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INTERNATIONAL JOURNAL OF EDUCATION AND INFORMATION TECHNOLOGIES Volume 9, 2015 Principle and Computer Simulation Model of Variation of Delastell’s cipher BIFID M. Musilek and S. Hubalovsky resistance to the algorithm and subsequent programming. Abstract—An interesting possibility to develop system Rather than rewriting the mathematical task in the learning programmer thinking of students of computer science is integration of programming the new method based on introducing the digital technology to non-trivial pencil and paper cipher system. The system approach, modeling and simulation is used in learning computer support gives us the opportunity of experimenting and of students of Computer Support of Archives specialization at creative modifications of the original idea. The paper describe introducing method of system approach, modeling and computer Faculty of Art, University of Hradec Kralove (see e.g. [3] – simulation to learning of algorithm development and programming [7]). for student of Computer Support of Archives specialization. The The mentioned approach is demonstrated by case study of approach is based on creation of simulation program for encryption using of polygraphic Delastell’s cipher BIFID and its and decryption different types of ciphers. The paper describes the variation. The computer simulation of the case studies is principles of polygraphic Delastell’s cipher BIFID and its variation realized and visualized in Java Script programming language. as well as possibilities of encryption and decryption of the cipher using the computer simulation program. II. THEORETICAL BACKGROUND Keywords—Algorithmic thinking, Delastell’s cipher, education, historical encryption, programming. A. Principles of Polygraphic Substitution Cipher BIFID Polygraphic substitution cipher BIFID [8] combines I. INTRODUCTION fractionation of substitution tables with transposition. The HE ability to create mathematical model and transform it result is polygraphic substitution cipher. The specified cipher Tto algorithm as well as to computer simulation program operations are performed with a group of five digits in the develops system thinking, skills and imagination. Regarding basic variant, i.e. the substitution is clearly intended for a this fact the courses of algorithm development and group of five symbols by used encryption table (generally programming are an inseparable part of study skills of students called Polybius square) and by agreed manner of transposition specializing in “Informatics” at high schools and secondary of given numerical mid-text. Below example will clarify the schools [1]. situation. Learning of algorithm development and programming was/is The following message has to be encrypt by polygraphic often explained by the mathematical tasks, which can be ciphers BIFID: "Both men are employed on the Faculty of clearly described, defined and developed by algorithm. Science." The message has to be first rewritten to five-letter’s Altogether, the exercises are based on rewriting the groups, ignoring the spaces between the words and the last mathematical equations and formulas using algorithms and group is complete to five characters: practicing the standard algorithm. The complexity and integration of system approach to learning of algorithm BOTHM ENARE EMPLO YEDON development and programming is missing [2]. Students, who FACUL TYOFS CIENC EKLMN do not have sufficient mathematical experience, do not Encryption table (Polybius square) will be input based on understand algorithm as well as programming task. In such passwords University of Hradec Kralove, where letters I and J type of learning the students cannot see the context with are connected (as is common in English) to one field of square problems that occur in real life. Learning of algorithm – see Table 1: development escapes them, and the result is indifference or Table 1 Encryption table Stepan Hubalovsky is assoc. prof. at University of Hradec Kralove, Department of informatics, Faculty of Science, Hradec Kralove 500 38, 1 2 3 4 5 Rokitanskeho 62, Czech republic, [email protected]. 1 U N I/J V E Michal Musilek is assistant professor at University of Hradec Kralove, Department of informatics, Faculty of Science, Hradec Kralove 500 38, 2 R S T Y O Rokitanskeho 62, Czech republic, [email protected]. 