# Lecture 5: Arithmetic Modulo m, Primes and Greatest Common Divisors Lecturer: Lale Özkahya

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1 BBM 205 Discrete Mathematics Hacettepe University bbm205 Lecture 5: Arithmetic Modulo m, Primes and Greatest Common Divisors Lecturer: Lale Özkahya Resources: Kenneth Rosen, Discrete Mathematics and App /1term/ma006a/#notes

2 9/28/2014 Our Problem: Encryption Alice needs to send bob a message. Eve can read the communications. Alice encrypts the message. Message to Bob Alice Eve Bob Classic Cryptography Alice and Bob exchange some information in advance, in a secure way. Encrypts Message to Bob Decrypts Alice Eve Bob 3

3 9/28/2014 Example: Atbash Cipher Replace each letter with a symbol, according to the sequence (key): My hovercraft is full of eels Nb slevixizug rh ufoo lu vvoh Other Historical Ciphers Scytale transposition cipher, used by the Spartan military. The Enigma machine in World War II. Cipher runes. 4

4 9/28/2014 The Internet Problem. When performing a secret transaction over the internet, we cannot securely exchange information in advance. Encrypts Message to Bob Decrypts Alice Eve Bob Public-key Cryptography Idea. Use a public key which is used for encryption and a private key used for decryption. Bob generates both keys. Keeps the private key and publishes the public one. This is my public key Alice Eve Bob 5

5 9/28/2014 Public-key Cryptography Idea. Use a public key which is used for encryption and a private key used for decryption. Alice encrypts her message with Bob s public key and sends it. Encrypts Message to Bob Decrypts Alice Bob Public-key Cryptography Eve has the public key and the encrypted message. We need an action that is easy to do (encrypt using a public key) but very difficult to reverse (decrypt using a public key). 6

6 9/28/2014 Public-key Cryptography Eve has the public key and the encrypted message. We need an action that is easy to do (encrypt using a public key) but very difficult to reverse (decrypt using a public key). Bad example. The public key is the number k. We encrypt a number a as a k. The adversary can divide by k Public-key Cryptography Eve has the public key and the encrypted message. We need an action that is easy to do (encrypt using a public key) but very difficult to reverse (decrypt using a public key). 7

7 9/28/2014 Integers We consider the set of integers Z =, 2, 1,0,1,2,. The expression a Z means that a is in the set Z. For example, we have 1 Z, 10 2 Z. On the other hand 0.3 Z, 2 Z. Division Given two integers a, b Z, we say that a divides b (or a b) if there exists s Z such that b = sa. True or false:

8 9/28/2014 Our First Proof Claim. If a b an b c then a c. Proof. There exists s Z such that b = as. There exists t Z such that c = bt. Therefore, c = ast. Setting r = st, we have c = ar. Our Second Proof Claim. If a b and b a then a = ±b. Proof. There exists s Z such that a = sb. There exists t Z such that b = ta. That is, a = sta. st = 1 so either s = t = 1 or s = t = 1. 9

9 9/28/2014 Prime Numbers A natural number is an integer that is non-negaive. The set of natural numbers: N = 0,1,2,3,. A number of N 0,1 is said to be prime if its only positive divisors are one at itself. Proof by Induction Claim (Prime decomposition). Every natural number n 2 is either a prime or a product of primes. Proof. Induction basis: The claim holds for 2. Induction step: Assume that the claim holds for every natural number smaller than n. If n is a prime, the claim holds for n. Otherwise, we can write n = ab. By the induction hypothesis, both a and b are either primes or a product of primes. Thus, n is a product of primes. 10

10 9/28/2014 Proof by Contradiction Claim. There exist infinitely many prime numbers. Proof. Assume, for contradiction, that there exists a finite set of primes P = p 1, p 2, p 3,, p n. The number p 1 p 2 p n + 1 is not prime, since it is not in P. The number p 1 p 2 p n + 1 is prime, since it cannot be divided by any of the primes of P. Contradiction! So there must be infinitely many primes! More Division Properties Claim. Given two numbers a, b N, there are unique q, r N such that r < b and a = qb + r. We say that q and r are the quotient and the remainder of dividing a with b. We write r = a mod b. Proof by algorithm! 11

