# Mathematics of Cryptography Part I

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1 CHAPTER 2 Mathematics of Cryptography Part I (Solution to Odd-Numbered Problems) Review Questions 1. The set of integers is Z. It contains all integral numbers from negative infinity to positive infinity. The set of residues modulo n is Z n. It contains integers from 0 to n 1. The set Z has non-negative (positive and zero) and negative integers; the set Z n has only non-negative integers. To map a nonnegative integer from Z to Z n, we need to divide the integer by n and use the remainder; to map a negative integer from Z to Z n, we need to repeatedly add n to the integer to move it to the range 0 to n The number 1 is an integer with only one divisor, itself. A prime has only two divisors: 1 and itself. For example, the prime 7 has only two divisor 7 and 1. A composite has more than two divisors. For example, the composite 42 has several divisors: 1, 2, 3, 6, 7, 14, 21, and A linear Diophantine equation of two variables is of the form ax + by = c. We need to find integer values for x and y that satisfy the equation. This type of equation has either no solution or an infinite number of solutions. Let d = gcd (a, b). If d does not divide c then the equation have no solitons. If d divides c, then we have an infinite number of solutions. One /. of them is called the particular solution; the rest, are called the general solutions. 7. A residue class [a] is the set of integers congruent modulo n. It is the set of all integers such that x = a (mod n). In each set, there is one element called the least (nonnegative) residue. The set of all of these least residues is Z n. 9. A matrix is a rectangular array of l m elements, in which l is the number of rows and m is the number of columns. If a matrix has only one row (l = 1), it is called a row matrix; if it has only one column (m = 1), it is called a column matrix. A square matrix is a matrix with the same number of rows and columns (l = m). The determinant of a square matrix A is a scalar defined in linear algebra. The multiplicative inverse of a square matrix exists only if its determinant has a multiplicative inverse in the corresponding set. 1

2 2 Exercises 11. a. It is false because 26 = b. It is true because 123 = c. It is true because 127 is a prime. d. It is true because 21 = 3 7. e. It is false because 96 = f. It is false because 8 is greater than a. gcd (a, b, 16) = gcd (gcd (a, b), 16) = gcd (24, 16) = 8 b. gcd (a, b, c, 16) = gcd (gcd (a, b, c), 16) = gcd (12, 16) = 4 c. gcd (200, 180, 450) = gcd (gcd (200, 180), 450) = gcd (20, 450) = 10 d. gcd (200, 180, 450, 600) = gcd (gcd (200, 180, 450), 600) = gcd (10, 600) = a. gcd (3n + 1, 2n + 1) = gcd (2n + 1, n) = 1 b. gcd (301, 201) = gcd ( , ) = 1 gcd (121, 81) = gcd ( , ) = a. 22 mod 7 = 1 b. 291 mod 42 = 39 c. 84 mod 320 = 84 d. 400 mod 60 = a. (125 45) mod 10 = (125 mod mod 10) mod 10 = (5 5) mod 10 = 5 mod 10 b. (424 32) mod 10 = (424 mod mod 10) mod 10 = (4 2) mod 10 = 8 mod 10 c. (144 34) mod 10 = (144 mod mod 10) mod 10 = (4 4) mod 10 = 6 mod 10 d. (221 23) mod 10 = (221 mod mod 10) mod 10 = (1 3) mod 10 = 3 mod a mod 5 = (a n 10 n + + a a 0 ) mod 5 = [(a n 10 n ) mod (a ) mod 5 + a 0 mod 5] mod 5 = [ a 0 mod 5] = a 0 mod 5