3 F H A D C ISSN: 2074-1316 186 INTERNATIONAL JOURNAL OF EDUCATION AND INFORMATION TECHNOLOGIES Volume 9, 2015 A B C D E F G Table 2 Frequency of characters in plain text 382 104 132 248 646 114 97 H I J K L M N The effectiveness of encryption algorithm may be calculated 324 323 0 32 194 99 326 based on index of coincidence. The index of coincidence was introduced to cryptanalysis by William Frederick Friedman [9] O P Q R S T U (1891-1969). If the frequencies of individual letters of the 370 106 3 300 302 432 130 alphabet is ni, the number of different characters forming the V W X Y Z alphabet k and total number of characters of the analyzed 56 114 11 72 2 text N, then the index of coincidence define IC is given by 4 K L B G M formula (1): 5 P Q W X Z k First, the text will be encrypt into to numeric mid-text: nn ii −⋅ )1( IC = (1) ∑ i=1 NN −⋅ )1( BOTHM ENARE EMPLO YEDON 42234 11321 14542 21321 The approximate formula is used in cryptoanalysis that 35325 52315 55125 45452 gives a more accurate value of the IC, for long analyzed text. We have also used the following approximate formula, FACUL TYOFS CIENC EKLMN because we analyzed the texts of length of the thousands of characters. The value of pi is the relative frequency (posteriori 33314 22232 31113 14441 th 13512 34512 53525 51252 probability) of occurrence of the i character of alphabet: The cipher text is reached from the mid-text in the second k = pIC 2 phase of encryption. The principle of the creation of the cipher ∑ i (2) from the mid-text is as follows: i=1 Twice two letters are taken from the first row of five-digit group; Another variable that we can be used for measurement of Then last letter of the first row is connected with the first the effectiveness of an encryption algorithm informatics letters of the second row; entropy is so called Shannon entropy Claude Elwood Shannon Finally twice two letters are taken from the second row. (1916-2001)). The index of coincidence of the information For the second phase of the encryption the same table as in entropy H is defined as follows: the first phase will be used: k ⋅−= log ppH 42234 11321 14542 21321 ∑ i 2 i (3) 35325 52315 55125 45452 i=1 LTKWO UHETE VXOPO RHVXQ Where pi is again the relative frequency of occurrence of the ith character of alphabet. 33314 22232 31113 14441 The index of coincidence for given plaintext (Oliver Twist 13512 34512 53525 51252 by Charles Dickens) is IC = 0.0657, informatics entropy text is AFKCN STTMN FUCCO VGENQ H = 4.16. After application of the above procedure to BIFID cipher It is clear from the example that this type of encryption is and to Polybius square obtained by using password University more complicated than the encryption of other substitution of Hradec Kralove, the frequency of the character is shown in ciphers, (simple substitution, bigram substitution of type Table 3. Playfair cipher or Four-square cipher). The complexity of the BIFID cipher can be proved by Table 3 Frequency of characters in cipher text increasing of information entropy of the ciphertext, e.g. if this cipher is generally worse decipherable. A B C D E F G 316 129 179 125 189 246 50 B. Index of Coincidence and Information Entropy H I J K L M N The text of the first chapter of the novel Oliwer Twist by 427 288 0 111 109 83 326 Charles Dickens has been chosen to calculate the information O P Q R S T U entropy. Table 2 shows frequency analysis of the plain text. 269 184 137 238 290 392 223 V W X Y Z 135 137 71 147 118 ISSN: 2074-1316 187 INTERNATIONAL JOURNAL OF EDUCATION AND INFORMATION TECHNOLOGIES Volume 9, 2015 Table 6 Encryption table for Czech alphabet The index of coincidence of the ciphertext is IC = 0.0500, text informatics entropy increases to H = 4.47. For 1 2 3 4 5 6 A L E X N D comparison, the plaintext was encrypt by bigram substitution 1 cipher Playfair using the same Polybiova squares. The resulted 2 R V I K Y ! ciphertext has length of 5080 characters whose extension was 3 B C Č Ď F G due to completion of double consonants. The index of 4 H J K L M Ň coincidence is H = 0.0524 and informatics entropy text is 5 O P Q Ř S Š H = 4.39, which is consistent with the expectation that bigram 6 T Ť U V W Z substitution will have for the same plaintext higher index of coincidence and lower informatics entropy than polygrams For both tables was chosen password "Александр substitution. Великий", respectively "Alexander Veliký“. Ideally random text, which uses 25 English letters (the letter Other variations of Polybius cipher are based on J is not present even in our chosen plaintext, nor in any of the permutations of used transposition. The letters can be cipher text), is minimal coincidence index IC = 1/25 = 0.0400 associated not only horizontally but also upward and and informatics maximum entropy H = log225 = 4.64. downward – see [*]. The message “Both men are employed on Let us summarize the values of the statistical characteristics the Faculty of Science." Is in this case encrypt as follows: of the four previously mentioned texts in Table 4. • The first phase is in all three cases the same.
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