is each letter for privacy and data security. In today's world, there are many in the plaintext is replaced with another letter of the alphabet. applications such as secret message transferring in military The mapping between plaintext and ciphertext is always one systems, payments to private organizations, personal emails, to one mapping. data storage in personal devices and secure exchange of There are lots of ways to decrypt the message. In password, where the information security issues are more Monoalphabetic cipher, there are 26! possible keys, so Brute challenging and complex. One of the methods for ensuring Force attack becomes infeasible. The most sophisticated security is Cryptography- deals with hiding the real technique for the cryptanalysis of Monoalphabetic cipher is information. called frequency analysis. It is based on the language we used Here the main term is Cryptology; it is a science and for encryption. In English, e is the most common letter, the secret communication involves cryptography and followed by t, then a and so on [10]. The cracker can calculate cryptanalysis. Cryptography is the science and study of hiding the occurrence of the letters appears in the ciphertext and information text to make them secure and immune to attack. relates them to the frequency of occurrence of the language The word cryptography is a Greek term 'crypto' means secret we used. and 'graphy' means writing. The basic goal of cryptography is 14 International Journal of Applied Engineering Research ISSN 0973-4562 Volume 13, Number 4 (2018) Spl. © Research India Publications. http://www.ripublication.com Polyalphabetic Cipher Vigenere Table is a 26x26 table with A to Z as row Polyalphabetic cipher uses multiple Monoalphabetic cipher heading and column heading. Each row comprises all the 26 substitutions so that same plaintext alphabet mapped into alphabets of English. The first row has 26 letters in alphabetic different alphabets. And the key is used to specify the order. From the second row, each row has the letters shifted to mapping. Polyalphabetic substitution ciphers, the plaintext left by one position in a cyclic way. For example suppose the letters are enciphered differently depending upon their message to be encrypted is CRYPTOGRAPHY and uses the placement in the text [2]. For encryption, the plaintext is keyword TIME, here the keyword repeats until it is the same divided into different groups, each group having m elements length as the plaintext. where m is the length of the keyword. Here the keyword repeats until it is equal to the length of the plaintext. The Table 1: Encryption Process elements in each group are encrypted using the corresponding keyword letter. If the same letter is repeating in a group, it Plaintext C R Y P T O G R A P H Y will be encrypted as a different element, depending upon the Keyword T I M E T I M E T I M E key letter. The Polyalphabetic ciphers have the advantage of Ciphertext V Z K T M W S V T X T C hiding the letter frequency of the underlying language [10]. So the cracker cannot use single letter frequency analysis to break And decryption is performed by using the letter in the the code. keyword to select the row index and find the position of the Let the plaintext P = P1, P2, P3… Pn is encrypting using the ciphertext letter in the corresponding row. The letter resides key stream K =K1, K2, K3 …Km, and then the encryption and on the top of the column that contains ciphertext letter will be decryption algorithm can be defined as: the decrypted plaintext letter [7]. The number of possible Encryption: Ci = Pi + Ki (mod m) (mod26) solutions for this cryptosystem grows with the length of the Decryption: Pi = Ci + Ki (mod m) (mod26), (1) text by a power of 26. The strength of this cipher is that the where Ci = C1, C2, C3 …Cn is the ciphertext [12]. The best same letter in the plaintext can be encrypted in different ways well-known algorithm in Polyalphabetic cipher is referred to depending on the key we used. Vigenere cipher is one of the as Vigenere cipher. great breakthroughs in the world of cryptography; it was unbreakable for hundreds of years. Existing System Blaise de Vigenere a French cryptographer of the 1500s Cryptanalysis of Vigenere Cipher created this cipher called as Vigenere Cipher. Vigenere cipher Cryptanalysis of classical ciphers is made possible because of has a similar method of encryption as Caesar cipher but uses a the redundancy in the linguistic structure of natural languages far more complex encryption key. In this method, the plaintext [5]. In the Monoalphabetic substitution cipher, the most is encrypted using a table of alphabets, termed as Vigenere frequent letters in the ciphertext correspond to the most Tableau, Vigenere square, or Vigenere Table. frequent letters in the plaintext. So the cracker can easily break the code by performing frequency analysis on the letters in the ciphertext. The Vigenere cipher masks the frequency with which a character appears in a language, which makes the use of frequency analysis more difficult [4]. The frequency distribution of the ciphertext is much more flat. The primary weakness of the Vigenere cipher is the repeating nature of its key [8]. Vigenere cipher is easily broken if the cracker discovers the length of the keyword. So the security of this cipher relies on having the key length unknown. Suppose the length of the keyword is m, once it is known to the cracker, he can split the ciphertext into a different block of size m. Every mth character of the ciphertext is encrypted using the same shift. So the cracker can write these blocks into a matrix in a way that each row is filled with letters of each block. Now the letters in the column have been encrypted using the same key. The cracker can easily break this code by performing frequency analysis on each column.
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