3 3 23. a mod 4 = (a n 10 n + + a a 0 ) mod 4 = [(a n 10 n ) mod (a ) mod 4 + a 0 mod 4] mod 4 = [ (a ) mod 4 + a 0 mod 4] = (a a 0 ) mod a mod 9 = (a n 10 n + + a a 0 ) mod 9 = [(a n 10 n ) mod (a ) mod 9 + a 0 mod 9] mod 9 = (a n + + a 1 + a 0 ) mod a mod 11 = (a n 10 n + + a a 0 ) mod 11 = [(a n 10 n ) mod (a ) mod 11 + a 0 mod 11] mod 11 = + a 3 ( 1) + a 2 (1) + a 1 ( 1) + a 0 (1)] mod 11 For example, mod 11 = [(1)6 + ( 1)3 + (1)1 + ( 1)4 + (1)5 + ( 1)3 + (1)6 + ( 1)7 + (1)2] mod 11 = 8 mod 11 = 5 mod a. (A + N) mod 26 = (0 + 13) mod 26 = 13 mod 26 = N b. (A + 6) mod 26 = (0 + 6) mod 26 = 6 mod 26 = G c. (Y 5) mod 26 = (24 5) mod 26 = 19 mod 26 = T d. (C 10) mod 26 = (2 10) mod 26 = 8 mod 26 = 18 mod 26 = S 31. (1, 1), (3, 7), (9, 9), (11, 11), (13, 17), (19, 19) 33. a. We have a = 25, b = 10 and c = 15. Since d = gcd (a, b) = 5 divides c, there is an infinite number of solutions. The reduced equation is 5x + 2y = 3. We solve the equation 5s + 2t = 1 using the extended Euclidean algorithm to get s =1 and t = 2. The particular and general solutions are x 0 = (c/d) s = 3 x = k y 0 = (c/d) t = 6 y = 6 5 k (k is an integer) b. We have a = 19, b = 13 and c = 20. Since d = gcd (a, b) = 1 and divides c, there is an infinite number of solutions. The reduced equation is 19x + 13y = 20. We solve the equation 19s + 13t =1 to get s = 2 and t = 3. The particular and general solutions are x 0 = (c/d) s = 40 x = k y 0 = (c/d) t = 60 y = k (k is an integer) c. We have a = 14, b = 21 and c = 77. Since d = gcd (a, b) = 7 divides c, there is an infinite number of solutions. The reduced equation is 2x + 3y = 11. We solve the equation 2s + 3t =1 to get s = 1 and t = 1. The particular and general solutions are x 0 = (c/d) s = 11 x = k y 0 = (c/d) t = 11 y = 11 2 k (k is an integer)

4 4 d. We have a = 40, b = 16 and c = 88. Since d = gcd (a, b) = 8 divides c, there is an infinite number of solutions. The reduced equation is 5x + 2y = 11. We solve the equation 5s + 2t =1 to get s = 1 and t = 2. The particular and general solutions are x 0 = (c/d) s = 11 x = k y 0 = (c/d) t = 22 y = 22 5 k (k is an integer) 35. We have the equation 39x + 15y = 270. We have a = 39, b = 15 and c = 270. Since d = gcd (a, b) = 3 divides c, there is an infinite number of solutions. The reduced equation is 13x + 5y = 90. We solve the equation 13s + 5t =1: s = 2 and t = 5. The particular and general solutions are x 0 = (c/d) s = 180 x = k y 0 = (c/d) t = 450 y = k To find an acceptable solution (nonnegative values) for x and y, we need to start with negative values for k. Two acceptable solutions are k = 35 x = 5 and y = 5 k = 36 x = 0 and y = a. 3x (mod 5) 3x ( 5 + 4) (mod 5) 3x 4 (mod 5) a = 3, b = 4, n = 5 d = gcd (a, n) = 1 Since d divides b, there is only one solution. Reduction: 3x 4 (mod 5) x 0 = (3 1 4) (mod 5) = 2 b. 4x (mod 6) 4x ( 6 + 4) (mod 6) 4x 4 (mod 6) a = 4, b = 4, n = 6 d = gcd (a, n) = 2 Since d divides b, there are two solutions. Reduction: 2x 2 (mod 3) x 0 = (2 1 2) (mod 3) = 1 x 1 = / 2 = 4 c.

5 5 9x (mod 7) 9x ( ) (mod 7) 9x 1 (mod 7) a = 9, b = 1, n = 7 d = gcd (a, n) = 1 Since d divides b, there is only one solution. Reduction: 9x 1 (mod 7) x 0 = (9 1 1) (mod 7) = 4 d. 232x (mod 50) 232x 206 (mod 50) a = 232, b = 206, n = 50 d = gcd (a, n) = 2 Since d divides b, there are two solutions. Reduction: 116x 103 (mod 25) 16x 3 (mod 25) x 0 = (16 1 3) (mod 25) = 8 x 1 = /2 = a. The determinant and the inverse of matrix A are shown below: 3 0 A = det(a) = 3 mod (det(a)) 1 = 7 mod A 1 = 7 A 1 = adj(a) b. Matrix B has no inverse because det(b) = ( ) mod = 2 mod 10, which has no inverse in Z 10. c. The determinant and the inverse of matrix C are shown below: C = det(c) = 3 mod C 1 = (det(c)) 1 = 7 mod 10 In this case, det(c) = 3 mod 10; its inverse in Z 10 is 7 mod 10. It can proved that C C 1 = I (identity matrix).

6 6

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