11 9/28/2014 Our First Algorithm Input. Two numbers a, b N. Output. Two number q, r N such that a = qb + r and r < b. q 0 and n a. While n b: n n b. q q + 1. r n a = 12 b = 5 q = 0 n = 12 r =? q = 1 n = 7 r =? q = 2 n = 2 r =? r = 2 Greatest Common Divisor We say that d is a common divisor of a and b (where a, b, d N) if d a and d b. The greatest common divisor of a and b, denoted GCD(a, b), is a common divisor c of a and b, such that If d a and d b then d c. Equivalently, if d a and d b then d c. 12

12 9/28/2014 Examples: GCD What is GCD 18,42? 6 What is GCD 50,100? 50 What is GCD , ? GCD , ? = = What can we do when dealing with numbers that are too large to factor? GCD Property Claim. If a = bq + r then GCD a, b = GCD b, r Example. 66 = GCD 66,21 = GCD 21,3 = 3. 13

13 9/28/2014 Computing GCD: General approach Problem. Compute GCD(a, b). Find q 1, r 1 Z such that a = q 1 b + r 1. Since GCD a, b = GCD(b, r 1 ), it suffices to compute the latter. Find q 2, r 2 Z such that b = q 2 r 1 + r 2. Since GCD b, r 1 = GCD(r 1, r 2 ), it suffices to compute the latter. Continue until obtaining a zero remainder (then the divider is the required GCD). The Euclidean Algorithm Input. Two numbers a, b N. Output. GCD a, b. r a mod b. While r 0: a b. b r. r a mod b. Output b. a = 78 b = 45 a = 78 b = 45 r = 33 a = 45 b = 33 r = 12 a = 33 b = 12 r = 9 a = 12 b = 9 r = 3 a = 9 b = 3 r = 0 14

14 9/28/2014 Proof of GCD Property Claim. If a = bq + r then GCD a, b = GCD b, r Proof. Since r = a bq, every common divisor of a and b is also a divisor of r. Thus, GCD a, b GCD b, r Since a = bq + r, every common divisor of b and r is also a common divisor of a. Thus, GCD b, r GCD a, b. The End The Voynich manuscript: 15

15 9/30/2014 Warm-up: The Fibonacci Numbers Fibonacci numbers: F 0 = F 1 = 1 F i = F i 1 + F i 2. 1,1,2,3,5,8,13,21,34, How many rounds of the algorithm are required to compute GCD F n, F n 1? Round 1: r = F n F n 1 = F n 2. Round 2: r = F n 1 F n 2 = F n 3. Round n: r = F 1 F 0 = 0. More GCDs Theorem. For any a, b N, there exist s, t Z such that GCD a, b = as + bt. GCD 18,27 = = 9 GCD 25,65 = = 5 3

16 9/30/2014 The Extended Euclidean Algorithm Build a matrix: First two rows are a, 1,0 and b, 0,1. Every other row is obtained by subtracting the two rows above it, to obtain the next value of b a = 78 b = 45 a = 78 b = 45 r = 33 a = 45 b = 33 r = 12 a = 33 b = 12 r = 9 a = 12 b = 9 r = 3 a = 9 b = 3 r = 0 The Extended Euclidean Algorithm Build a matrix: First two rows are a, 1,0 and b, 0,1. Every other row is obtained by subtracting the two rows above it, to obtain the next value of b In every step, we have a = qb + r, and then a b, b r. If R i denotes the i th row: R i = R i 2 qr i 1. 4

17 9/30/2014 The Extended Euclidean Algorithm Build a matrix: First two rows are a, 1,0 and b, 0,1. Every other row is obtained by subtracting the two rows above it, to obtain the next value of b = so R 5 = R 3 2R 4 Proof by Algorithm! Theorem. If c = GCD(a, b), then there exist s, t Z such that as + bt = c = = = = = =

18 9/30/2014 Algorithm Correctness Proof Sketch. By induction. Induction basis. Trivial for the first two rows. Induction step. R i R i+1 = R i+2 s 1 s 2 s 3 t 1 t 2 t 3 u 1 u 2 u 3 s 1 = a s 2 + b s 3, t 1 = a t 2 + b t 3, u 1 = s 1 qt 1 = a s 2 qt 2 + b s 3 qt 3 = a u 2 + b u 3. Induction hypothesis Scales Problem We need to verify the weights of various objects by using scales. We have an unlimited amount of weights in two different integer sizes - a and b. For which values of a and b can we measure every possible integer weight? Answer. Whenever GCD a, b = 1. 6

19 9/30/2014 Number Theory Number theory: the study of integers. Some famous theorems: Euclid. There are infinitely many prime numbers. Fermat s last theorem. The equation x n + y n = z n has no integer solutions when n > 2. Lagrange Every natural number can be represented as the sum of four integer squares. 15 = = Number Theory (2) A couple of famous open problems: Twin prime conjecture. There are infinitely many pairs of prime numbers that differ by two (5 and 7, 17 and 19, 41 and 43, ). Goldbach's conjecture. Every even integer greater than 2 can be expressed as the sum of two primes. 7

20 9/30/2014 Congruences Recall. The remainder of dividing a by m can be written as r = a mod m. If also r = b mod m, we say that a is congruent to b modulo m, and write a b mod m. Equivalently, m a b. The numbers 3, 10, 17, 73, 1053 are all congruent modulo 7. Congruence Classes If m = 2, numbers are congruent if they have the same parity. If m = 3, there are three distinct classes of numbers mod mod mod 3 In general, we have exactly m equivalence classes of numbers. 8

21 9/30/2014 Congruency is Transitive Claim 1. If a b mod m and b c mod m then a c mod m mod mod mod 10 Congruency is Transitive Claim 1. If a b mod m and b c mod m then a c mod m. Proof. If m (a b) and m b c then m a c since a c = a b + b c. 9

22 9/30/2014 Congruency and Addition Claim 2. If a b mod m and c d mod m, then a + c b + d mod m mod mod mod 12 Congruency and Addition Claim 2. If a b mod m and c d mod m, then a + c b + d mod m. Proof. If m a b and m c d then m a + c (b + d). 10

23 9/30/2014 Congruency and Multiplication Claim 3. If a b mod m and c d mod m, then ac bd mod m mod mod mod 12 Congruency and Multiplication Claim 3. If a b mod m and c d mod m, then ac bd mod m. Proof. We have ac bd = ac cb + cb bd = c a b + b c d. That is, m ac bd. 11

24 9/30/2014 Relatively Prime Numbers Two integers m, n Z are relatively prime if GCD m, n = 1. Claim 4. If a and m are relatively prime, then there exists b Z such that ab 1 mod m. GCD 6,17 = = 1 mod 17 Relatively Prime Numbers Two integers m, n Z are relatively prime if GCD m, n = 1. Claim 4. If a and m are relatively prime, then there exists b Z such that ab 1 mod m. Proof. There exist s, t Z such that as + mt = 1. Taking b = s, we have m ab + mt 1 m ab 1. 12

25 9/30/2014 A Cancellation Law Claim 5. If k, m are relatively prime, and ak bk mod m, then a b mod m. GCD 5,9 = mod mod 9 A Cancellation Law Claim 5. If k, m are relatively prime and ak bk mod m, then a b mod m. Proof. By Claim 4 there exist s Z such that ks 1 mod m. a a 1 aks bks b 1 b mod m. 13

26 9/30/2014 A Cancellation Law Claim 5. If k, m are relatively prime and ak bk mod m, then a b mod m. What happens when GCD k, m 1? k = 4 m = 8 2 k 4 k 0 mod 8 Latin Squares Claim 6. Let a, b, m Z and let a, m be relatively prime. Then there is a unique x (mod m) such that ax b mod m. m = 11, a = 5, b = 6 5 x 6 mod 11 x =

27 9/30/2014 Latin Squares Claim 6. Let a, b, m Z and let a, m be relatively prime. Then there is a unique x (mod m) such that ax b mod m. Proof. By Claim 4 there exists s Z such that as 1 mod m. Thus, x = sb is one valid solution. Assume, for contradiction, that there are two distinct solutions x, x. Then ax ax mod m. x = sax = sax = x. Problem: Large Powers Problem. Compute mod 7. 15

28 9/30/2014 Problem: Large Powers Problem. Compute mod 7. Modest beginning mod mod mod mod mod mod mod 7 Problem: Large Powers mod mod mod mod mod mod mod mod mod 7 16

29 10/2/2014 Reminder: Some Congruent Properties Addition. If a b mod m and c d mod m, then a + c b + d mod m. Products. If a b mod m and c d mod m, then ac bd mod m. Cancellation. If GCD k, m = 1 and ak bk mod m, then a b mod m. Inverse. If GCD a, m = 1, then there exists b Z such that ab 1 mod m. Warm-up: Division by Nine Claim. A number a N is divisible by 9 if and only if the sum of its digits is divisible by 9. Is divisible by 9? = = 9 3

