crypto/rand: support generation of 2-5 bit primes, also document the error return...

Fixes #6849.
Fixes #6867.

R=golang-dev, agl
CC=golang-dev
https://golang.org/cl/35870043

diff --git a/src/pkg/crypto/rand/util.go b/src/pkg/crypto/rand/util.go
index 0cd5e0e022f7bbb3095c6828e9a582d8eb130acc..5f74407850c55f161493c2f6f311090e5761d7e2 100644 (file)
--- a/src/pkg/crypto/rand/util.go
+++ b/src/pkg/crypto/rand/util.go
@@ -27,9 +27,11 @@ var smallPrimesProduct = new(big.Int).SetUint64(16294579238595022365)
 
 // Prime returns a number, p, of the given size, such that p is prime
 // with high probability.
+// Prime will return error for any error returned by rand.Read or if bits < 2.
 func Prime(rand io.Reader, bits int) (p *big.Int, err error) {
-       if bits < 1 {
-               err = errors.New("crypto/rand: prime size must be positive")
+       if bits < 2 {
+               err = errors.New("crypto/rand: prime size must be at least 2-bit")
+               return
        }
 
        b := uint(bits % 8)
@@ -79,7 +81,7 @@ func Prime(rand io.Reader, bits int) (p *big.Int, err error) {
                for delta := uint64(0); delta < 1<<20; delta += 2 {
                        m := mod + delta
                        for _, prime := range smallPrimes {
-                               if m%uint64(prime) == 0 {
+                               if m%uint64(prime) == 0 && (bits > 6 || m != uint64(prime)) {
                                        continue NextDelta
                                }
                        }

diff --git a/src/pkg/crypto/rand/util_test.go b/src/pkg/crypto/rand/util_test.go
new file mode 100644 (file)
index 0000000..33f9820
--- /dev/null
+++ b/src/pkg/crypto/rand/util_test.go
@@ -0,0 +1,26 @@
+// Copyright 2013 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 rand_test
+
+import (
+       "crypto/rand"
+       "testing"
+)
+
+// http://golang.org/issue/6849.
+func TestPrimeSmall(t *testing.T) {
+       for n := 2; n < 10; n++ {
+               p, err := rand.Prime(rand.Reader, n)
+               if err != nil {
+                       t.Fatalf("Can't generate %d-bit prime: %v", n, err)
+               }
+               if p.BitLen() != n {
+                       t.Fatalf("%v is not %d-bit", p, n)
+               }
+               if !p.ProbablyPrime(32) {
+                       t.Fatalf("%v is not prime", p)
+               }
+       }
+}