=Paper=
{{Paper
|id=Vol-3200/paper2
|storemode=property
|title=Using the Sum of Real Type Functions to Encrypt Messages
|pdfUrl=https://ceur-ws.org/Vol-3200/paper2.pdf
|volume=Vol-3200
|authors=Viktor Avramenko,Mykyta Bondarenko
}}
==Using the Sum of Real Type Functions to Encrypt Messages==
Using the Sum of Real Type Functions to Encrypt Messages Viktor Avramenko1, Mykyta Bondarenko2 1,2 Sumy State University, Rymskogo-Korsakova st. 2., Sumy, 40007, Ukraine Abstract This paper presents a symmetric key cryptosystem using the sum of real type functions which allows to increase the cryptographic strength. Both transmitter and receiver choose Key Functions with the same argument, the interval for setting the argument, and the step for changing it. The symbol of the transmitted message is encrypted in an array where each element is the sum of Key Functions with random amplitudes. This sum includes those Key Functions for which the corresponding bit is one. Decryption uses disproportion functions. The system is suitable for encrypting both discrete and continuous messages. Keywords 1 Cryptosystems, disproportion functions, function of real variable, key functions, encryption, decryption, text messages 1. Introduction systems, but it’s not promising due to the intensive development of the quantum computers [7], which will significantly affect the Widely used cryptosystems are based on the cryptographic strength of existing cryptosystems set of integers. They implement symmetric and [8]. The ordinal brute force has complexity O(2k), asymmetric encryption algorithms. In symmetric meanwhile Grover's quantum algorithm [9] systems, the same key is used for both encryption reduces it to O(2k/2) [9]. and decryption. The most famous symmetric Implementing quantum algorithms will also systems are AES [1] and GOST 28147-89 [2, 3]. reduce the robustness of asymmetric systems. The To hack such a system, an enumeration of RSA system uses the super polynomial possible keys is required. The brute-force computational complexity of the factorization of complexity is O(2k), where k is the key length in natural numbers. At the same time, there is a bits. For symmetric systems, if the quantum algorithm whose complexity is communication channel is open, there is a polynomial O(n3) [10]. It means the cryptographic problem of secure key transmission. This problem strength of asymmetric systems can be reduced as does not exist for asymmetric open key systems. a result of the implementation of Shor's quantum In these systems, the most widely used algorithms algorithm for computing the discrete logarithm. In are RSA and El-Gamal [4, 5]. The RSA algorithm [11], Shor's algorithm is given for the group of is based on the computational complexity of the points of an elliptic curve over the field GF(p) integer factorization problem. El-Gamal's with complexity O(n3). Implementing quantum algorithm is based on the difficulty of computing algorithms will also reduce the robustness of the discrete logarithm, especially over a group of asymmetric systems. A method for increasing the points of an elliptic curve [6]. For breaking crypto resistance of the system under these asymmetric cryptosystems, there are conditions is proposed in [12]. Along with the cryptanalysis methods which are faster than full search for ways to hack cryptosystems, methods search. This circumstance makes it necessary to for detecting signals of means of secretly use longer keys compared to keys in symmetric III International Scientific And Practical Conference “Information Security And Information Technologies”, September 13–19, 2021, Odesa, Ukraine EMAIL: vv.avramenko@cs.sumdu.edu.ua (A.1); nikbond97@gmail.com (A. 2) ORCID: 0000-0002-6317-6711 (A. 1); 0000-0002-8849-7378 (A. 2) © 2021 Copyright for this paper by its authors. Use permitted under Creative Commons License Attribution 4.0 International (CC BY 4.0). CEUR Workshop Proceedings (CEUR-WS.org) obtaining information are also being developed The cryptosystems [15-18] in the process of [13]. computer modeling have shown high The above analysis shows that one should look cryptographic strength when trying to guess the for other ways to create cryptosystems. In parameters of keys functions, even if their form is particular, to complicate the selection of keys known. To further complicate the work of using the simple enumeration method, one should cryptanalysts, the task is to develop a switch from using integers to real ones. It is cryptosystem that could combine the advantages known [14] the set of real numbers has a higher of the systems considered in [15-17] and the cardinality compared to the set of natural system [18]. So, it’s necessary to develop the numbers, so one can expect the cryptographic algorithms for encryption and decryption of strength of a cryptosystem based on real numbers analog and discrete messages, using several will be higher. The possibilities of creating functions of a real variable