# Cryptography and Network Security Number Theory

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1 Cryptography and Network Security Number Theory Xiang-Yang Li

2 Introduction to Number Theory Divisors b a if a=mb for an integer m b a and c b then c a b g and b h then b (mg+nh) for any int. m,n Prime number P has only positive divisors 1 and p Relatively prime numbers No common divisors for p and q except 1

3 GCD Greatest common divisor gcd(a,b) The largest number that divides both a and b Euclid's algorithm Find the GCD of two numbers a and b, a<b Use fact if a and b have divisor d so does a- b, a-2b

4 Cont. GCD (a,b) is given by: let g 0 =b g 1 =a g i+1 = g i-1 mod g i when g i =0 then gcd(a,b) = g i-1 The algorithm terminates in O(log b) rounds Why?

5 Properties For any two integers a and b Exist integers m and n: gcd(a,b) =ma+bn Example: a=2, b=3; we choose m=-1, n=1 so 2+3=1 a=6, b=11; we choose m=2, n=-1 so 2*6-11=1 Simple proof? Integer a can be factored as a=p 1 a 1 p 2 a 2 p 3 a 3. p n a n where p i is prime number

6 Modular Arithmetic Congruence a b mod n says when divided by n that a and b have the same remainder It defines a relationship between all integers a a a b then b a a b, b c then a c

7 Cont. addition (a+b) mod n (a mod n) + (b mod n) subtraction a-b mod n a+(-b) mod n multiplication a*b mod n derived from repeated addition Possible: a*b 0 where neither a, b 0 mod n

8 Cont. Division a/b mod n multiplied by inverse of b: a/b = a*b -1 mod n b -1* b 1 mod n mod 10 because 3*7 1 mod 10 Inverse does not always exist! Only when gcd(b,n)=1

9 Addition and Multiplication Integers modulo n with addition and multiplication form a commutative ring with the laws of Associativity (a+b)+c a+(b+c) mod n Commutativity a+b b+a mod n Distributivity (a+b)*c (a*c)+(b*c) mod n

10 Galois Field If n is constrained to be a prime number p then this forms a Galois field modulo p denoted GF(p) and all the normal laws associated with integer arithmetic work Exponentiation b = a e mod p Discrete Logarithms find x where a x = b mod p

11 Inverses and Euclid's Extended GCD Routine If (a,n)=1 then the inverse always exists Can extend Euclid's algorithm to find inverse by keeping track of g i = u i.n + v i.a Extended Euclid's (or binary GCD) algorithm to find inverse of a number a mod n (where (a,n)=1) is:

12 Inverse Inverse(a,n) is given by: X=(x1,x2,x3)=(1,0,n); Y=(y1,y2,y3)=(0,1,a) If y3=0 return x3=gcd(a,n); no inverse If y3=1 return y3=gcd(a,n); y2=a -1 mod n Q=[x3/y3] T=X-Q*Y X=Y; Y=T Goto 2 nd step

13 Proof Assume we compute gcd(x 0,y 0 ) Let X i =(x i,y i ); 0 x i -q i+1 y i < y i Then X i =M i X i-1, where M i =(0,1; 1,-q i ) Assume the gcd algorithm terminates in n steps We have M n M n-1 M 1 X 0 =(gcd(x 0,y 0 ), 0) T Assume M n M n-1 M 1 =( ) Then ax 0 +by 0 =gcd(x 0,y 0 ) a c b d The above algorithm is to keep track of a,b,c,d, and x i,y i values.

14 Euler Totient Function If consider arithmetic modulo n, then a reduced set of residues is a subset of the complete set of residues modulo n which are relatively prime to n eg for n=10, the complete set of residues is {0,1,2,3,4,5,6,7,8,9} the reduced set of residues is {1,3,7,9} The number of elements in the reduced set of residues is called the Euler Totient function f(n)

15 Euler's Theorem Let gcd(a,n)=1 then a φ(n) mod n = 1 Proof: consider all reduced residues x i Then ax i also form reduced residues set Using Π ax i = Π x i mod n and Π x i has inverse Compute φ(n) If factoring of n is known φ(n)=n Π(1-1/p i ) where p i is its prime factor Otherwise It is expensive!

