# FACTORING LARGE NUMBERS, A GREAT WAY TO SPEND A BIRTHDAY

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1 FACTORING LARGE NUMBERS, A GREAT WAY TO SPEND A BIRTHDAY LINDSEY R. BOSKO I would like to acknowledge the assistance of Dr. Michael Singer. His guidance and feedback were instrumental in completing this project. Abstract. To keep private information secure, messages are encrypted and then decrypted using a number of different algorithms. One of the most famous encryption algorithms relies on the knowledge that factoring large numbers is difficult. We describe an attack on this algorithm, known as Pollard s Rho Method and relate it to a curiosity of probability known as the Birthday Problem. 1. Introduction Example 1.1. Suppose two algebra students, Alice and Bob, are sending encrypted messages back and forth in the following manner. They agree to map the alphabet onto Z 26 with A 0, B 1, C 2,..., Z 25. Alice wishes to send the message F ERMAT to Bob. She begins by converting each letter to its integer modulo 26. Thus, F 5, E 4, R 17, M 12, A 0, T 19. All students in the class choose primes p, q with p q and an encryption exponent, a such that gcd (a, m) = 1 where m = (p 1)(q 1). The class publishes a listing of n = pq and a for each student. Alice uses the list to find Bob s information to be (n = 31921, a = 47) and encrypts the message further. Letter x Z 26 x a mod n F E R M A 0 0 T THE AUTHOR WAS PARTIALLY SUPPORTED BY NSF GRANT CCR

2 FACTORING LARGE NUMBERS, A GREAT WAY TO SPEND A BIRTHDAY 2 Alice then sends Bob the six numbers in the last column of the table. Using a computer algebra system Bob finds b such that ab = 1 mod (p 1)(q 1). For this example, b = Bob then calculates the following modulo 31921: y y b mod n Letter F E R M 0 0 A T Thus, the encrypted message Bob receives converts back into Alice s original message of F ERM AT. This introductory example depicts the Rivest-Shamir-Adleman (RSA), one of the most widely used publickey cryptosystems. Exercise 1.2. Recall, p, q are primes, n = pq, and m = (p 1)(q 1). a and b are chosen such that ab = 1 mod m and gcd (a, m) = 1. Describe how the RSA works in general by encrypting and decrypting x Z n. In our example, the factorization of n = was not known. If an intruder factors this into n = (233)(137), then he could calculate m = (232)(136) = and determine b such that ab = 1 mod m. The calculation of b can be done on any number of computer algebra systems. From here, Alice s intercepted, encrypted message could be raised to the power b, simplified modulo n and decrypted. Thus, the RSA relies on the known difficulty of factoring large numbers. Its secureness would fail if one could discover a way to factor large numbers efficiently, numbers much larger than n = With a and n public, one only need factor n into the product of p and q to easily calculate m. With this information now known, an intercepted message could be decrypted by the intruder. Exercise 1.3. Using p = 11, q = 5, and a = 27 determine n, m, and b. Then encrypt message RHO and decrypt to verify your work is correct. 2. Pollard s Rho Method The factoring of numbers is a unique curiosity because it is a problem known to elementary aged school children and renowned mathematicians alike. Mainly, given a positive integer N, how do we find its prime divisors? Young children may recommend that one could simply test

3 FACTORING LARGE NUMBERS, A GREAT WAY TO SPEND A BIRTHDAY 3 all integers less than N until a divisor is found and this is certainly the way most of us first learn to factor. We can improve on this technique when we come to realize we need only test the integers up to N for divisors. Additionally, we strengthen our factoring techniques further by learning divisibility tests. We all know that the a number is divisible by three if the sum of its digits is divisible by three. Eventually, this may come to include the more obscure tests for divisibility by 7, 11, and perhaps 13. Even armed with this arsenal, a large enough number cannot be conquered by divisibility tests. For this note we explore a method for factoring beyond the naive guess and check. In 1975 Pollard devised a novel factorization method to find a proper divisor of an integer, provided one exists (see [2]). It is of interest to note that the computer algebra system, Mathematica, uses Pollard s rho method in its FactorInteger function. We describe Pollard s method here. Suppose N > 0 and p N where p is prime. We begin by randomly selecting with replacement from the set S 1 = {0, 1, 2,... N 1} to form a sequence x 1, x 2, x 3,.... Defining x i = x i mod p our sequence is now x 1, x 2, x 3,... with each x i S 2 = {0, 1, 2,... p 1}. Because the sets S 1, S 2 are finite, eventually there will be a repeated integer in both sequences. We expect to achieve a repeat in the the sequence (x i ) i=1 prior to (x i ) i=1. Say, x i = x j for some i j. Then, x i x j mod p and by definition, p (xi x j ) if x i x j. Thus, gcd( x i x j, N) 1. As long as gcd( x i x j, N) N, we have found a proper divisor for N, namely the gcd of x i x j and N. It is vital to note that in executing this algorithm, we do not need to know the value of p. We only need to calculate gcd( x i x j, N) and know that there exists a prime divisor of N. A question may arise from the description above. Mainly, what is the best way in which to choose random numbers from the set {0, 1, 2,..., N 1}? Pollard proposes using a function f : Z N Z N, choosing an initial element x 0 {0, 1, 2,..., N 1}, and then recursively defining x i+1 = f(x i ). With a proper choice of f, the sequence generated will be pseudo-random, which is sufficient for the algorithm. Example 2.1. Given N = is not prime, we use Pollard s rhomethod with the function f(x) = x mod N, f(x i ) = x i+1, and x 0 = 631 to determine a prime factor of N.

