Math 609/597: Cryptography 1

Math 609/597: Cryptography 1 The Solovay-Strasse Primality Test 12 October, 1993 Burt Roseberg Revised: 6 October, 2000 1 Itroductio We describe the Solovay-Strasse primality test. There is quite a bit of umber-theoretic backgroud ecessary to the full uderstadig of the algorithm, however, i practice it is very simple. It is also curious because it works icredibly quickly to give you a probably correct aswer, however o oe has foud a less tha expoetial-time algorithm to tell you for certai whether a umber is prime. The algorithm works by selectig radom itegers ad computig large powers of them i the rig Z/Z, where is the umber you wat to test. Also, the so called Jacobi symbol is calculated for these itegers. If ever these calculatios disagree, the is composite. For if were prime, the Jacobi symbol would i fact be the Legedre symbol, ad for the Legedre symbol equality of the two methods of calculatio is a theorem. It is ot ecessary for the two calculatios to disagree whe is composite, but it is likely. Half of the itegers betwee 1 ad 1 which are relatively prime to will make the calculatios disagree. (If we happe to choose a iteger which is ot relatively prime to, we are eve better off: we ot oly kow is composite, we have a o-trival factor!) Hece, the probablity that after k choices of a radom iteger you would wrogly proclaim a composite to be prime is less tha 1/2 k. I practice, the results are eve better. We by explaiig the Legedre Symbol, the exted the defiitio to the Jacobi Symbol. From there we apply the Jacobi Symbol to primality testig. Fially, a Pascal program is preseted. 2 Quadratic Residues Suppose p is a odd prime. I (Z/pZ), the group of ivertible elemets mod p, that is to say, Z/pZ without 0, half of the itegers are squares ad the rest are ot. This is quickly see by cosiderig the map x x 2. Each elemet a i the rage of this map receives exactly two elemets, amely, if b 2 a the ( b) 2 a, that is, if we ca assume that b b, which is equivalet to assumig p 2. The elemets which are squares are called quadratic residues, the rest are quadratic o-residues. Defiitio 1 (Legedre Symbol) For p a prime, ad b a positive iteger, The Legedre Symbol is defied by, [ ] b 0 if b ad p are ot relatively prime, 1 if b is a quadratic residue mod p, p 1 if b is a quadratic o-residue mod p If p is two, the the value of the Legedre Symbol is oe for ay odd b ad 0 else. For p a odd prime, we ca use this theorem: Theorem 1 For ay odd prime p ad ay positive iteger b, [ ] b b (p 1)/2 (mod p). p

Math 609/597: Cryptography 2 Proof: If (b, p) 1 the b 0 (mod p) ad the equality follows. We heceforth cosider itegers b relatively prime to p. It is quickly see that the set, A { a (Z/pZ) a (p 1)/2 1 (mod p) }, form a subgroup of (Z/pZ). Ay quadratic residue b a 2 is i A, sice, b (p 1)/2 a 2(p 1)/2 1 (mod p), by Little Fermat. We remarked above that half the elemets of (Z/pZ) are quadratic residues, hece A is of size either (p 1) ad (p 1)/2 (the order of a subgroup must divide the order of the group). The subgroup A does ot iclude ay geerator of the group, hece its size is ot (p 1). Therefore A cotais exactly the quadratic residues. O the other had, by Little Fermat, the square of b (p 1)/2 is 1, hece for those itegers relatively prime to p but outside of A, the power must evaluate to the oly other root of oe, that is, 1. There are several rules for computig with Legedre symbols, for istace: that the Legedre symbol depeds oly o the residue of b mod p, that the Legedre symbol of 1 over p is always 1, ad that, [ ] [ ] [ ] b1 b 2 b1 b2. p p p These are easy to verify. Two rules which are more difficult to show are: Theorem 2 For p a odd prime, [ ] 2 ( 1) (p2 1)/8 p { 1 if p ±1 (mod 8) 1 if p ±3 (mod 8) Theorem 3 (Law of Quadratic Reciprocity) For p ad q odd primes, [ ] [ ] { p q ( 1) (p 1)(q 1)/4 1 if p q 3 (mod 4) q p 1 else. For proofs see Neal Koblitz s A Course i Number Theory ad Cryptography, or for a echatigly elemetary proof, Adré Weil s Number Theory for Begiers. 3 Jacobi Symbols The Jacobi Symbol exteds the defiitio of the Legedre Symbol to deomiators other tha primes. I doig so, it loses the umber-theoretic iterpretatio, it o loger idicates which itegers are quadratic residues, ad it is o loger possible to calculate it by takig the umerator to a certai power mod the deomiator.