30 10/2/2014 Warm-up: Division by Nine (2) Claim. A number a N is divisible by 9 if and only if the sum of its digits is divisible by 9. Proof. Write a as a k a k;1 a 1 a 0 where a i is the (i + 1) th rightmost digit of a. a a 0 + a a k = a a a a a k = a a a That is, 9 a a 0 + a a k Warm-up: Division by Nine (3) Claim. A number a N is divisible by 9 if and only if the sum of its digits is divisible by 9. Proof. Write a as a k a k;1 a 1 a 0 where a i is the (i 1) th rightmost digit of a. We have: 9 a a 0 + a a k. Equivalently, a a 0 + a a k mod 9. 4

31 10/2/2014 Casting Out Nines Problem. Is the following correct? 54,321 98,765 = 5,363,013,565. If this is correct, then 54,321 98,765 5,363,013,565 mod mod mod mod mod 9 x Casting Out Nines (cont.) Is the casting out nines technique always correct in verifying whether a b = c? If the calculation mod 9 is wrong, the original calculation must be wrong. If the calculation mod 9 is correct, the original calculation might still be wrong! mod 9. 5

32 10/2/2014 Casting Out Nines Crank In the 1980 s, a crank wrote a 124-page book explaining the law of conservations of numbers that he developed for 24 years. This law was perfected with 100% effectiveness. The book is basically 124 pages about the casting out nines trick. It does not mention that the method can sometimes fail. Fermat s Little Theorem Theorem. For any prime p and integer a, a p a mod p. Examples: mod mod mod 1009 Pierre de Fermat 6

33 10/2/2014 Fermat s Little Theorem Theorem. For any prime p and integer a, a p a mod p. Proof. By induction on a: We now prove only the case of a 0. Induction basis: Obviously holds for a = 0. Induction step: Assume that the claim holds for a. In a later lecture we prove a + b p a p + b p mod p. Thus: a + 1 p a p + 1 a + 1 mod p. A Corollary Corollary. If a N is not divisible by a prime p then a p;1 1 mod p. Proof. We have GCD a, p = 1. Fermat s little theorem: a p a mod p. Combine with cancelation property: If GCD k, m = 1 and ak bk mod m, then a b mod m. 7

34 10/2/2014 Euler s Totient Function Euler s totient φ(n) is defined as follows: Given n N 0, then φ n = x 1 x < n and GCD x, n = 1. In more words: φ n is the number of natural numbers 1 x n such that x and n are relatively prime. φ 12 = 1,5,7,11 = 4 Leonhard Euler The Totient of a Prime Observation. If p is a prime number, then φ p = p 1. The first thousand values of φ n : 8

35 10/2/2014 Euler s Theorem Theorem. For any pair a, n N such that GCD a, n = 1, we have φ n a 1 mod n. This is a generalization of the claim a p;1 1 mod p (when p is prime). The RSA Algorithm Public key cryptosystem. Discovered in 1977 by Rivest, Shamir, and Adleman. Still extremely common! Ron Rivest Adi Shamir Leonard Adleman 9

36 10/2/2014 RSA Public and Private Keys 1. Choose two LARGE primes p, q (say, 500 digits). 2. Set n = pq. 3. Compute φ n, and choose 1 < e < φ n such that GCD e, φ n = Find d such that de 1 mod φ n. Public key. n and e. Private information. p, q, and d. Preparing for Secure Communication Bob generates p, q, n, d, e, and transmits only e and n. This is my public key (e, n) Alice Eve Bob 10

37 10/2/2014 Encrypting a Message Alice wants to send Bob the number m < n without Eve deciphering it. Alice uses n, e to calculate X = m e mod n, and sends X to Bob. m e mod n Alice Eve Bob Decrypting a Message Bob receives message X = m e mod n from Alice. Then he calculates: X d mod n m ed mod n m 1:k φ n mod n m mod n. de 1 mod φ n Euler s Theorem: m φ(n) 1 mod n Slightly cheating since the theorem requires GCD m, n = 1 11

38 10/2/2014 RSA in One Slide Bob wants to generate keys: Arbitrarily chooses primes p and q. sets n = pq and finds φ n. Chooses e such that GCD φ n, e = 1. Find d such that de 1 mod φ n. Alice wants to pass bob m. Receives n, e from Bob. Returns X m e mod n. Bob receives X. Calculates X d mod n. Example: RSA (with small numbers) Bob wants to generate keys: Arbitrarily chooses primes p = 61 and q = 53. n = = φ 3233 = Chooses e = 17 (GCD 3120,17 = 1). For de 1 mod 3120, we have d = Alice wants to pass bob m = 65. Receives n, e from Bob. Returns m e = mod Bob receives X 2790 mod Calculates X d = mod

39 10/2/2014 Some Details Bob needs to: Find two large primes p, q. Calculate n, d, e. Alice needs to Use n, e to calculate X = m e mod n. Eve must not be able to Use n, e, X to find m. Bob needs to: Use n, d, X to find m. That is: Easy to compute a large power mod n. Hard to compute a large root mod n. Taking Large Roots Eve has n, e, and Alice s message X m e mod n. If Eve can compute X 1/e mod n, she can read the message! (i.e., if she can factor n). So far nobody knows how to compute this in a reasonable time. Or do they? 13

40 10/2/2014 Computing a Large Power Problem. How can we compute A naïve approach: mod 9721? mod mod mod 9721 This approach requires (about ) steps. Impossible! Computing a Large Power Fast! Problem. How can we compute mod 9721? mod mod mod mod 9721 Only 4000 steps. Easy! 14

41 10/2/2014 A Small Technical Issue What if we calculate a b where b is not a power of two? We calculate a, a 2, a 4, a 8, a 16, a 32, Every number can be expressed as a sum of distinct powers of = a 57 = a 32 a 16 a 8 a What is Left to Do? Bob wants to generate keys: Arbitrarily chooses primes p and q.? n = pq find φ n.? Chooses e such that GCD φ n, e = 1. Find d such that de 1 mod φ n. Alice wants to pass bob m. Receives n, e from Bob. Returns X m e mod n. Bob receives X. Calculates X d mod n.? 15

42 11/18/2014 Reminder: Euler s Totient Function Euler s totient φ(n) is defined as follows: Given n N, then φ n = x 1 x < n and GCD x, n = 1. In more words: φ n is the number of natural numbers 1 x n such that x and n are coprime. φ 12 = 1,5,7,11 = 4. Reminder #2: The RSA Algorithm Bob wants to generate keys: Arbitrarily chooses primes p and q.? n = pq find φ n.? Chooses e such that GCD φ n, e = 1. Find d such that de 1 mod φ n.? Alice wants to pass bob m. Receives n, e from Bob. Returns X m e mod n. Bob receives X. Calculates X d mod n.? 2

43 11/18/2014 Finding φ n Problem. Given n = pq, where p, q are large primes, find φ n. We need the number of elements in 1,2,, n that are not multiplies of p or q. There are n = q numbers are divisible by p. p There are n = p numbers are divisible by q. q Only n = pq is divided by both. Thus: φ n = n p q + 1. The RSA Algorithm Bob wants to generate keys: Arbitrarily chooses primes p and q.? n = pq find φ n. Chooses e such that GCD φ n, e = 1. Find d such that de 1 mod φ n.? Alice wants to pass bob m. Receives n, e from Bob. Returns X m e mod n. Bob receives X. Calculates X d mod n.? 3

44 11/18/2014 Choose e s.t. GCD φ n, e = 1 Problem. Given n = pq, where p, q are large primes, find e N such that GCD φ n, e = 1. We can choose arbitrary numbers until we find one that is relatively prime to φ n. For the worst values of φ n, a random number is good with probability 1/ log log n. The RSA Algorithm Bob wants to generate keys: Arbitrarily chooses primes p and q.? n = pq find φ n. Chooses e such that GCD φ n, e = 1. Find d such that de 1 mod φ n.? Alice wants to pass bob m. Receives n, e from Bob. Returns X m e mod n. Bob receives X. Calculates X d mod n. 4

45 11/18/2014 Find d such that de 1 mod φ n Recall. Since GCD e, φ n = 1 then there exist s, t Z such that se + tφ n = 1. That is, se 1 mod φ n. We can find s, t by the extended Euclidean algorithm from lecture 2. The RSA Algorithm Bob wants to generate keys: Arbitrarily chooses primes p and q.? n = pq find φ n. Chooses e such that GCD φ n, e = 1. Find d such that de 1 mod φ n. Alice wants to pass bob m. Receives n, e from Bob. Returns X m e mod n. Bob receives X. Calculates X d mod n. 5

46 11/18/2014 Quantum Computing A bit of a computer contains a value of either 0 or 1. A quantum computer contains qubits, which can be in superpositions of states. Theoretically, a quantum computer can easily factor numbers and decipher almost any known encryption. Should We Stop Ordering Things Online? 6

