Introduction to Cryptography with Coding Theory, 3rd edition Wade Trappe Solution Manual

INSTRUCTOR’S SOLUTIONS MANUAL INTRODUCTION TO CRYPTOGRAPHY WITH CODING THEORY THIRD EDITION

Wade Trappe Rutgers University

Lawrence C. Washington University of Maryland

Chapter 10 - Exercises 1. (a) Perhaps the easiest way to do this is to list the powers of 2 mod 13 until we get 3: 2, 22 ≡ 4, 23 ≡ 8, 24 ≡ 16 ≡ 3 (mod 13). Therefore L2 (3) = 4. (b) 27 = 128 ≡ 11 (mod 13), which implies that L2 (11) = 7.

3. (a) 62 ≡ 3 (mod 11), 64 ≡ 32 ≡ 9, 65 ≡ 6 · 64 ≡ 6 · 9 ≡ 10. (b) Since 2 is a primitive root, 25 = 2(11−1)/2 ≡ −1 (mod 11). Therefore, −1 ≡ 65 ≡ (2x )5 ≡ (−1)x , so x is odd.

5. Since 5 is a primitive root, 5611 ≡ −1 (mod 1223). Therefore, 1 ≡ 3611 ≡ 611x 5 ≡ (−1)x , so x is even.

7. (a) L2 (24) ≡ 3L2 (2) + L2 (3) ≡ 3 + 69 ≡ 72 (mod 100). Therefore, L2 (24) = 72. (b) L2 (24) ≡ 3L2 (5) ≡ 3 · 24 ≡ 72 (mod 100). Therefore, L2 (24) = 72.

9. Since 11 = 44/22 , and since 3136 ≡ 1 (mod 137), we have 3x ≡ 36−2·10 ≡ −14 3 ≡ 3−14+136 ≡ 3122 . Therefore, x = 122.

11. (a) If someone knows 2x (mod p), it is difficult to find x since this is the discrete logarithm problem. (b) If p has only 5 digits, it is easy to compute 2k (mod p) for k = 1, 2, . . . p − 1 until the number 2x (mod p) is found. Then k = x is the password.

13. Alice computes 33 ≡ 10 (mod 17) and sends 10 to Bob. Bob computes 5 3 ≡ 5 (mod 17) and sends 5 to Alice. Alice computes 53 ≡ 6 (mod 17) and Bob computes 105 ≡ 6 (mod 17). The shared secret is 6.

15. Eve computes b1 with bb1 ≡ 1 (mod p − 1). Then xb21 ≡ αbb1 ≡ α1 ≡ α (mod p).

17. (a) Write d in base N , so d = a0 + a1 N with 0 ≤ ai < N . Then m ≡ cd ≡ ca0 +a1 N implies that mc−a1 N ≡ ca0 . Therefore, there is a match for j = a0 and k = a1 . This gives a0 + a1 N as a candidate for d. (b) c = m = 1 gives a match for every j, k, so it is unlikely that the first match yields the correct d. √ (c) The lists are of length approximately N . This is approximately the time √ required to factor n by dividing by all the primes up to N .

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