4 FACTORING LARGE NUMBERS, A GREAT WAY TO SPEND A BIRTHDAY 4 i x i mod N We can determine gcd( x i x j, N) for 0 x i < x j 10. The first time we achieve a result other than 1 is for i = 3 and j = 10. In this instance, gcd( x 3 x 10, N) = gcd(63222, 82123) = 41. Thus, we have a prime divisor, p = 41, which allows us to completely factor N = = Exercise 2.2. Determine the gcd( x 4 x 11, N), gcd( x 5 x 12, N), and gcd( x 6 x 13, N). What do you notice? Why might this be occurring? In the example above, we wisely chose not to display the work involved in checking gcd( x i x j, N) for 0 x i < x j 10. The actual work involves checking j gcd s for each j 1 until a gcd greater than 1 is achieved. For the above example, this meant calculating over 45 different gcd s where operation counts for determining gcd s are O((log 2 uv) 2 ) with u, v as the bit sizes of the two inputs. Luckily for us, there exists a more efficient way to implement Pollard s rho-method that eliminates many of these costly gcd calculations. 3. Floyd s Cycle Detection Method Before describing the new method we inspect Pollard s method further by expanding upon Example 2.1. Recall N = and p = 41.

5 FACTORING LARGE NUMBERS, A GREAT WAY TO SPEND A BIRTHDAY 5 i x i mod N x i = x i mod p i x i mod N x i = x i mod p From our previous work, the smallest such t, l for which gcd( x t x t+l, N) 1 occurs when t = 3 and l = 7. Notice that x j = x j+l for j 3. Thus, the sequence, (x i ) i=1 cycles every 7 iterations starting with x 3. The cycling occurs as a result of x 3 = x 10, which implies that x 3 = x 10 mod p. So, f(x 3 ) = f(x 10 ) mod p. Thus, x 4 = x 11, x 5 = x 12, and so on. A clever depiction of this sequence forms the shape of the lowercase Greek letter rho, ρ. Because of this cycling we also will have gcd( x j x j+l, N) 1 for each j 3. In executing Pollard s rho-method with the goal of efficiency, we want to find the first indication of this cycling without checking each gcd. To achieve this, we employ Floyd s cycle detection method. We first describe an overview of the method using a common analogy. Suppose there exists a sequence which eventually lapses into a cycle of some finite length. To find this cycle, we send a tortoise and a hare through the sequence. Assume the tortoise moves through each element, one

6 FACTORING LARGE NUMBERS, A GREAT WAY TO SPEND A BIRTHDAY 6 at a time in order, and the hare starts at the same point, but moves twice as fast as the tortoise. We compare the positions of the animals after each move. The hare will be in cycle first, and may go through it several times, but, eventually, both will be in the cycle. Since the cycle has finite length, they will meet. This should occur before the tortoise s makes a full revolution in the cycle. Mathematically, if f : Z N Z N and x 0 = y 0 Z N then define x i+1 = f(x i ) and y i+1 = f(f(y i )). Here, the x i s mark the tortoise s path and the y j s mark the hare s. We identify t and l as the smallest such integers for which x t+l = x t mod p. We have already seen that x j+l = x j mod p for j t. So, there are many instances for which we obtain equality modulo p. We claim that the first time x i = y i mod p occurs when i t + l. Proof of Claim. Suppose y i = x i. Then x i = x 2i and we have i t since t was defined to be the smallest such integer. Also, l (2i i) = i and there exists k Z such that (k + 1)l = i. We depict this as: 0 l 2l kl t (k + 1)l = i Then, i = (k + 1)l = t + ( t mod l) t + l if t > 0. If t = 0, then i = l. Thus, we have detected the existence of the cycle before completing more than one loop. Now, let us rework Example 2.1 with this new technique. Example 3.1. Recall that N = and it originally took checking over 45 different gcd s before finding a nontrivial divisor, p, of N. i x i mod N y i mod N gcd( x i y i, N) Exercise 3.2. Complete the last two columns and prove that after checking just seven gcd s, we determine a prime divisor for N = Note that this is an improvement over the initial execution of the method without Floyd s cycle detection.

