hkdf: improve example

Change-Id: I540c699baf1f7cbf27da458961d581773f442864
Reviewed-on: https://go-review.googlesource.com/c/144397
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>

Author: FiloSottile

hkdf/example_test.go

@@ -9,49 +9,44 @@ import (
 	"crypto/rand"
 	"crypto/sha256"
 	"fmt"
-	"golang.org/x/crypto/hkdf"
 	"io"
+
+	"golang.org/x/crypto/hkdf"
 )
 
-// Usage example that expands one master key into three other cryptographically
-// secure keys.
+// Usage example that expands one master secret into three other
+// cryptographically secure keys.
 func Example_usage() {
-	// Underlying hash function to use
+	// Underlying hash function for HMAC.
 	hash := sha256.New
 
-	// Cryptographically secure master key.
-	master := []byte{0x00, 0x01, 0x02, 0x03} // i.e. NOT this.
+	// Cryptographically secure master secret.
+	secret := []byte{0x00, 0x01, 0x02, 0x03} // i.e. NOT this.
 
-	// Non secret salt, optional (can be nil)
-	// Recommended: hash-length sized random
+	// Non-secret salt, optional (can be nil).
+	// Recommended: hash-length random value.
 	salt := make([]byte, hash().Size())
-	n, err := io.ReadFull(rand.Reader, salt)
-	if n != len(salt) || err != nil {
-		fmt.Println("error:", err)
-		return
+	if _, err := rand.Read(salt); err != nil {
+		panic(err)
 	}
 
-	// Non secret context specific info, optional (can be nil).
-	// Note, independent from the master key.
-	info := []byte{0x03, 0x14, 0x15, 0x92, 0x65}
-
-	// Create the key derivation function
-	hkdf := hkdf.New(hash, master, salt, info)
-
-	// Generate the required keys
-	keys := make([][]byte, 3)
-	for i := 0; i < len(keys); i++ {
-		keys[i] = make([]byte, 24)
-		n, err := io.ReadFull(hkdf, keys[i])
-		if n != len(keys[i]) || err != nil {
-			fmt.Println("error:", err)
-			return
+	// Non-secret context info, optional (can be nil).
+	info := []byte("hkdf example")
+
+	// Generate three 128-bit derived keys.
+	hkdf := hkdf.New(hash, secret, salt, info)
+
+	var keys [][]byte
+	for i := 0; i < 3; i++ {
+		key := make([]byte, 16)
+		if _, err := io.ReadFull(hkdf, key); err != nil {
+			panic(err)
 		}
+		keys = append(keys, key)
 	}
 
-	// Keys should contain 192 bit random keys
-	for i := 1; i <= len(keys); i++ {
-		fmt.Printf("Key #%d: %v\n", i, !bytes.Equal(keys[i-1], make([]byte, 24)))
+	for i := range keys {
+		fmt.Printf("Key #%d: %v\n", i+1, !bytes.Equal(keys[i], make([]byte, 16)))
 	}
 
 	// Output: