Paper Title (use style: paper title) P-ISSN : 2715-2448 | E-ISSN : 2715-7199 Vol.1 No.2, 10 July 2020 Buana Information Tchnology and Computer Sciences (BIT and CS) 42 | Vol.1 No.2, 10 July 2020 Buana Information Tchnology and Computer Sciences (BIT and CS) Combination of Hill Cipher Algorithm and Caesar Cipher Algorithm for Exam Data Security Agung Susilo Yuda Irawan 1 Study Program Technical Information Faculty of Computer Science, Universitas Singaperbangsa Karawang Email:agung@unsika.ac.id Nono Heryana 2 Study Program System Information Faculty of Computer Science, Universitas Singaperbangsa Karawang Email: nono@unsika.ac.id ‹β› Arip Solehudin 3 Study Program Technical Information Faculty of Computer Science, Universitas Singaperbangsa Karawang Email: arip.solehudin@staff.unsika.ac.id Abstract—The progress of communication technology has had a positive impact on human life, including in the field of education. The education office is currently implementing computer-based exams starting from the State Higher Education Entrance Joint Selection (SBMPTN) exam to the School Final Examination. But with the implementation of computer-based exams this is of course the less secure the level, therefore the authors make this research with the aim of securing the exam data that will be tested with Hill cipher cryptography and Caesar cipher. Cryptography is a technique of hiding data that is done to secure data, in this case cryptography aims to secure data on exam questions. Keywords : Kriptografi, Hill Cipher, Caesar Cipher Abstrak — Kemajuan teknologi komunikasi telah memberikan dampak positif pada kehidupan manusia, termasuk di bidang pendidikan. Kantor pendidikan saat ini sedang melaksanakan ujian berbasis komputer mulai dari ujian Seleksi Bersama Masuk Perguruan Tinggi Negeri (SBMPTN) hingga Ujian Akhir Sekolah. Tetapi dengan implementasi ujian berbasis komputer ini tentu saja semakin tidak aman levelnya, oleh karena itu penulis membuat penelitian ini dengan tujuan mengamankan data ujian yang akan diuji dengan kriptografi Hill cipher dan cipher Caesar. Kriptografi adalah teknik menyembunyikan data yang dilakukan untuk mengamankan data, dalam hal ini kriptografi bertujuan untuk mengamankan data pada pertanyaan ujian. Kata kunci: Kriptografi, Hill Cipher, Caesar Cipher I. INTRODUCTION Currently technology has developed very rapidly, including in the field of education, an example of application technological development that is on the exam. The education office is currently implementing a computer-based exam starting from the Joint Higher Education Entrance Examination (SBMPTN) exams to the Final School Exams. But with the implementation of computer-based exams, of course the less the level of security, then of The authors make this research with the aim of securing the data on the exam questions to be tested with Hill cipher and Caesar cipher cryptography [1]. It can be interpreted that cryptography is hidden tulidan [2]. There are several algorithms or methods on cryptography includes Hill cipher, Caesar cipher, Vernam Cipher, Advanced Encryption Standard (AES), and so forth. II. METHOD The method used in this study is the Caesar Cipher and Hill Cipher method for the process cryptography on data security exam questions, to combine the two methods there are several process that must be done. Figure 2 shows the process carried out. Fig 1 The cryptographic process of Caesar Cipher and Hill Cipher In Figure 1 can be seen the cryptographic process of Caesar Cipher and Hill Cipher methods. The first process is the problem the test (plain text) is