as keys without the cryptosystems using one or more functions of a necessity to calculate derivatives. real variable as keys are considered in [15-18]. So, in [15], characters from the ASCII code 2. Mathematical formulation of the table are encrypted by the sum of 10 functions of a real variable, which are keys. Each key-function problem is preceded by a coefficient, which, depending on the character being encrypted, is equal to zero or The message that is encrypted is a sequence of one. The amplitudes of these functions are random T numeric character codes from the ASCII table for each new symbol. The resulting sum of the (or numeric values of the pixel brightness values of the functions is transmitted over the components in the case of a graphic image communication channel. On the receiving side, transmission). Each of them is encrypted using fragments of key functions are recognized, which one-dimensional arrays of length N values. These are represented in the received encrypted signal. arrays are obtained using one and the same step h This allows you to decrypt the symbol transmitted of changing the argument of m Key Functions of at the current time using the disproportion the real variable. In this case, the value y(j, i) of functions [19-22]. the matrix y(T, N) has the form: In [16, 17], a variant is proposed when , , (1) symbols for transferring binary codes are encoded with the help of three key functions of a real where: variable. "1", "0", "space", "new line" are j is the number of the character in the transmitted encoded. Any other character is recognized as a message; new line. For unauthorized access to the fq(i) = fq(ih), (i = 1, 2, … N > m), (q = 1, 2, … m) intercepted message, you need to select the type - an array of values of the q-th Key Function; and parameters of the key functions. kqj - coefficients that are generated during In [15-17], the disproportion functions over the encryption of the j-th element and can be either first-order derivative were used. In this case, it is equal to zero or represent random numbers which necessary to apply numerical methods for are unknown to the recipient. calculating the current values of the first Key Functions can be either continuous or derivatives. The need for these calculations led to discrete. These functions should be the same for the fact that the ciphertext significantly exceeded the transmitting and receiving sides and have the the length of the encrypted message. same numbering. Also, the step h of changing the A completely different encryption principle argument of the Key Functions should be the was proposed in [18]. One function of the real same. An encrypted message in the form of a variable is used as the key. The disproportion matrix y(T,N) is transmitted over an open function of the numerical representation of the communication channel. The task is to decrypt the encrypted process is calculated with respect to the message using the matrix received at the receiving key function. The obtained values of the end. To solve it, the integral disproportion of the disproportion function are an encrypted message first order is used [23]. and are transmitted over the communication channel. To avoid calculating the derivatives, the integral disproportions of the first order is used 3. Disproportion functions [23]. One of the first publications in which In this case, y(x) and f(x) are represented by disproportion functions were proposed was [19]. one-dimensional arrays. If the approximate values In particular, the disproportion with respect to the of the integrals in (6) are calculated using the n-th-order derivative of the function y(x) with trapezoid formula, then for the one and the same respect to x is described by the expression: step h for y(x) and f(x), disproportion (6) takes the 1 following form (7): (2) @ ⋅ , @ , (7) ! Here the @ symbol is chosen to denote the operation of calculating disproportion. The symbol "d" stands for "derivative". The order is 4. Encrypting and decrypting indicated in parentheses. The left part (2) reads "et messages d n y with respect to x". The order n ≥ 1 is an integer. If for any value of x, the function y(x) has The transmitting and receiving sides must have the form y = kxn, then disproportion (2) is equal to the same system of m Key Functions of the real zero regardless of the value of the coefficient k. variable, their numbering, the interval of changing For the case when n = 1, the argument and step h of its change. The number (3) @ , of elements N of the one-dimensional array corresponding to the encrypted character must For defining the functions parametrically, also be set. These can be both characters from the when, x = φ(t), y = ψ(t), where t is a parameter, ASCII table, and components of pixel brightness disproportion (3) is described by the expression when transmitting color graphic images. Each of (4) them is represented by an integer. The required @ , number of Key Functions