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Source file src/math/big/prime.go

  // Copyright 2016 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  
  package big
  
  import "math/rand"
  
  // ProbablyPrime reports whether x is probably prime,
  // applying the Miller-Rabin test with n pseudorandomly chosen bases
  // as well as a Baillie-PSW test.
  //
  // If x is prime, ProbablyPrime returns true.
  // If x is chosen randomly and not prime, ProbablyPrime probably returns false.
  // The probability of returning true for a randomly chosen non-prime is at most ¼ⁿ.
  //
  // ProbablyPrime is 100% accurate for inputs less than 2⁶⁴.
  // See Menezes et al., Handbook of Applied Cryptography, 1997, pp. 145-149,
  // and FIPS 186-4 Appendix F for further discussion of the error probabilities.
  //
  // ProbablyPrime is not suitable for judging primes that an adversary may
  // have crafted to fool the test.
  //
  // As of Go 1.8, ProbablyPrime(0) is allowed and applies only a Baillie-PSW test.
  // Before Go 1.8, ProbablyPrime applied only the Miller-Rabin tests, and ProbablyPrime(0) panicked.
  func (x *Int) ProbablyPrime(n int) bool {
  	// Note regarding the doc comment above:
  	// It would be more precise to say that the Baillie-PSW test uses the
  	// extra strong Lucas test as its Lucas test, but since no one knows
  	// how to tell any of the Lucas tests apart inside a Baillie-PSW test
  	// (they all work equally well empirically), that detail need not be
  	// documented or implicitly guaranteed.
  	// The comment does avoid saying "the" Baillie-PSW test
  	// because of this general ambiguity.
  
  	if n < 0 {
  		panic("negative n for ProbablyPrime")
  	}
  	if x.neg || len(x.abs) == 0 {
  		return false
  	}
  
  	// primeBitMask records the primes < 64.
  	const primeBitMask uint64 = 1<<2 | 1<<3 | 1<<5 | 1<<7 |
  		1<<11 | 1<<13 | 1<<17 | 1<<19 | 1<<23 | 1<<29 | 1<<31 |
  		1<<37 | 1<<41 | 1<<43 | 1<<47 | 1<<53 | 1<<59 | 1<<61
  
  	w := x.abs[0]
  	if len(x.abs) == 1 && w < 64 {
  		return primeBitMask&(1<<w) != 0
  	}
  
  	if w&1 == 0 {
  		return false // n is even
  	}
  
  	const primesA = 3 * 5 * 7 * 11 * 13 * 17 * 19 * 23 * 37
  	const primesB = 29 * 31 * 41 * 43 * 47 * 53
  
  	var rA, rB uint32
  	switch _W {
  	case 32:
  		rA = uint32(x.abs.modW(primesA))
  		rB = uint32(x.abs.modW(primesB))
  	case 64:
  		r := x.abs.modW((primesA * primesB) & _M)
  		rA = uint32(r % primesA)
  		rB = uint32(r % primesB)
  	default:
  		panic("math/big: invalid word size")
  	}
  
  	if rA%3 == 0 || rA%5 == 0 || rA%7 == 0 || rA%11 == 0 || rA%13 == 0 || rA%17 == 0 || rA%19 == 0 || rA%23 == 0 || rA%37 == 0 ||
  		rB%29 == 0 || rB%31 == 0 || rB%41 == 0 || rB%43 == 0 || rB%47 == 0 || rB%53 == 0 {
  		return false
  	}
  
  	return x.abs.probablyPrimeMillerRabin(n+1, true) && x.abs.probablyPrimeLucas()
  }
  
  // probablyPrimeMillerRabin reports whether n passes reps rounds of the
  // Miller-Rabin primality test, using pseudo-randomly chosen bases.
  // If force2 is true, one of the rounds is forced to use base 2.
  // See Handbook of Applied Cryptography, p. 139, Algorithm 4.24.
  // The number n is known to be non-zero.
  func (n nat) probablyPrimeMillerRabin(reps int, force2 bool) bool {
  	nm1 := nat(nil).sub(n, natOne)
  	// determine q, k such that nm1 = q << k
  	k := nm1.trailingZeroBits()
  	q := nat(nil).shr(nm1, k)
  
  	nm3 := nat(nil).sub(nm1, natTwo)
  	rand := rand.New(rand.NewSource(int64(n[0])))
  
  	var x, y, quotient nat
  	nm3Len := nm3.bitLen()
  
