# Introduction to Cryptography. Lecture 6

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1 Introduction to Cryptography Lecture 6 Benny Pinkas page 1

2 Public Key Encryption page 2

3 Classical symmetric ciphers Alice and Bob share a private key k. System is secure as long as k is secret. Major problem: generating and distributing k. k Alice k Bob page 3

4 Diffie and Hellman: New Directions in Cryptography, We stand today on the brink of a revolution in cryptography. The development of cheap digital hardware has freed it from the design limitations of mechanical computing such applications create a need for new types of cryptographic systems which minimize the necessity of secure key distribution theoretical developments in information theory and computer science show promise of providing provably secure cryptosystems, changing this ancient art into a science. page 4

5 Diffie-Hellman Came up with the idea of public key cryptography public key Bob Alice Bob secret key Bob Everyone can learn Bob s public key and encrypt messages to Bob. Only Bob knows the decryption key and can decrypt. Key distribution is greatly simplified. page 5

6 Plan Basic number theory Divisors, modular arithmetic The GCD algorithm Groups References: Many books on number theory Almost all books on cryptography Cormen, Leiserson, Rivest, (Stein), Introduction to Algorithms, chapter on Number-Theoretic Algorithms. page 6

7 Divisors, prime numbers We work over the integers A non-zero integer b divides an integer a if there exists an integer c s.t. a=cb. Denoted as b a I.e. b divides a with no remainder Examples Trivial divisors: 1 a, a a Each of {1,2,3,4,6,8,12,24} divides 24 5 does not divide 24 Prime numbers An integer a is prime if it is only divisible by 1 and by itself. 23 is prime, 24 is not. page 7

8 Modular Arithmetic Modular operator: a mod b, (or a%b) is the remainder of a when divided by b I.e., the smallest r 0 s.t. integer q for which a = qb+r. (Thm: there is a single choice for such q,r) Examples 12 mod 5 = 2 10 mod 5 = 0-5 mod 5 = 0-1 mod 5 = 4 April 2011, 3 page 8

9 Modular congruency a is congruent to b modulo n (a b mod n) if (a-b) = 0 mod n Namely, n divides a-b In other words, (a mod n) = (b mod n) E.g., mod mod 5 There are n equivalence classes modulo n [3] 7 = {,-11,-4,3,10,17, } page 9

10 Greatest Common Divisor (GCD) d is a common divisor of a and b, if d a and d b. gcd(a,b) (Greatest Common Divisor), is the largest integer that divides both a and b. (a,b >= 0) gcd(a,b) = max k s.t. k a and k b. Examples: gcd(30,24) = 6 gcd(30,23) = 1 If gcd(a,b)=1 then a and b are denoted relatively prime. page 10

11 Facts about the GCD gcd(a,b) = gcd(b, a mod b) (interesting when a>b) Since (e.g., a=33, b=15) If c a and c b then c (a mod b) If c b and c (a mod b) then c a If a mod b = 0, then gcd(a,b)=b. Therefore, gcd(19,8) = gcd(8, 3) = gcd(3,2) = gcd(2,1) = 1 gcd(20,8) = gcd(8, 4) = 4 page 11

12 Euclid s algorithm Input: a>b>0 Output: gcd(a,b) Algorithm: 1. if (a mod b) = 0 return (b) 2. else return( gcd(b, a mod b) ) Complexity: O(log a) rounds Each round requires O(log 2 a) bit operations Actually, the total overhead can be shown to be O(log 2 a) page 12

13 The extended gcd algorithm Finding s, t such that gcd(a,b) = a s + b t Extended-gcd(a,b) /* output is (gcd(a,b), s, t) 1. If (a mod b=0) then return(b,0,1) 2. (d,s,t ) = Extended-gcd(b, a mod b) 3. (d,s,t) = (d, t, s - a/b t ) 4. return(d,s,t) Note that the overhead is as in the basic GCD algorithm page 13

14 Extended gcd algorithm Given a,b finds s,t such that gcd(a,b) = a s + b t In particular, if p is prime and a<p then gcd(a,p)=1, and therefore a s+p t=1. April 2011, 3 page 14

15 Extended gcd algorithm Given a,b finds s,t such that gcd(a,b) = a s + b t In particular, if p is prime and a<p then gcd(a,p)=1, and therefore a s+p t=1. This implies that (a s 1 mod p) April 2011, 3 page 15

16 Extended gcd algorithm Given a,b finds s,t such that gcd(a,b) = a s + b t In particular, if p is prime and a<p then gcd(a,p)=1, and therefore a s+p t=1. This implies that (a s 1 mod p) THM: There is no positive integer smaller than gcd(a,b) which can be represented as a linear combination of a,b. For example, a=12, b=8. 4= There are no s,t for which 2=s 12 + t 8 Therefore if we find s,t such that as+tb=1, then we know that gcd(a,b)=1 April November 3, , 2009 page 16

17 Groups Definition: a set G with a binary operation :G G G is called a group if: (closure) a,b G, it holds that a b G. (associativity) a,b,c G, (a b) c = a (b c). (identity element) e G, s.t. a G it holds that a e =a. (inverse element) a G a -1 G, s.t. a a -1 = e. A group is Abelian (commutative) if a,b G, it holds that a b = b a. Examples: Integers under addition (Z,+) = {,-3,-2,-1,0,1,2,3, } page 17

