Dan Boneh

Intro. Number Theory

Modular e’th roots

Online Cryptography Course Dan Boneh

Dan Boneh

Modular e’th rootsWe know how to solve modular linear equations:

a x + b = 0 ⋅ in ZN Solution: x = −b a⋅ -1 in ZN

What about higher degree polynomials?

Example: let p be a prime and c Z∈ p . Can we solve:

x2 – c = 0 , y3 – c = 0 , z37 – c = 0 in Zp

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Modular e’th rootsLet p be a prime and c Z∈ p .

Def: x Z∈ p s.t. xe = c in Zp is called an e’th root of c .

Examples: 71/3 = 6 in

31/2 = 5 in

11/3 = 1 in

21/2 does not exist in

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The easy caseWhen does c1/e in Zp exist? Can we compute it efficiently?

The easy case: suppose gcd( e , p-1 ) = 1Then for all c in (Zp)*: c1/e exists in Zp and is easy to

find.

Proof: let d = e-1 in Zp-1 . Then

d e = 1 in Z⋅ p-1 ⇒

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The case e=2: square rootsIf p is an odd prime then gcd( 2, p-1) ≠ 1

Fact: in , x x⟶ 2 is a 2-to-1 function

Example: in :

Def: x in is a quadratic residue (Q.R.) if it has a square root in

p odd prime the # of Q.R. in is (p-1)/2 + 1 ⇒

1 10

1

2 9

4

3 8

9

4 7

5

5 6

3

x −x

x2

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Euler’s theoremThm: x in (Zp)* is a Q.R. x⟺ (p-1)/2 = 1 in Zp (p odd prime)

Example:

Note: x≠0 x⇒ (p-1)/2 = (xp-1)1/2 = 11/2 { 1, -1 } in Z∈ p

Def: x(p-1)/2 is called the Legendre Symbol of x over p (1798)

in : 15, 25, 35, 45, 55, 65, 75, 85, 95, 105

= 1 -1 1 1 1, -1, -1, -1, 1, -1

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Computing square roots mod pSuppose p = 3 (mod 4)

Lemma: if c (Z∈ p)* is Q.R. then √c = c(p+1)/4 in Zp

Proof:

When p = 1 (mod 4), can also be done efficiently, but a bit harder

run time ≈ O(log3 p)

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Solving quadratic equations mod pSolve: a x⋅ 2 + b x + c = 0 in Z⋅ p

Solution: x = (-b ± √b2 – 4 a c ) / 2a in Z⋅ ⋅ p

• Find (2a)-1 in Zp using extended Euclid.

• Find square root of b2 – 4 a c ⋅ ⋅ in Zp (if one exists)

using a square root algorithm

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Computing e’th roots mod N ??Let N be a composite number and e>1

When does c1/e in ZN exist? Can we compute it efficiently?

Answering these questions requires the factorization of N(as far as we know)

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