Math 609/597: Cryptography 3 Defiitio 2 (Jacobi Symbol) For ay positive iger, we defie the Jacobi symbol accordig to the prime decompositio of by, r ( α if p i m ) r [ ] αi m i, the. i1 Note immediately that the Jacobi symbol has values either 1, 1 or 0, ad it is zero oly if oe of its factors is zero, that is, m ad are ot relatively prime. Sice the Euclidea algorithm efficietly determies if two itegers are relatively prime, from the poit of view of calculatig the Jacobi symbol, attetio focuses o the case of m ad relatively prime. Theorem 4 For itegers ad m, ad factorizatios of 1 2 ad m m 1 m 2, the Jacobi symbols obeys: ( m1 m ) ( 2 m1 ) ( m2 ), ad ( ) ( ) ( ) m m m, 1 2 1 2 Proof: For the first equivalece, write out the Jacobi symbol as a product of Legedre symbols, apply the rule, [ 1 ] [ 2 1 ] [ 2 ], m m m for Legedre symbols, the rearrage ad collect terms. For the secod equivalece, write out the Jacobi symbol accordig to its defiitio, the collect terms. Theorem 5 For ad m relatively prime ad odd, ( ) ( m ) ( 1) (m 1)( 1)/4. m Proof: Let i be the umber of prime factors i ad j the umber of prime factors i m, coutig multiplicity. We use iductio o i ad j. Whe i j 1, the basis case, the theorem is exactly the law of quadratic reciprocity. Assume ow that the theorem is true for ay i < I ad j < J, with I ad J greater tha oe. We show it is true for ay i < I + 1 ad j < J + 1. Let be a iteger with I + 1 factors. Write it as the product of two itegers 1 2 each havig less tha I factors, ad apply the iductio hypothesis to the factored Jacobi symbols: ( ) ( ) ( ) ( ) ( ) ( ) 1 2 m 1 2 m m m 1 2 m m 1 2 i1 ( 1) (1 1)(m 1)/4 ( 1) (2 1)(m 1)/4 p i ( 1) ( 1+ 2 2)(m 1)/4. Hece our product is 1 if both m ad 1 + 1 are 3 mod 4, ad 1 else. Note that, sice 1 ad 2 are both odd, ( 1 1)( 2 1) is divisible by 4. Multiplig this out, we get 1 + 2 1 mod 4. So the product is 1 if ad m are 3 mod 4, ad 1 else. That is, ( 1) ( 1+ 2 2)(m 1)/4 ( 1) ( 1)(m 1)/4,

