hkdf

Documentation

Overview

Package hkdf implements the HMAC-based Extract-and-Expand Key Derivation Function (HKDF) as defined in RFC 5869.

HKDF is a cryptographic key derivation function (KDF) with the goal of expanding limited input keying material into one or more cryptographically strong secret keys.

Example (Usage)

Usage example that expands one master secret into three other cryptographically secure keys.

package main

import (
	"bytes"
	"crypto/rand"
	"crypto/sha256"
	"fmt"
	"io"

	"github.com/dacohen/crypto/hkdf"
)

func main() {
	// Underlying hash function for HMAC.
	hash := sha256.New

	// Cryptographically secure master secret.
	secret := []byte{0x00, 0x01, 0x02, 0x03} // i.e. NOT this.

	// Non-secret salt, optional (can be nil).
	// Recommended: hash-length random value.
	salt := make([]byte, hash().Size())
	if _, err := rand.Read(salt); err != nil {
		panic(err)
	}

	// 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)
	}

	for i := range keys {
		fmt.Printf("Key #%d: %v\n", i+1, !bytes.Equal(keys[i], make([]byte, 16)))
	}

}

Output:

Key #1: true
Key #2: true
Key #3: true

Index

• func Expand(hash func() hash.Hash, pseudorandomKey, info []byte) io.Reader
• func Extract(hash func() hash.Hash, secret, salt []byte) []byte
• func New(hash func() hash.Hash, secret, salt, info []byte) io.Reader

Examples

• Package (Usage)

Functions

func Expand

func Expand(hash func() hash.Hash, pseudorandomKey, info []byte) io.Reader

Expand returns a Reader, from which keys can be read, using the given pseudorandom key and optional context info, skipping the extraction step.

The pseudorandomKey should have been generated by Extract, or be a uniformly random or pseudorandom cryptographically strong key. See RFC 5869, Section 3.3. Most common scenarios will want to use New instead.

func Extract

func Extract(hash func() hash.Hash, secret, salt []byte) []byte

Extract generates a pseudorandom key for use with Expand from an input secret and an optional independent salt.

Only use this function if you need to reuse the extracted key with multiple Expand invocations and different context values. Most common scenarios, including the generation of multiple keys, should use New instead.

func New

func New(hash func() hash.Hash, secret, salt, info []byte) io.Reader

New returns a Reader, from which keys can be read, using the given hash, secret, salt and context info. Salt and info can be nil.