encrypted by the Caesar cipher method and produces Cipher text. Then the cipher text converted to decimal. The result of decimal is re-encrypted using the Hill Cipher method generate Cipher text [3].To process the description or return to the original message with the process carried out it is the opposite of the encryption process, if in the first encryption process with the Caesar Cipher method then on the first description process is the Hill Cipher method [4]. III. RESULTS AND DISCUSSION The cryptographic process using the Hill Cipher and Caesar Cipher method is done by entering an example Exam 43 | Vol.1 No.2, 10 July 2020 Buana Information Tchnology and Computer Sciences (BIT and CS) questions to be encrypted and make a key. First the message will be encrypted with using the Caesar Cipher method, then the results are converted to decimal and re-encrypted with the method Hill Cipher with a single process and a key [5]. Examples of exam questions to be encrypted are English exam questions "I am so. I want to eat "with key (key) = 5. The first stage that will be done is encryption with the Caesar Cipher method. As for the process it is as follows: Plaintext = I am so . I want to eat Key = 5 Then change the plaintext and key to binary data, can be seen in Table 1. TABLE I Conversion from Plaintext to Binary Plain Text Biner I 01001001 a 01100001 m 01101101 s 01110011 o 01101111 . 00101110 I 01001001 w 01110111 a 01100001 n 01101110 t 01110100 t 01110100 o 01101111 e 01100101 a 01100001 t 01110100 Then do the encryption process by shifting the binary number 5 steps to the right, can be seen in Table 2. TABLE II Encryption Process with Caesar Cipher Biner Cipher Text 01001001 01001010 01100001 00001011 01101101 01101011 01110011 10011011 01101111 01111011 00101110 01110001 01001001 01001010 01110111 10111011 01100001 00001011 01101110 01110011 01110100 10100011 01110100 10100011 01101111 01111011 01100101 00101011 01100001 00001011 01110100 10100011 In Table 2 the encryption results obtained by the Caesar Cipher method are still in the form of binary numbers. Then do the conversion to decimal to facilitate the next encryption process. TABLE III Binary to Decimal Conversion Process Cipher Text Decimal 01001010 74 00001011 11 01101011 107 10011011 155 01111011 123 01110001 113 01001010 74 10111011 187 00001011 11 01110011 115 10100011 163 10100011 163 01111011 123 00101011 43 00001011 11 10100011 163 In Table 3 can be seen from the results of the conversion to decimal where the results will be directly encrypted with Hill Cipher method. In the Hill Cipher method, the key used is a matrix in which the matrix is used is 2x2 by using the same key in the encryption process with the method before that is 5 [6] [7]. So that the existing key can be used for the encryption process using the Hill Cipher method, the key will be formed 2x2 matrix by performing a simple calculation process [8]. Key = 5 key k = [ 𝑘𝑒𝑦 𝑘𝑒𝑦 − 1 𝑘𝑒𝑦 + 1 𝑘𝑒𝑦 + 2 ] k = [ 5 5 − 1 5 + 1 5 + 2 ] So from the above calculation results obtained 2x2 matrix key with numbers k = [ 5 4 6 7 ] Next divide the row of decimal numbers in ciphertext2 into a matrix with the number of key matrix columns (key = 2x2). [ 74 11 ] [ 107 155 ] [ 123 113 ] [ 74 187 ] [ 11 115 ] [ 163 163 ] [ 123 43 ] [ 11 163 ] Then do the key matrix multiplication with the matrix that has been made. [ 5 4 6 7 ] [ 74 11 ] = [ 414 521 ] 𝑀𝑜𝑑 255 [ 159 11 ] [ 5 4 6 7 ] [ 107 155 ] = [ 1155 1727 ] 𝑀𝑜𝑑 255 [ 135 197 ] [ 5 4 6 7 ] [ 123 113 ] = [ 1067 1529 ] 𝑀𝑜𝑑 255 [ 47 254 ] [ 5 4 6 7 ] [ 74 187 ] = [ 1118 1753 ] 𝑀𝑜𝑑 