depends on the maximum value of this number. For example, to encrypt characters from the ASCII table, m = 8 For ψ(t) = kφ(t) disproportion (4) is equal to Key Functions are required. They can be either zero in the entire area of existence x = φ(t), continuous or discrete. If the Key Functions are regardless of the value of k. In [19], the case was continuous, it is necessary to calculate N elements considered when of one-dimensional arrays of their values, ⋯ (5) changing the argument from the initial xmin to the , final xmax value with a step h. When encrypting where f1(x), f2(x), … fm(x) are known characters from the ASCII table or the pixel functions; k1, k2, … km are coefficients whose brightness, their numerical representations differ values are unknown. by one. In these cases, the step h of changing the It is shown that the disproportion functions argument must be equal to one. allow calculating the values of the unknown An m-bit binary code corresponds to each coefficients in (5) from the data obtained for the encrypted character. Each bit in this code is current value of the argument. This opportunity is associated with a specific number of the Key used to create cryptosystems [15-17]. Function. If the bit is zero, the value of the In practice, often the first derivative of the corresponding Key Function is also zero. If the bit function does not exist or is equal to zero on some is equal to one, then a random value of the interval. This excludes the possibility of using amplitude of the corresponding Key Function is disproportions over the first-order derivative (2- played. The character to be encrypted is 4). In this case, it is advisable to use the integral represented by the sum (1). disproportion of the first order [23]. This disproportion of the function y(x) with respect to 4.1. Encrypting messages f(x) has the form: (6) 1. The following is a character encryption @ , algorithm: 2. Calculate arrays of N > m values of Key where h - is the preset time interval. In the discrete Functions: fq(x), q = 1, 2… m. representation of signals, this is a time 3. Enter the encrypted j-th character and quantization step. calculate its cipher in the form of values of the one-dimensional array y(j, i), i = 1, 2... N , @ , (13) , according to (1). , 1 , , 4. Repeat this point for all characters of the , message of length T. , 1 , 5. A sequence of T arrays is an encrypted message transmitted over an open communication Taking into account that the disproportion of channel. F21(j, i) with respect to F21(j, i) is equal to zero, we get: 4.2. Decrypting messages F j, i k j, i , (14) Third level: The disproportion of F0121(j, i) Pre-compute the arrays fq(i) = fq(ih), with respect to F3121(j,i) is calculated in the (q = 1, 2, ... m), (i = 1, 2, ... N > m), of Key following way Functions and to receive T one-dimensional , @ , (15) arrays y(j , i), j = 1, 2, ... T, i = 1, 2, ... N over the , communication channel. Further, in order to , 1 , simplify the description of the decryption process, , 1 , an example is given when only three functions are , used in the cryptosystem - the keys: f1(x), f2(x), 0, f3(x). In this case m = 3. Accordingly, the j-th It is equal to zero because, as can be seen from character of the message is encrypted as (14), there is a proportional relationship between , , (8) F0121(j, i) and F3121(j, i). This fact allows 1, 2, … 3, calculating from (14) k3j and k2j, k1j for the j-th The process consists of m = 3 levels in message symbol. accordance with the number of Key Functions. , (16) First level: It is necessary to calculate the array , of disproportions (7) y(j,i) with respect to any of the Key Functions , for example, f1(i): , @ , (9) , , (17) , , 1 , , , 1 where i = 2, 3…N. , (18) , Also calculate the disproportions (7) of the remaining key functions with respect to f1(i): Depending on which of these coefficients are , @ , (10) nonzero and which are equal to zero, the j-th , 1 , , message symbol is decrypted. In practice, it must , be taken into account that there are calculation 1 errors. where r = 2, 3. Therefore, it is necessary to compare the Considering that the disproportion of the disproportion (15) calculated at the last level in function relative to itself is zero, we get: modulus not strictly with zero, but with an , , , , (11) approximate number ɛ. For example, it could be Second level: It is necessary to select any ɛ=10-4. In this case, if | F01213121(j, i) | <= ɛ, then it disproportion from right-hand of (11), for should be assumed that it is zero. example F21(j, i). It is used to calculate next The value of ɛ is determined during testing of disproportions: the cryptosystem. Theoretically, this , @ , (12) disproportion is equal to zero for all i = 2, 3, ... N, , , 1 , , but, taking into account the calculation errors, it is , recommended to do calculations using formulas , 1 , (16-18) for i, at which the modulus of disproportion (15) is minimal. 