18 More examples of groups Addition modulo N (G, ) = ({0,1,2,,N-1}, +) page 18

19 More examples of groups Z p * Multiplication modulo a prime number p (G, ) = ({1,2,,p-1}, ) E.g., Z 7* = ( {1,2,3,4,5,6}, ) Closure (the result of the multiplication is never divisible by p): sa+tp = 1 s b+t p = 1 ss (ab)+(sat +s bt+ tt p)p = 1 Therefore 1=gcd(ab,p). Trivial: associativity, existence of identity element. The extended GCD algorithm shows that an inverse always exists: sa+tp = 1 sa = 1-tp sa 1 mod p page 19

20 More examples of groups Z N * Multiplication modulo a composite number N (G, ) = ({a s.t. 1 a N-1 and gcd(a,n)=1}, ) E.g., Z 10* = ( {1,3,7,9}, ) Closure: sa+tn = 1 s b+t N = 1 ss (ab)+(sat +s bt+ tt N)N = 1 Therefore 1=gcd(ab,N). Associativity: trivial Existence of identity element: 1. Inverse element: as in Z p * page 20

21 Subgroups Let (G, ) be a group. (H, ) is a subgroup of G if (H, ) is a group H G For example, H = ( {1,2,4}, ) is a subgroup of Z 7*. Lagrange s theorem: If (G, ) is finite and (H, ) is a subgroup of (G, ), then H divides G 7 In our example: 3 6. page 21

22 Cyclic Groups Exponentiation is repeated application of a 3 = a a a. a 0 = 1. a -x = (a -1 ) x A group G is cyclic if there exists a generator g, s.t. a G, i s.t. g i =a. I.e., G= <g> = {1, g, g 2, g 3, } For example Z * 7 = <3> = {1,3,2,6,4,5} Not all a G are generators of G, but they all generate a subgroup of G. E.g. 2 is not a generator of Z * 7 The order of a group element a is the smallest j>0 s.t. a j =1 Lagrange s theorem for x Z p*, ord(x) p-1. page 22

23 Fermat s theorem Corollary of Lagrange s theorem: if (G, ) is a finite group, then a G, a G =1. Proof: a G generates a subgroup H of G. A H = 1 Lagrange theorem: H G A G = (A H ) G / H = 1 page 23

24 Fermat s theorem Corollary of Lagrange s theorem: if (G, ) is a finite group, then a G, a G =1. Corollary (Fermat s theorem): a Z * p, a p-1 =1 mod p. E.g., for all a Z 7*, a 6 =1, a 7 =a. Computing inverses: Given a G, how to compute a -1? Fermat s theorem: a -1 = a G -1 (= a p-2 in Z * p ) Or, using the extended gcd algorithm (for Z p * or Z N *): gcd(a,p) = 1 sa + tp = 1 sa = -tp + 1 s is a -1!! Which is more efficient? page 24

25 Computing in Z p * page 25

26 Groups we will use Z p * Multiplication modulo a prime number p (G, ) = ({1,2,,p-1}, ) E.g., Z 7* = ( {1,2,3,4,5,6}, ) Z * N Multiplication modulo a composite number N (G, ) = ({a s.t. 1 a N-1 and gcd(a,n)=1}, ) E.g., Z 10* = ( {1,3,7,9}, ) page 26

27 Euler s phi function Lagrange s Theorem: a in a finite group G, a G =1. Euler s phi function (aka, Euler s totient function), φ(n) = number of elements in Z * n φ(p) = p-1 for a prime p. n= i=1..k p e(i) i φ(n) = n i=1..k (1-1/p i ) φ(p 2 ) = p(p-1) for a prime p. n=pq φ(n) =(p-1)(q-1) (i.e. {x gcd(x,n)=1, 1 x n} Corollary: For Z n* (n=pq), Z n* = φ(n) =(p-1)(q-1). a Z n* it holds that a φ(n) =1 mod n For Z p* (prime p), a p-1 =1 mod p (Fermat s theorem). For Z n* (n=pq), a (p-1)(q-1) =1 mod n page 27

28 Finding prime numbers page 28

29 Finding prime numbers Prime number theorem: #{primes x} x / lnx as x How can we find a random k-bit prime? Choose x at random in {2 k,,2 k+1-1} (How many numbers in that range are prime? About 2 k+1 / ln2 k+1-2 k / ln2 k numbers, i.e. a 1/ ln(2 k ) fraction.) Test if x is prime (more on this later in the course) The probability of success is 1/ln(2 k ) = O(1/k). The expected number of trials is O(k). page 29

30 Finding generators How can we find a generator of Z p*? Pick a random number a [1,p-1], check if is a generator Naively, check whether 1 i p-2 a i 1 But we know that if a i =1 mod p then i p-1. Therefore need to only check i for which i p-1. Easy if we know the factorization of (p-1). In that case For all a Z p*, the order of a divides (p-1) For every integer divisor b of (p-1), check if a b =1 mod p. If none of these checks succeeds, then a is a generator, since its order must be p-1. page 30

31 Finding prime numbers of the right form How can we know the factorization of p-1? Easy, for example, if p=2q+1, and q is prime. How can we find a k-bit prime of this form? 1. Search for a prime number q of length k-1 bits. (Will be successful after about O(k) attempts.) 2. Check if 2q+1 is prime (we will see how to do this later in the course). 3. If not, go to step 1. page 31

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