16 Fermat's Theorem Let p be a prime and gcd(a,p)=1 then a p-1 mod p = 1 Proof: similar to the proof of Euler s theorem But consider all integers in Z p Generally, for any prime number p a p mod p = a

17 Chinese Remainder Theorem Let m 1,m 2,.m k be pair-wise relative prime numbers Assume integer A= a i mod m i Then A= Σ a i C i mod M Where M=Π m i ; M i =M/ m i C i = M i * (M i -1 mod m i )

18 Primality Testing To check if exists integer a such that a p Primary school method Two slow! Check n 0.5 numbers At least around (n/ln n) 0.5 numbers need be checked Any improvement?

19 Simple Fact Equation x 2 1 mod p has only solutions1,-1 If p is prime number Simple proof: (x+1)(x-1) 0 mod p So if we find another solution, then p can not be prime number! Miller and Rabin 1975,1980 Randomly chosen integer a If a 2 1 mod p then p is not prime number Integer a is called the witness Otherwise p maybe, or maybe not a prime number

20 Witness Algorithm Witness(a,n) Let b k b k-1 b 1 b 0 be the binary code of n-1 Let d=1 For i=k downto 0 x=d; d=d*d mod n If d=1 and x 1, and x n-1 return TRUE If b i =1 then d=d*a mod n Endfor If d 1 then return TRUE Return FALSE

21 Facts Analysis the result of witness If returns TRUE, then n is not prime number Find other solutions for x 2 1 mod n Otherwise, n maybe prime number Given odd n and random a Witness fails with probability less than 0.5 Run witness algorithm s times If one time, it is TRUE Then n is not prime number Otherwise, Pr(n is prime)>1-2 -s

22 Randomized Methods Las Vegas Method Always produces correct results Runs in expected polynomial time Monte Carlo Method Runs in polynomial time May produce incorrect results with bounded probability No-Biased Monte Carlo Method Answer yes is always correct, but the answer no may be wrong Yes-biased Monte Carlo Method Answer no is always correct, but the answer yes may be wrong

23 Witness Algorithm Witness Algorithm is based on Monte Carlo Method It actually test compositeness, not primality When it reports yes, the number is always composite When it reports no, input may be composite, prime Probability Result Pr(input=composite ans=composite)= 1 Pr(ans=no input=composite)<1/2 Pr(input=composite ans=no) 1/4

24 Time Complexity Each round of witness cost O(log n) Unit: integer multiplication and modular arithmetic So the primality testing cost O(slog n) The confidence is 1-2 -s if report prime The confidence is 1 if report non-prime

25 Primitive Root Order of integer The order of a modulo n is the smallest positive k such that a k 1 mod n Primitive Root Integer a is a primitive root of n if the order of a modulo n is φ(n) Not all integers have primitive root Example n=pq for primes p and q Prime p has φ(p-1) primitive roots

26 Discrete Logarithms Y g x mod p Compute x Time complexity O(e (ln p)1/3 (ln ln p) 2/3 )

27 Quadratic Residue Quadratic Residue Integer b is a quadratic residue of integer n if and only if x 2 b mod n has a solution for x Otherwise b is called quadratic nonresidue Given odd prime p, b is quadratic residue, iff b (p-1)/2 1 mod n b is quadratic nonresidue, iff b (p-1)/2-1 mod n

28 Complexity Theory The input length of a problem is the number n of symbols used to characterize it Function f(n) is order O(g(n)) if f(n)<=c* g(n), for all n>=n 0, for some c Polynomial time algorithm (P) solves any instance of a particular problem with input length n in time O(p(n)), where p is a polynomial

29 Cont. Non-deterministic polynomial time algorithm (NP) - is one for which any guess at the solution of an instance of the problem may be checked for validity in polynomial time. NP-complete problems - are a subclass of NP problems for which it is known that if any such problem has a polynomial time solution, then all NP problems have polynomial solutions. Co-NP: the complements of NP problems.

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