crypto/rsa: add 3-prime support.

R=golang-dev, rsc1
CC=golang-dev
https://golang.org/cl/4365041

src/pkg/crypto/rsa/rsa.go

@@ -92,19 +92,21 @@ type PublicKey struct {
 type PrivateKey struct {
 	PublicKey          // public part.
 	D         *big.Int // private exponent
-	P, Q      *big.Int // prime factors of N
+	P, Q, R   *big.Int // prime factors of N (R may be nil)
 
 	rwMutex    sync.RWMutex // protects the following
-	dP, dQ  *big.Int     // D mod (P-1) (or mod Q-1) 
+	dP, dQ, dR *big.Int     // D mod (P-1) (or mod Q-1 etc) 
 	qInv       *big.Int     // q^-1 mod p
+	pq         *big.Int     // P*Q
+	tr         *big.Int     // pq·tr ≡ 1 mod r
 }
 
 // Validate performs basic sanity checks on the key.
 // It returns nil if the key is valid, or else an os.Error describing a problem.
 
 func (priv *PrivateKey) Validate() os.Error {
-	// Check that p and q are prime. Note that this is just a sanity
+	// Check that p, q and, maybe, r are prime. Note that this is just a
-	// check. Since the random witnesses chosen by ProbablyPrime are
+	// sanity check. Since the random witnesses chosen by ProbablyPrime are
 	// deterministic, given the candidate number, it's easy for an attack
 	// to generate composites that pass this test.
 	if !big.ProbablyPrime(priv.P, 20) {
@@ -113,16 +115,26 @@ func (priv *PrivateKey) Validate() os.Error {
 	if !big.ProbablyPrime(priv.Q, 20) {
 		return os.ErrorString("Q is composite")
 	}
+	if priv.R != nil && !big.ProbablyPrime(priv.R, 20) {
+		return os.ErrorString("R is composite")
+	}
 
-	// Check that p*q == n.
+	// Check that p*q*r == n.
 	modulus := new(big.Int).Mul(priv.P, priv.Q)
+	if priv.R != nil {
+		modulus.Mul(modulus, priv.R)
+	}
 	if modulus.Cmp(priv.N) != 0 {
 		return os.ErrorString("invalid modulus")
 	}
-	// Check that e and totient(p, q) are coprime.
+	// Check that e and totient(p, q, r) are coprime.
 	pminus1 := new(big.Int).Sub(priv.P, bigOne)
 	qminus1 := new(big.Int).Sub(priv.Q, bigOne)
 	totient := new(big.Int).Mul(pminus1, qminus1)
+	if priv.R != nil {
+		rminus1 := new(big.Int).Sub(priv.R, bigOne)
+		totient.Mul(totient, rminus1)
+	}
 	e := big.NewInt(int64(priv.E))
 	gcd := new(big.Int)
 	x := new(big.Int)
@@ -131,7 +143,7 @@ func (priv *PrivateKey) Validate() os.Error {
 	if gcd.Cmp(bigOne) != 0 {
 		return os.ErrorString("invalid public exponent E")
 	}
-	// Check that de ≡ 1 (mod totient(p, q))
+	// Check that de ≡ 1 (mod totient(p, q, r))
 	de := new(big.Int).Mul(priv.D, e)
 	de.Mod(de, totient)
 	if de.Cmp(bigOne) != 0 {
@@ -140,7 +152,7 @@ func (priv *PrivateKey) Validate() os.Error {
 	return nil
 }
 
-// GenerateKeyPair generates an RSA keypair of the given bit size.
+// GenerateKey generates an RSA keypair of the given bit size.
 func GenerateKey(rand io.Reader, bits int) (priv *PrivateKey, err os.Error) {
 	priv = new(PrivateKey)
 	// Smaller public exponents lead to faster public key
@@ -196,6 +208,77 @@ func GenerateKey(rand io.Reader, bits int) (priv *PrivateKey, err os.Error) {
 	return
 }
 
+// Generate3PrimeKey generates a 3-prime RSA keypair of the given bit size, as
+// suggested in [1]. Although the public keys are compatible (actually,
+// indistinguishable) from the 2-prime case, the private keys are not. Thus it
+// may not be possible to export 3-prime private keys in certain formats or to
+// subsequently import them into other code.
+//
+// Table 1 in [2] suggests that size should be >= 1024 when using 3 primes.
+//
+// [1] US patent 4405829 (1972, expired)
+// [2] http://www.cacr.math.uwaterloo.ca/techreports/2006/cacr2006-16.pdf
+func Generate3PrimeKey(rand io.Reader, bits int) (priv *PrivateKey, err os.Error) {
+	priv = new(PrivateKey)
+	priv.E = 3
+
+	pminus1 := new(big.Int)
+	qminus1 := new(big.Int)
+	rminus1 := new(big.Int)
+	totient := new(big.Int)
+
+	for {
+		p, err := randomPrime(rand, bits/3)
+		if err != nil {
+			return nil, err
+		}
+
+		todo := bits - p.BitLen()
+		q, err := randomPrime(rand, todo/2)
+		if err != nil {
+			return nil, err
+		}
+
+		todo -= q.BitLen()
+		r, err := randomPrime(rand, todo)
+		if err != nil {
+			return nil, err
+		}
+
+		if p.Cmp(q) == 0 ||
+			q.Cmp(r) == 0 ||
+			r.Cmp(p) == 0 {
+			continue
+		}
+
+		n := new(big.Int).Mul(p, q)
+		n.Mul(n, r)
+		pminus1.Sub(p, bigOne)
+		qminus1.Sub(q, bigOne)
+		rminus1.Sub(r, bigOne)
+		totient.Mul(pminus1, qminus1)
+		totient.Mul(totient, rminus1)
+
+		g := new(big.Int)
+		priv.D = new(big.Int)
+		y := new(big.Int)
+		e := big.NewInt(int64(priv.E))
+		big.GcdInt(g, priv.D, y, e, totient)
+
+		if g.Cmp(bigOne) == 0 {
+			priv.D.Add(priv.D, totient)
+			priv.P = p
+			priv.Q = q
+			priv.R = r
+			priv.N = n
+
+			break
+		}
+	}
+
+	return
+}
+
 // incCounter increments a four byte, big-endian counter.
 func incCounter(c *[4]byte) {
 	if c[3]++; c[3] != 0 {
@@ -336,6 +419,14 @@ func (priv *PrivateKey) precompute() {
 	priv.dQ.Mod(priv.D, priv.dQ)
 
