crypto/internal/fips140/rsa: do trial divisions in key generation

This is optimized to be cheap in terms of extra code and complexity,
rather than performance, so we reuse the GCD we have for inverting d.

Recovers most of the performance loss since CL 630516, although
benchmarking key generation is by nature extremely noisy.

goos: darwin
goarch: arm64
pkg: crypto/rsa
cpu: Apple M2
                   │ 3b42687c56  │           b3d018a1e8-dirty           │
                   │   sec/op    │    sec/op     vs base                │
GenerateKey/2048-8   104.1m ± 7%   139.7m ± 20%  +34.10% (p=0.000 n=20)

Updates #69799
For #69536

Change-Id: I00347610935db8feb0597529a301ad7ace5b2f22
Reviewed-on: https://go-review.googlesource.com/c/go/+/632479
Reviewed-by: Roland Shoemaker <roland@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/crypto/internal/fips140/rsa/keygen.go

@@ -128,6 +128,18 @@ func isPrime(w []byte) bool {
 		return false
 	}
 
+	primes, err := bigmod.NewNat().SetBytes(productOfPrimes, mr.w)
+	// If w is too small for productOfPrimes, key generation is
+	// going to be fast enough anyway.
+	if err == nil {
+		_, hasInverse := primes.InverseVarTime(primes, mr.w)
+		if !hasInverse {
+			// productOfPrimes doesn't have an inverse mod w,
+			// so w is divisible by at least one of the primes.
+			return false
+		}
+	}
+
 	// iterations is the number of Miller-Rabin rounds, each with a
 	// randomly-selected base.
 	//
@@ -183,6 +195,20 @@ func isPrime(w []byte) bool {
 	}
 }
 
+// productOfPrimes is the product of the first 74 primes higher than 2.
+//
+// The number of primes was selected to be the highest such that the product fit
+// in 512 bits, so to be usable for 1024 bit RSA keys.
+//
+// Higher values cause fewer Miller-Rabin tests of composites (nothing can help
+// with the final test on the actual prime) but make InverseVarTime take longer.
+var productOfPrimes = []byte{
+	0x10, 0x6a, 0xa9, 0xfb, 0x76, 0x46, 0xfa, 0x6e, 0xb0, 0x81, 0x3c, 0x28, 0xc5, 0xd5, 0xf0, 0x9f,
+	0x07, 0x7e, 0xc3, 0xba, 0x23, 0x8b, 0xfb, 0x99, 0xc1, 0xb6, 0x31, 0xa2, 0x03, 0xe8, 0x11, 0x87,
+	0x23, 0x3d, 0xb1, 0x17, 0xcb, 0xc3, 0x84, 0x05, 0x6e, 0xf0, 0x46, 0x59, 0xa4, 0xa1, 0x1d, 0xe4,
+	0x9f, 0x7e, 0xcb, 0x29, 0xba, 0xda, 0x8f, 0x98, 0x0d, 0xec, 0xec, 0xe9, 0x2e, 0x30, 0xc4, 0x8f,
+}
+
 type millerRabin struct {
 	w *bigmod.Modulus
 	a uint