Math 609/597: Cryptography 4 which proves the theorem for i I ad ad ay j < J. Swappig the role of ad m gives that the theorem is true for i I ad j J. Cotiuig by iductio, the theorem is true for all i ad j. I calculatig the Jacobi symbol we use the previous theorem ad reductio of the umerator modulo the deomiator to reduce step by step the calculatio, except if the umerator or deomiator is eve. If both are eve, the the result is zero. We ca throw out all factors of two of the deomiator, sice ay odd is a quadratic residue mod 2. If the umerator is eve, its factors of two are treated usig the followig theorem. Theorem 6 For ay odd iteger, ( ) 2 ( 1) (2 1)/8. Proof: Similar to the previous theorem, we use a iductio o the umber of elemets i the prime decompositio of. The basis case is is a prime, whe the Jacobi symbol equals the Legedre symbol ad the theorem is true by defiitio. Suppose the theorem is true for all which are the product of less tha I primes. Write, a product of I primes, as 1 2 ad calculate, usig the iductio hypothesis: ( ) ( ) ( ) 2 2 2 1 2 1 2 ( 1) (12 + 2 2 2)/8 Note that a odd iteger is either 1 or 3 mod 4, so ay square of a odd iteger is 1 mod 4. Therefore ( 1 2 1)( 2 2 1) is zero mod 16. Multiplig this out we fid, 2 1 2 + 2 2 1 (mod 16). Recall that m 2 1 is divisible by 8 for ay odd m, so 2 1 ad 1 2 + 2 2 2 are equal ad both either 0 or 8 mod 16. I ay case, ( 1) (12 + 2 2 2)/8 ( 1) (2 1)/8, provig the theorem for ay iteger a product of I primes. Proceedig the iductio, we have the theorem for all odd. 4 Applicatio to Primality Testig The Jacobi symbol is used to test for primality of a give iteger by testig for agreemet betwee two calculatios, ( ) b (?) b ( 1)/2 (mod ), which, if is a prime, is a idetity for the Legedre symbol. If is ot a prime, however, either b will ot be relatively prime to, or the two calculatios might ot agree. How ofte they do ot agree is discussed ext.

Math 609/597: Cryptography 5 Theorem 7 For ay prime p, there is a geerator for (Z/p 2 Z). Proof: For ay prime p, there is a geerator g for (Z/pZ). Let h equal g or g(1 + p), depedig o whether or ot g p 1? 1 (mod p 2 ). If g p 1 1 (mod p 2 ), the (g(1 + p)) p 1 1 + (p 1)p + p 2 w 1 + p(p 1) (mod p 2 ), where w is some iteger. Hece h ca be chose so that its p 1 power mod p 2 is ot oe. Sice i either case h g (mod p), h is a geerator of (Z/pZ), We show that h is a geerator of (Z/p 2 Z). Let h j 1 (mod p 2 ). This this cogruece remais true modulo p, therefore (p 1) j. This beig so, we ca write j as j (p 1)j. But j must also divide the order of the group, j p(p 1), hece j p. Sice h p 1 is ot oe, j caot be oe, so it must be p. Therefore the order of h is the size of the group. Theorem 8 For ay odd composite, there is a b relatively prime to such that, ( ) b b ( 1)/2 (mod ). Proof: If p 2, for a prime p, let g geerate (Z/p 2 Z). Select a b such that b g (mod p 2 ) ad b 1 (mod q) for ay other distict prime q dividig. The existece of b is assured by the Chiese Remaider Theorem. If the equatio were true, the b 1 1 (mod ), which beig a cogruece remaiig true i Z/p 2 Z, would imply that p(p 1) ( 1). However, the p would divide both ad 1. So we ca suppose is square free. Let g be a quadratic o-residue i some Z/pZ where p. Select a b, agai by the Chiese Remaider theorem, such that b g (mod p) ad b 1 (mod q) for ay other prime q dividig. The it is impossible for b ( 1)/2 1 (mod ), else this would be true Z/qZ for those primes where b is oe. However, the rules of calculatio for the Jacobi symbol give: ( ) b ( b p ) q ( ) b 1. q We used that is odd i two places, that there are quadratic o-residues ad that 1 1 (mod q). Theorem 9 If is a odd composite, the for at least half of the itegers b relatively prime to i the iterval [1, 1], ( ) b b ( 1)/2 (mod ).

Math 609/597: Cryptography 6 Proof: Let A be the set of a for which (a, ) 1 ad the equality holds. Sice there is a b relatively prime to for which the equality does ot hold, take ay a A ad cosider ab. It is agai relatively prime to ad, ( a ) ( ) b a ( 1)/2 b ( 1)/2 (mod ) because we are iside the group (Z/Z). Hece we have that all of ba does ot satisfy the equality, ad hece A caot accout for more tha half the elemets i [1, 1]. 5 Program program SolovayStrasse (iput,output) ; fuctio gcd( a, b : iteger ) : iteger ; var t : iteger ; if a < 0 the a : - a ; if b < 0 the b : - b ; if a < b the t : a ; a : b ; b : t ; while b<>0 do t : a mod b ; a : b ; b : t ; gcd : a fuctio twofactor( var a : iteger ) : iteger ; var i : iteger ; i : 0 ; while ((a mod 2) 0 ) do i : i + 1 ; a : a div 2 ; twofactor : i ;

Math 609/597: Cryptography 7 fuctio jacobi( m, : iteger ) : iteger ; { assume (m,)1, is odd, 0 < m <. } var i, j, d : iteger ; i : 1 ; while (m>1) do { it could be zero or 1 to exit} j : twofactor( m ) ; if ( j mod 2 ) 1 the d : mod 8 ; if ( d 3 ) or ( d 5 ) the i : - i ; if ( (m mod 4) 3 ) AND ( ( mod 4 ) 3 ) the i : - i ; d : mod m ; : m ; m : d ; jacobi : i ; fuctio multiply( a, b, c : iteger ) : iteger ; { retur a * b mod c, without overflow by repeated doublig } { assume 0 < a, b < c } var i : iteger ; if (a0) the i : 0 else if (a mod 2) 1 the i : multiply( (a-1) div 2, b, c ) ; if ( (c - i ) > i ) the i : i + i else i : ( i - c ) + i ; if ( (c - i ) > b ) the i : i + b else i : ( i - c ) + b ; ed else

Math 609/597: Cryptography 8 i : multiply( a div 2, b, c ) ; if ( (c - i ) > i ) the i : i + i else i : ( i - c ) + i ; ed multiply : i ; fuctio fastexp( b, j, : iteger ) : iteger ; { take b to the j mod } var i : iteger32 ; if (j0) the i : 1 else if ( j mod 2 ) 1 the i : fastexp( b, (j-1) div 2, ) ; i : multiply( multiply( b, i, ), i, ) ; ed else i : fastexp( b, j div 2, ) ; i : multiply( i, i, ) ; fastexp : i fuctio primality( p, i : iteger ) : iteger ; { give a iteger p, test for primality, usig i iteratios of some umbers, here they are the umber 2, 3,..., i+1. Returs 0 if o cotradictio betwee the jacobi ad legedre symbols was foud. Else returs the evidece that p is composite, either a factor or a iteger such that the sumbols differ. } var j : iteger ; dl, dj : iteger ; b : boolea ; if (p<2) the j : 1 { ot prime } else if (p2) the j : 0 { prime } else if (p mod 2)0 the j : 2 { ot prime } else {Precoditio: p is odd, 3 or larger}

Math 609/597: Cryptography 9 {Check if i is uecessarily large, ad correct} if (i+1)>p the i : p-2 ; j : 1 ; b : false ; repeat j : j + 1 ; if (j > (i+1) ) the b : true else { test for o-trivial gcd } if gcd(j,p)>1 the b : true else { apply tests } dl : fastexp( j, (p-1) div 2, p ) ; if (dl<>1) the dl : dl - p ; dj : jacobi( j, p ) ; if ( dj<>dl) the b : true ; util b ; if (j > (i+1)) the {test ra to completio} j : 0 ; primality : j ; var i,j,k : iteger ; c : char ; write( Quit [y/]? ) ; readl(c) ; while (c<> y ) do writel( Primality test of p, i iteratios, ) ; write( p? ) ; readl(j) ; write( i? ) ; readl(i) ; k : primality( j, i ) ; if ( k 0 ) the writel(j:0, might be a prime. ) else writel(j:0, is ot a prime, fails test usig,k:0) ; write( Quit [y/]? ) ; readl(c) ; ed.