4.3. An example of encrypting and select the form of eight Key Functions and the values of their parameters. decrypting characters from an ASCII table Table 1 Encrypted and decrypted characters “Hello” Eight Key Functions are used (m =8): y ‘H’ ‘e’ ‘l’ ‘l’ ‘o’ 1. f1(x) = 1000 sin((α1 - β1)x) cos(wβ1x) 0 1 ‐323.36050 257.702939 ‐1096.0141 167.391848 ‐872.47149 1051.01033 37.134528 532.400561 ‐112.93721 427.740614 2. f2(x) = 1000 exp(0.1α2x) sin(wβ2x) 2 57.298613 175.907791 ‐408.37541 ‐216.26334 ‐116.65218 3 ‐165.32821 126.358160 ‐324.75198 ‐162.19800 ‐197.22270 cos((α2 + β2)x) 4 ‐186.82906 ‐394.77504 ‐929.02530 ‐439.94548 ‐449.01146 3. f3(x) = 1000 exp(-α3x) sin(wβ3x) 5 6 ‐163.70378 37.446166 ‐392.33753 299.880685 ‐1059.2853 370.310135 ‐385.85981 74.675455 ‐170.75848 44.746439 4. f4(x) = 1000 cos((α1x - β1)x) sin(wβ1x) 7 8 ‐110.70494 ‐2.714026 426.787248 ‐59.278796 ‐218.90900 115.564604 ‐238.68403 ‐2.371129 ‐314.75860 ‐165.23427 5. f5(x) = 1000 exp(0.1sin(α2x)) sin(w 9 9.436954 152.916970 116.697501 ‐23.361501 281.220083 10 42.465400 ‐412.02347 ‐203.82595 84.424717 150.029168 cos(βx)) cos((α2+β2) x) 11 ‐24.295297 615.002251 349.117675 ‐42.371452 ‐231.55086 6. f6(x) = 1000 sin(-cos(α3x)) cos(w 12 13 101.570285 195.422364 95.754178 132.219196 543.764259 855.514365 227.452661 432.359247 ‐122.68102 754.345004 sin(β3x)) 14 15 ‐11.067358 214.445079 ‐507.14921 264.682389 ‐252.27430 659.731238 ‐29.696351 467.369970 228.457327 ‐63.039751 7. f7(x) = 1000 sin(wx + α1) exp(-β1x2) 8. f8(x) = 1000 cos(wγx2) Table 2 where α1 = 1, α2 = 0.12, α3 = 0.5, β1 = 0.1, β2 = 1.5, β3 = 0.7, γ = 0.5, w = 400 are constants. Encrypted and decrypted characters ‘A’ y ‘A’ ‘A’ ‘A’ ‘A’ ‘A’ A sequence of numbers corresponding to the 0 ‐597.135343 ‐762.540347 ‐609.489179 ‐1245.052456 ‐917.400855 transmitted characters from the ASCII code table 1 9.473961 ‐6.667141 ‐2.760240 ‐37.310266 ‐17.407304 is encrypted. Each character is encoded by a sum 2 3 274.695430 15.193014 368.229254 87.216540 291.933312 60.428145 625.796927 237.898008 451.736269 138.849052 of Key Functions 4 5 ‐123.497929 ‐58.830278 ‐217.377766 ‐107.584825 ‐165.579209 ‐81.548078 ‐438.953490 ‐221.367775 ‐291.370618 ‐145.669069 y x x x x (19) 6 ‐8.280530 ‐11.911812 ‐9.337867 ‐21.332769 ‐14.999968 7 199.488556 342.700766 261.876769 683.403833 456.291669 x x 8 ‐76.316724 ‐131.227388 ‐100.265637 ‐261.818697 ‐174.769533 x x 9 10 ‐60.158608 ‐125.104506 103.377062 ‐215.011307 78.993047 ‐164.292499 ‐206.183725 ‐428.868345 137.653657 ‐286.313719 x , 11 12 214.641127 ‐14.047252 368.892513 24.142272 281.874942 18.447384 735.803375 ‐48.154847 491.224813 ‐32.148341 where: 13 14 6.530344 ‐223.052958 11.223364 ‐383.349601 8.575899 ‐292.921754 22.386440 ‐764.640006 14.945262 ‐510.476209 x = ih – argument; 15 169.891087 291.983039 223.107534 582.397666 388.810617 h = 1 – step of changing the argument. i – is the ordinal number of the element of the Below is an example that illustrates the one-dimensional array for each of the Key resistance of the system obtaining keys, even if Functions, as well as the array y0, y1, ..., yN-1, somehow it was possible to find out the forms of which is the character cipher; Key Functions. Suppose that the above sequence N – a number of elements of each one- of characters is encrypted using functions (7), and dimensional array. Based on the requirement of decrypted using the same kind of functions, but N > m, the amount of array elements N = 16. the constant w was guessed incorrectly. Instead of Table 1 shows the transmitted characters in the w = 400 was used w = 399.999 during decryption. upper horizontal line. The corresponding ciphers In this case, the disproportion at the last eighth are given in the form of arrays arranged vertically. level in absolute value exceeds the permissible The decrypted characters are located horizontally deviation ɛ from zero. That is, decryption is on the bottom line. impossible. Only if w = 399.9999, the message It is obvious that the received message matches may be decrypted. This result shows that even the transmitted one. It should be noted that the such a slight deviation of one of the parameters of ciphers (arrays) of the adjacent symbols `t` are the Key Functions does not allow decryption of completely different. the transmitted character. The codes of the other adjacent identical symbols in the message are given in Table 2. The 5. Requirements for Key Functions above results indicate that the ciphers of the adjacent identical symbols in the message differ from each other. This circumstance greatly 1. The Key Functions must be of real type. complicates the "hacking" of the cryptosystem. In 2. They can’t be constant and must not take order to "crack" the message, it is required to zero values. 3. When using the key function, there should be no situation where division by a number close to zero occurs, which leads to an [1] National Institute of Standards and unacceptable calculation error. 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