  NextRandom:
  	for i := 0; i < reps; i++ {
  		if i == reps-1 && force2 {
  			x = x.set(natTwo)
  		} else {
  			x = x.random(rand, nm3, nm3Len)
  			x = x.add(x, natTwo)
  		}
  		y = y.expNN(x, q, n)
  		if y.cmp(natOne) == 0 || y.cmp(nm1) == 0 {
  			continue
  		}
  		for j := uint(1); j < k; j++ {
  			y = y.mul(y, y)
  			quotient, y = quotient.div(y, y, n)
  			if y.cmp(nm1) == 0 {
  				continue NextRandom
  			}
  			if y.cmp(natOne) == 0 {
  				return false
  			}
  		}
  		return false
  	}
  
  	return true
  }
  
  // probablyPrimeLucas reports whether n passes the "almost extra strong" Lucas probable prime test,
  // using Baillie-OEIS parameter selection. This corresponds to "AESLPSP" on Jacobsen's tables (link below).
  // The combination of this test and a Miller-Rabin/Fermat test with base 2 gives a Baillie-PSW test.
  //
  // References:
  //
  // Baillie and Wagstaff, "Lucas Pseudoprimes", Mathematics of Computation 35(152),
  // October 1980, pp. 1391-1417, especially page 1401.
  // http://www.ams.org/journals/mcom/1980-35-152/S0025-5718-1980-0583518-6/S0025-5718-1980-0583518-6.pdf
  //
  // Grantham, "Frobenius Pseudoprimes", Mathematics of Computation 70(234),
  // March 2000, pp. 873-891.
  // http://www.ams.org/journals/mcom/2001-70-234/S0025-5718-00-01197-2/S0025-5718-00-01197-2.pdf
  //
  // Baillie, "Extra strong Lucas pseudoprimes", OEIS A217719, https://oeis.org/A217719.
  //
  // Jacobsen, "Pseudoprime Statistics, Tables, and Data", http://ntheory.org/pseudoprimes.html.
  //
  // Nicely, "The Baillie-PSW Primality Test", http://www.trnicely.net/misc/bpsw.html.
  // (Note that Nicely's definition of the "extra strong" test gives the wrong Jacobi condition,
  // as pointed out by Jacobsen.)
  //
  // Crandall and Pomerance, Prime Numbers: A Computational Perspective, 2nd ed.
  // Springer, 2005.
  func (n nat) probablyPrimeLucas() bool {
  	// Discard 0, 1.
  	if len(n) == 0 || n.cmp(natOne) == 0 {
  		return false
  	}
  	// Two is the only even prime.
  	// Already checked by caller, but here to allow testing in isolation.
  	if n[0]&1 == 0 {
  		return n.cmp(natTwo) == 0
  	}
  
  	// Baillie-OEIS "method C" for choosing D, P, Q,
  	// as in https://oeis.org/A217719/a217719.txt:
  	// try increasing P ≥ 3 such that D = P² - 4 (so Q = 1)
  	// until Jacobi(D, n) = -1.
  	// The search is expected to succeed for non-square n after just a few trials.
  	// After more than expected failures, check whether n is square
  	// (which would cause Jacobi(D, n) = 1 for all D not dividing n).
  	p := Word(3)
  	d := nat{1}
  	t1 := nat(nil) // temp
  	intD := &Int{abs: d}
  	intN := &Int{abs: n}
  	for ; ; p++ {
  		if p > 10000 {
  			// This is widely believed to be impossible.
  			// If we get a report, we'll want the exact number n.
  			panic("math/big: internal error: cannot find (D/n) = -1 for " + intN.String())
  		}
  		d[0] = p*p - 4
  		j := Jacobi(intD, intN)
  		if j == -1 {
  			break
  		}
  		if j == 0 {
  			// d = p²-4 = (p-2)(p+2).
  			// If (d/n) == 0 then d shares a prime factor with n.
  			// Since the loop proceeds in increasing p and starts with p-2==1,
  			// the shared prime factor must be p+2.
  			// If p+2 == n, then n is prime; otherwise p+2 is a proper factor of n.
  			return len(n) == 1 && n[0] == p+2
  		}
  		if p == 40 {
  			// We'll never find (d/n) = -1 if n is a square.
  			// If n is a non-square we expect to find a d in just a few attempts on average.
  			// After 40 attempts, take a moment to check if n is indeed a square.
  			t1 = t1.sqrt(n)
  			t1 = t1.mul(t1, t1)
  			if t1.cmp(n) == 0 {
  				return false
  			}
  		}
  	}
  
  	// Grantham definition of "extra strong Lucas pseudoprime", after Thm 2.3 on p. 876
  	// (D, P, Q above have become Δ, b, 1):
  	//
  	// Let U_n = U_n(b, 1), V_n = V_n(b, 1), and Δ = b²-4.
  	// An extra strong Lucas pseudoprime to base b is a composite n = 2^r s + Jacobi(Δ, n),
  	// where s is odd and gcd(n, 2*Δ) = 1, such that either (i) U_s ≡ 0 mod n and V_s ≡ ±2 mod n,
  	// or (ii) V_{2^t s} ≡ 0 mod n for some 0 ≤ t < r-1.
  	//
  	// We know gcd(n, Δ) = 1 or else we'd have found Jacobi(d, n) == 0 above.
  	// We know gcd(n, 2) = 1 because n is odd.
  	//
  	// Arrange s = (n - Jacobi(Δ, n)) / 2^r = (n+1) / 2^r.
  	s := nat(nil).add(n, natOne)
  	r := int(s.trailingZeroBits())
  	s = s.shr(s, uint(r))
  	nm2 := nat(nil).sub(n, natTwo) // n-2
  