 	priv.qInv = new(big.Int).ModInverse(priv.Q, priv.P)
+
+	if priv.R != nil {
+		priv.dR = new(big.Int).Sub(priv.R, bigOne)
+		priv.dR.Mod(priv.D, priv.dR)
+
+		priv.pq = new(big.Int).Mul(priv.P, priv.Q)
+		priv.tr = new(big.Int).ModInverse(priv.pq, priv.R)
+	}
 }
 
 // decrypt performs an RSA decryption, resulting in a plaintext integer. If a
@@ -402,6 +493,19 @@ func decrypt(rand io.Reader, priv *PrivateKey, c *big.Int) (m *big.Int, err os.E
 		m.Mod(m, priv.P)
 		m.Mul(m, priv.Q)
 		m.Add(m, m2)
+
+		if priv.dR != nil {
+			// 3-prime CRT.
+			m2.Exp(c, priv.dR, priv.R)
+			m2.Sub(m2, m)
+			m2.Mul(m2, priv.tr)
+			m2.Mod(m2, priv.R)
+			if m2.Sign() < 0 {
+				m2.Add(m2, priv.R)
+			}
+			m2.Mul(m2, priv.pq)
+			m.Add(m, m2)
+		}
 	}
 
 	priv.rwMutex.RUnlock()

src/pkg/crypto/rsa/rsa_test.go

@@ -21,15 +21,37 @@ func TestKeyGeneration(t *testing.T) {
 	if err != nil {
 		t.Errorf("failed to generate key")
 	}
+	testKeyBasics(t, priv)
+}
+
+func Test3PrimeKeyGeneration(t *testing.T) {
+	if testing.Short() {
+		return
+	}
+
+	size := 768
+	priv, err := Generate3PrimeKey(rand.Reader, size)
+	if err != nil {
+		t.Errorf("failed to generate key")
+	}
+	testKeyBasics(t, priv)
+}
+
+func testKeyBasics(t *testing.T, priv *PrivateKey) {
+	if err := priv.Validate(); err != nil {
+		t.Errorf("Validate() failed: %s", err)
+	}
+
 	pub := &priv.PublicKey
 	m := big.NewInt(42)
 	c := encrypt(new(big.Int), pub, m)
 	m2, err := decrypt(nil, priv, c)
 	if err != nil {
 		t.Errorf("error while decrypting: %s", err)
+		return
 	}
 	if m.Cmp(m2) != 0 {
-		t.Errorf("got:%v, want:%v (%s)", m2, m, priv)
+		t.Errorf("got:%v, want:%v (%+v)", m2, m, priv)
 	}
 
 	m3, err := decrypt(rand.Reader, priv, c)
@@ -69,6 +91,29 @@ func BenchmarkRSA2048Decrypt(b *testing.B) {
 	}
 }
 
+func Benchmark3PrimeRSA2048Decrypt(b *testing.B) {
+	b.StopTimer()
+	priv := &PrivateKey{
+		PublicKey: PublicKey{
+			N: fromBase10("16346378922382193400538269749936049106320265317511766357599732575277382844051791096569333808598921852351577762718529818072849191122419410612033592401403764925096136759934497687765453905884149505175426053037420486697072448609022753683683718057795566811401938833367954642951433473337066311978821180526439641496973296037000052546108507805269279414789035461158073156772151892452251106173507240488993608650881929629163465099476849643165682709047462010581308719577053905787496296934240246311806555924593059995202856826239801816771116902778517096212527979497399966526283516447337775509777558018145573127308919204297111496233"),
+			E: 3,
+		},
+		D: fromBase10("10897585948254795600358846499957366070880176878341177571733155050184921896034527397712889205732614568234385175145686545381899460748279607074689061600935843283397424506622998458510302603922766336783617368686090042765718290914099334449154829375179958369993407724946186243249568928237086215759259909861748642124071874879861299389874230489928271621259294894142840428407196932444474088857746123104978617098858619445675532587787023228852383149557470077802718705420275739737958953794088728369933811184572620857678792001136676902250566845618813972833750098806496641114644760255910789397593428910198080271317419213080834885003"),
+		P: fromBase10("1025363189502892836833747188838978207017355117492483312747347695538428729137306368764177201532277413433182799108299960196606011786562992097313508180436744488171474690412562218914213688661311117337381958560443"),
+		Q: fromBase10("3467903426626310123395340254094941045497208049900750380025518552334536945536837294961497712862519984786362199788654739924501424784631315081391467293694361474867825728031147665777546570788493758372218019373"),
+		R: fromBase10("4597024781409332673052708605078359346966325141767460991205742124888960305710298765592730135879076084498363772408626791576005136245060321874472727132746643162385746062759369754202494417496879741537284589047"),
+	}
+	priv.precompute()
+
+	c := fromBase10("1000")
+
+	b.StartTimer()
+
+	for i := 0; i < b.N; i++ {
+		decrypt(nil, priv, c)
+	}
+}
+
 type testEncryptOAEPMessage struct {
 	in   []byte
 	seed []byte