add rand package, utilities for math/rand.

rand/rand.go

@@ -0,0 +1,49 @@
+// Package rand contains utilities for interacting with math/rand, including
+// seeding from a random sed.
+package rand
+
+import (
+	"crypto/rand"
+	"encoding/binary"
+	mrand "math/rand"
+)
+
+// CryptoUint64 generates a cryptographically-secure 64-bit integer.
+func CryptoUint64() (uint64, error) {
+	bs := make([]byte, 8)
+	_, err := rand.Read(bs)
+	if err != nil {
+		return 0, err
+	}
+
+	return binary.BigEndian.Uint64(bs), nil
+}
+
+// Seed initialises the non-cryptographic PRNG with a random,
+// cryptographically secure value. This is done just as a good
+// way to make this random. The returned 64-bit value is the seed.
+func Seed() (uint64, error) {
+	seed, err := CryptoUint64()
+	if err != nil {
+		return 0, err
+	}
+
+	// NB: this is permitted.
+	mrand.Seed(int64(seed))
+	return seed, nil
+}
+
+// Int is a wrapper for math.Int so only one package needs to be imported.
+func Int() int {
+	return mrand.Int()
+}
+
+// Intn is a wrapper for math.Intn so only one package needs to be imported.
+func Intn(max int) int {
+	return mrand.Intn(max)
+}
+
+// Intn2 returns a random value between min and max, inclusive.
+func Intn2(min, max int) int {
+	return Intn(max-min) + min
+}

rand/rand_test.go

@@ -0,0 +1,74 @@
+package rand
+
+import (
+	"fmt"
+	mrand "math/rand"
+	"testing"
+)
+
+func TestCryptoUint64(t *testing.T) {
+	n1, err := CryptoUint64()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	n2, err := CryptoUint64()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// This has such a low chance of occurring that it's likely to be
+	// indicative of a bad CSPRNG.
+	if n1 == n2 {
+		t.Fatalf("repeated random uint64s: %d", n1)
+	}
+}
+
+func TestIntn(t *testing.T) {
+	expected := []int{3081, 4887, 4847, 1059, 3081}
+	mrand.Seed(1)
+	for i := 0; i < 5; i++ {
+		n := Intn2(1000, 5000)
+
+		if n != expected[i] {
+			fmt.Printf("invalid sequence at %d: expected %d, have %d", i, expected[i], n)
+		}
+	}
+}
+
+func TestSeed(t *testing.T) {
+	seed1, err := Seed()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var seed2 uint64
+	n1 := Int()
+	tries := 0
+
+	for {
+		seed2, err = Seed()
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if seed1 != seed2 {
+			break
+		}
+
+		tries++
+
+		if tries > 3 {
+			t.Fatal("can't generate two unique seeds")
+		}
+	}
+
+	n2 := Int()
+
+	// Again, this not impossible, merely statistically improbably and a
+	// potential canary for RNG issues.
+	if n1 == n2 {
+		t.Fatalf("repeated integers fresh from two unique seeds: %d/%d -> %d",
+			seed1, seed2, n1)
+	}
+}