47 11/18/2014 Finding Large Primes Let n be a LARGE integer (e.g., ). The prime number theorem. The probability of a random p 1,, n being prime is about 1/ log n. If we randomly choose numbers from 1,, n, we expect to have about log n iterations before finding a prime. But how can we check whether our choice is a prime or not?! Primality Testing Given a LARGE q Z, how can we check whether q is prime? The naïve approach. Go over every number in 2,, q and check whether it divides q. But we chose our numbers to be too large for a computer to go over all of them! 7

48 11/18/2014 Recall: Fermat s Little Theorem For any prime p and integer a relatively prime to p, we have a p a mod p. Pick a random integer a and check whether a q a mod q. If not, q is not a prime! If yes,??? Pierre de Fermat Example: Fermat Primality Testing Is n = prime? mod n is not prime since 2 n 2 mod n. We can try 1000 different values of a and see if a n a mod n for each of them. 8

49 11/18/2014 Carmichael Numbers A number q N is said to be a Carmichael number if it is not prime, but still satisfies a q a mod q for every a that is relatively prime to q. The smallest such number is 561. Very rare about one in 50 trillion in the range R. D. Carmichael Example: Carmichael Numbers Claim. Let k N 0 such that 6k + 1, 12k + 1, and 18k + 1 are primes. Then n = 6k k k + 1 is a Carmichael number. Example. For k = 1, we have that 7,13,19 are primes = 1729 is a Carmichael number. 9

50 11/18/2014 Proof We need to prove that for any a that is relatively prime to n, we have a n a mod n. Recall. Since GCD a, n = 1, this is equivalent to a n 1 1 mod n. We rewrite n = 1296k k k + 1. For any such a, we have a n 1 = a 1296k3 +396k 2 +36k = a 6k 216k2 +66k+6. Proof (cont.) For any a relatively prime to n, we have a n 1 = a 6k 216k2 +66k+6. Recall. If a N is not divisible by a prime p then a p 1 1 mod p. Since a and 6k 1 are relatively prime a n k2 +66k+6 1 mod 6k + 1. Similarly, we have a n 1 1 mod 12k + 1 and a n 1 1 mod 18k + 1. Since a n 1 1 is divisible by the three pairwise coprime integers 6k + 1, 12k + 1, and 18k + 1, it is also divisible by their product n. That is, a n 1 1 mod n. 10

51 11/18/2014 Miller Rabin Primality Test The Miller Rabin primality test works on every number. Gary Miller Michael Rabin Root of Unity Claim. For any prime p, the only numbers a 1,, p such that a 2 1 mod p are 1 and p 1. Example. The solutions to a 2 1 mod 1009 are exactly the numbers satisfying a 1 or 1008 mod

52 11/18/2014 Root of Unity Claim. For any prime p, the only numbers a 1,, p such that a 2 1 mod p are 1 and p 1. Proof. a 2 1 mod p a mod p a + 1 a 1 0 mod p That is, either p a + 1 or p a 1. Roots of Unity Properties Given a prime p > 2, we write p 1 = 2 s d where d is odd and s 1. Claim. For any odd prime p and any 1 < a < p, one of the following holds. a d 1 mod p. There exists 0 r < s such that a 2rd 1 mod p. 12

53 11/18/2014 Roots of Unity Properties (2) Claim. For any odd prime p and any 1 < a < p, one of the following holds. a d 1 mod p. There exists 0 r < s such that a 2rd 1 mod p. Proof. By Fermat s little theorem a p 1 1 mod p. Consider a (n 1)/2, a (n 1)/4,, a (n 1)/2s. By the previous claim, each such root is ± 1 mod n. If all of these roots equal 1, we are in the first case. Otherwise, we are in the second. Composite Witnesses Given a composite (non-prime) odd number n, we again write n 1 = 2 s d where d is odd and s 1. We say that a 2,3,4,, n 2 is a witness for n if a d 1 mod n. For every 0 r < s, we have a 2rd 1 mod n. 13

54 11/18/2014 Example: Composite Witness Problem. Prove that 91 is not a prime. 90 = mod is a witness that 91 is not a prime. There are Many Witnsses Given an odd composite n, the probability of a number 2,, n 2 being a witness is at least ¾. Given an odd n N, take i numbers and check if they are witnesses. If we found a witness, n is composite. If we did not find a witness, n is prime with probability at least i. ` 14

55 11/18/2014 The End 15

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