7 FACTORING LARGE NUMBERS, A GREAT WAY TO SPEND A BIRTHDAY 7 4. The Birthday Problem To determine an operation count for Pollard s rho-method we examine a curiosity of probability known as the Birthday Problem. Suppose we question n people, then the Birthday Problem asks how likely is it that two people share the same birthday? We eliminate February 29 and assume all of the other 365 birthdays are equally likely. A generalization of the Birthday Problem involves selecting elements from the set S = {1, 2, 3,..., n} sequentially with replacement until two are the same. How long do we expect this experiment to take? We analyze this problem by defining A i as the outcome that we need to make exactly i selections. The expected value for which we are searching is then: P (A i )i = P (A 1 ) + 2P (A 2 ) + 3P (A 3 ) + i=1 = P (A 1 ) + P (A 2 ) + P (A 3 ) + + P (A 2 ) + P (A 3 ) + + P (A 3 ) + + = probability 1 or more are selections are made = + probability 2 or more are selections are made + probability 3 or more are selections are made + probability i+1 or more selections are made i=0 Note that the probability that i + 1 or more selections are made is the same as the probability that i selections are made and no two are the same. So, the probability that i selections are made and no two are the same is: {(a1, a 2,..., a i ) : a j S, a j a k for 1 j < k i} {(a 1, a 2,..., a i ) : a j S} = n(n 1)(n 2) (n i + 1) n n... n } {{ } n times

8 FACTORING LARGE NUMBERS, A GREAT WAY TO SPEND A BIRTHDAY 8 = 1 ( 1 1 n ) ( 1 2 n ) ( 1 i 1 n ) (4.1) As 1 + x e x by its Taylor series expansion, then 1 x e x and we have Eq. (4.1) e 1 2 n e n ( = exp = exp (i 1) e n ) i 1 ( 1 n j=0 j n ( 1 (i 1) i 2 ( ) 1 i 1 = exp j n j=0 )) ( ) i(i 1) = exp Suppose we want to find which i satisfy the inequality exp 1 i(i 1). This is equivalent to 2 i(i 1) ln 2. Thus, ln 2 i 2 i (ln 2) i 2 i (ln 2) 0 ( ) i(i 1) ( 1 ± ) 1 + 8n(ln 2). By the quadratic formula, equality is achieved when i = 1 2 ( So, i 2 i (ln 2) 0 when i ) 1 + 8n(ln 2). Here, we can 2 use n = 365 to conclude that after asking 23 people, we expect with over 50% probability that two people will have the same birthday. Return to our expected value, which we know to be bounded above by ( ) i(i 1) exp 1 + i=0 = where the integral is solved ( ) (i 1)(i 1) exp ( ) j 2 exp ( ) x 2 exp dx Let t = x. i=1 j=0 0 = 2 + = 2 + exp ( t 2) dt 0 π nπ 2 = 2 + 2

9 FACTORING LARGE NUMBERS, A GREAT WAY TO SPEND A BIRTHDAY 9 Thus, our expected value is of O( n). The birthday problem, with n = 365, results in an expected value of no more than 26 people. Relating this to Pollard s rho-method, where we are choosing with replacement from the set {0, 1, 2,... p 1}, we expect to have to test O( p) values before two are the same modulo p. Since p N, we can say the algorithm has O( 4 N) As a result, we expect to have a gcd > 1 and, in turn, a divisor of N.

10 FACTORING LARGE NUMBERS, A GREAT WAY TO SPEND A BIRTHDAY 10 References [1] Frank H. Mathis, A Generalized Birthday Problem, SIAM Reivew, Vol. 33, No. 2, (June 1991), [2] J.M Pollard, Kangaroos, A Monte Carlo Method for Factorization, BIT 15 (1975),

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