255 [ 98 223 ] 44 | Vol.1 No.2, 10 July 2020 Buana Information Tchnology and Computer Sciences (BIT and CS) [ 5 4 6 7 ] [ 11 115 ] = [ 515 871 ] 𝑀𝑜𝑑 255 [ 5 106 ] [ 5 4 6 7 ] [ 163 163 ] = [ 1467 2119 ] 𝑀𝑜𝑑 255 [ 192 79 ] [ 5 4 6 7 ] [ 123 43 ] = [ 787 1039 ] 𝑀𝑜𝑑 255 [ 22 19 ] [ 5 4 6 7 ] [ 11 163 ] = [ 707 1207 ] 𝑀𝑜𝑑 255 [ 197 187 ] In the last process of encryption change the results of this multiplication into the characters that can be seen in table 4. TABLE IV The Process of Converting Decimal to Character Decimal Character 159 Ÿ 11 VT 135 ‡ 197 Å 47 / 254 Þ 98 B 223 ß 5 ENQ 106 J 192 À 79 O 22 SYN 19 DC3 197 Å 187 » In Table 4 we can see the final ciphertext results from the Caesar Cipher and Hill Cipher method, the ciphertext in the form of ASCII numbers. So, the ciphertext from the example exam questions I am so. I want to eat is Ÿ VT ‡ Å / þ b ß ENQ J À O SYN DC3 Å ». Furthermore, a description process is carried out to find out whether this method is successful for securing data on the sample exam questions. The first thing that will be done for the description process is the Hill Cipher method by multiplying the inverse key matrix with the ciphertext block matrix. K = [ 5 4 6 7 ] 𝑑𝑒𝑡K = (5 ∗ 7) − (4 ∗ 6) = 11 invers modulo: 11-1 mod 255 11x = 1 mod 255 11x = 1+255k x = (1+255k)/11 Search for k = n with the result that x is an integer. K = 5; x = (1 + 255 * 5) / 11 = 116 (whole number) The inverse of 11 mod 255 is equivalent to 116 mod 255 which is 116. The determinant inverse modulo is used to find the martiks inverse. K = [ 5 4 6 7 ] then K-1 = determinan [ 𝑑 −𝑐 −𝑏 𝑎 ] So that K1 = 116 [ 7 −4 −6 5 ] = [ 812 −464 −696 580 ] 𝑚𝑜𝑑 255 = [ 47 46 69 70 ] Continue multiplying the matrix with ciphertext. [ 47 46 69 70 ] [ 159 11 ] = [ 797 11741 ] 𝑀𝑜𝑑 255 [ 74 11 ] [ 47 46 69 70 ] [ 135 197 ] = [ 15407 23105 ] 𝑀𝑜𝑑 255 [ 107 197 ] [ 47 46 69 70 ] [ 47 254 ] = [ 13893 21023 ] 𝑀𝑜𝑑 255 [ 123 113 ] [ 47 46 69 70 ] [ 98 223 ] = [ 14864 22372 ] 𝑀𝑜𝑑 255 [ 74 187 ] [ 47 46 69 70 ] [ 5 106 ] = [ 5111 7765 ] 𝑀𝑜𝑑 255 [ 11 115 ] [ 47 46 69 70 ] [ 192 79 ] = [ 12658 18778 ] 𝑀𝑜𝑑 255 [ 163 163 ] [ 47 46 69 70 ] [ 22 19 ] = [ 1908 2848 ] 𝑀𝑜𝑑 255 [ 123 43 ] [ 47 46 69 70 ] [ 197 187 ] = [ 17861 26683 ] 𝑀𝑜𝑑 255 [ 11 163 ] After getting the results ,, then do the conversion to binary to proceed to the next process. TABEL I Proses konversi Desimal ke Biner Decimal Biner 74 01001010 11 00001011 107 01101011 155 10011011 123 01111011 113 01110001 74 01001010 187 10111011 11 00001011 115 01110011 163 10100011 163 10100011 123 01111011 43 00101011 11 00001011 163 10100011 These binary numbers are then re-encrypted for the last time using the Caesar cipher method by shifting 5 times to the left, with the results that can be seen in Table 6. TABLE VI The process of converting Decimal to Binary 45 | Vol.1 No.2, 10 July 2020 Buana Information Tchnology and Computer Sciences (BIT and CS) Biner Cipher Text Plain Text 01001010 01001001 I 00001011 01100001 a 01101011 01101101 m 10011011 01110011 s 01111011 01101111 o 01110001 00101110 . 01001010 01001001 I 10111011 01110111 w 00001011 01100001 a 01110011 01101110 n 10100011 01110100 t 10100011 01110100 t 01111011 01101111 o 00101011 01100101 e 00001011 01100001 a 10100011 01110100 t In table 5 it can be seen that the results of the description that have been done produce a Plaintext "Iamso.Iwanttoeat" in accordance with the example of the exam questions used for this study, with this the merging of the Hill Cipher and Caesar Cipher methods has been completed. IV. 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