crypto/rand: add and update examples

Change-Id: I77406c22b82c9f8bc57323c783f63c4897486e7e
Reviewed-on: https://go-review.googlesource.com/c/go/+/665096
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>

src/crypto/rand/example_test.go

@@ -12,16 +12,30 @@ import (
 
 // ExampleInt prints a single cryptographically secure pseudorandom number between 0 and 99 inclusive.
 func ExampleInt() {
-	a, err := rand.Int(rand.Reader, big.NewInt(100))
-	if err != nil {
-		fmt.Println("error:", err)
-		return
-	}
+	// Int cannot return an error when using rand.Reader.
+	a, _ := rand.Int(rand.Reader, big.NewInt(100))
 	fmt.Println(a.Int64())
 }
 
+// ExamplePrime prints a cryptographically secure pseudorandom 64 bit prime number.
+func ExamplePrime() {
+	// Prime cannot return an error when using rand.Reader and bits >= 2.
+	a, _ := rand.Prime(rand.Reader, 64)
+	fmt.Println(a.Int64())
+}
+
+// ExampleRead prints a cryptographically secure pseudorandom 32 byte key.
 func ExampleRead() {
 	// Note that no error handling is necessary, as Read always succeeds.
 	key := make([]byte, 32)
 	rand.Read(key)
+	// The key can contain any byte value, print the key in hex.
+	fmt.Printf("% x\n", key)
+}
+
+// ExampleText prints a random key encoded in base32.
+func ExampleText() {
+	key := rand.Text()
+	// The key is base32 and safe to display.
+	fmt.Println(key)
 }