hkdf

hkdf.go

package main

import (
	"crypto/hmac"
	"crypto/sha256"
	"fmt"
	"hash"
	"io"

	"golang.org/x/crypto/hkdf"
)

func main() {
	ikm := make([]byte, 32)
	info := []byte("info")

	b := test2(sha256.New, ikm, nil, info)
	fmt.Printf("hkdf %x\n", b)

	a := test1(sha256.New, ikm, nil, info)
	fmt.Printf("hmac %x\n", a)
}

// test1 performs HMAC(key=HMAC(key=salt, data=ikm), data=info).
func test1(hash func() hash.Hash, ikm, salt, info []byte) []byte {
	if salt == nil {
		salt = make([]byte, hash().Size())
	}
	// HMAC(key=salt, data=ikm)
	extractor := hmac.New(hash, salt)
	extractor.Write(ikm)
	prk := extractor.Sum(nil)

	// HMAC(key=prk, data=info || 1)
	expander := hmac.New(hash, prk)
	expander.Write(info)
	expander.Write([]byte{1})
	return expander.Sum(nil)
}

// test2 returns HKDF-Extract and HKDF-Expand.
func test2(hash func() hash.Hash, ikm, salt, info []byte) []byte {
	key := make([]byte, sha256.Size)
	r := hkdf.New(hash, ikm, salt, info)
	if _, err := io.ReadFull(r, key); err != nil {
		panic(err)
	}
	return key
}