  	// We apply the "almost extra strong" test, which checks the above conditions
  	// except for U_s ≡ 0 mod n, which allows us to avoid computing any U_k values.
  	// Jacobsen points out that maybe we should just do the full extra strong test:
  	// "It is also possible to recover U_n using Crandall and Pomerance equation 3.13:
  	// U_n = D^-1 (2V_{n+1} - PV_n) allowing us to run the full extra-strong test
  	// at the cost of a single modular inversion. This computation is easy and fast in GMP,
  	// so we can get the full extra-strong test at essentially the same performance as the
  	// almost extra strong test."
  
  	// Compute Lucas sequence V_s(b, 1), where:
  	//
  	//	V(0) = 2
  	//	V(1) = P
  	//	V(k) = P V(k-1) - Q V(k-2).
  	//
  	// (Remember that due to method C above, P = b, Q = 1.)
  	//
  	// In general V(k) = α^k + β^k, where α and β are roots of x² - Px + Q.
  	// Crandall and Pomerance (p.147) observe that for 0 ≤ j ≤ k,
  	//
  	//	V(j+k) = V(j)V(k) - V(k-j).
  	//
  	// So in particular, to quickly double the subscript:
  	//
  	//	V(2k) = V(k)² - 2
  	//	V(2k+1) = V(k) V(k+1) - P
  	//
  	// We can therefore start with k=0 and build up to k=s in log₂(s) steps.
  	natP := nat(nil).setWord(p)
  	vk := nat(nil).setWord(2)
  	vk1 := nat(nil).setWord(p)
  	t2 := nat(nil) // temp
  	for i := int(s.bitLen()); i >= 0; i-- {
  		if s.bit(uint(i)) != 0 {
  			// k' = 2k+1
  			// V(k') = V(2k+1) = V(k) V(k+1) - P.
  			t1 = t1.mul(vk, vk1)
  			t1 = t1.add(t1, n)
  			t1 = t1.sub(t1, natP)
  			t2, vk = t2.div(vk, t1, n)
  			// V(k'+1) = V(2k+2) = V(k+1)² - 2.
  			t1 = t1.mul(vk1, vk1)
  			t1 = t1.add(t1, nm2)
  			t2, vk1 = t2.div(vk1, t1, n)
  		} else {
  			// k' = 2k
  			// V(k'+1) = V(2k+1) = V(k) V(k+1) - P.
  			t1 = t1.mul(vk, vk1)
  			t1 = t1.add(t1, n)
  			t1 = t1.sub(t1, natP)
  			t2, vk1 = t2.div(vk1, t1, n)
  			// V(k') = V(2k) = V(k)² - 2
  			t1 = t1.mul(vk, vk)
  			t1 = t1.add(t1, nm2)
  			t2, vk = t2.div(vk, t1, n)
  		}
  	}
  
  	// Now k=s, so vk = V(s). Check V(s) ≡ ±2 (mod n).
  	if vk.cmp(natTwo) == 0 || vk.cmp(nm2) == 0 {
  		// Check U(s) ≡ 0.
  		// As suggested by Jacobsen, apply Crandall and Pomerance equation 3.13:
  		//
  		//	U(k) = D⁻¹ (2 V(k+1) - P V(k))
  		//
  		// Since we are checking for U(k) == 0 it suffices to check 2 V(k+1) == P V(k) mod n,
  		// or P V(k) - 2 V(k+1) == 0 mod n.
  		t1 := t1.mul(vk, natP)
  		t2 := t2.shl(vk1, 1)
  		if t1.cmp(t2) < 0 {
  			t1, t2 = t2, t1
  		}
  		t1 = t1.sub(t1, t2)
  		t3 := vk1 // steal vk1, no longer needed below
  		vk1 = nil
  		_ = vk1
  		t2, t3 = t2.div(t3, t1, n)
  		if len(t3) == 0 {
  			return true
  		}
  	}
  
  	// Check V(2^t s) ≡ 0 mod n for some 0 ≤ t < r-1.
  	for t := 0; t < r-1; t++ {
  		if len(vk) == 0 { // vk == 0
  			return true
  		}
  		// Optimization: V(k) = 2 is a fixed point for V(k') = V(k)² - 2,
  		// so if V(k) = 2, we can stop: we will never find a future V(k) == 0.
  		if len(vk) == 1 && vk[0] == 2 { // vk == 2
  			return false
  		}
  		// k' = 2k
  		// V(k') = V(2k) = V(k)² - 2
  		t1 = t1.mul(vk, vk)
  		t1 = t1.sub(t1, natTwo)
  		t2, vk = t2.div(vk, t1, n)
  	}
  	return false
  }
  

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