Commit 322ef7fd authored by Péter Szilágyi's avatar Péter Szilágyi

go.crypto/hkdf: implement hash-based, key derivation.

Detailed description in RFC 5869.

LGTM=agl
R=agl, minux.ma
CC=golang-codereviews
https://codereview.appspot.com/7474049

Committer: Adam Langley <agl@golang.org>
parent c7b21322
// Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package hkdf_test
import (
"bytes"
"code.google.com/p/go.crypto/hkdf"
"crypto/rand"
"crypto/sha256"
"fmt"
"io"
)
// Usage example that expands one master key into three other cryptographically
// secure keys.
func Example_usage() {
// Underlying hash function to use
hash := sha256.New
// Cryptographically secure master key.
master := []byte{0x00, 0x01, 0x02, 0x03} // i.e. NOT this.
// Non secret salt, optional (can be nil)
// Recommended: hash-length sized random
salt := make([]byte, hash().Size())
n, err := io.ReadFull(rand.Reader, salt)
if n != len(salt) || err != nil {
fmt.Println("error:", err)
return
}
// 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
}
}
// 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)))
}
// Output:
// Key #1: true
// Key #2: true
// Key #3: true
}
// Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// 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.
//
// RFC 5869: https://tools.ietf.org/html/rfc5869
package hkdf
import (
"crypto/hmac"
"errors"
"hash"
"io"
)
type hkdf struct {
expander hash.Hash
size int
info []byte
counter byte
prev []byte
cache []byte
}
func (f *hkdf) Read(p []byte) (int, error) {
// Check whether enough data can be generated
need := len(p)
remains := len(f.cache) + int(255-f.counter+1)*f.size
if remains < need {
return 0, errors.New("hkdf: entropy limit reached")
}
// Read from the cache, if enough data is present
n := copy(p, f.cache)
p = p[n:]
// Fill the buffer
var input []byte
for len(p) > 0 {
input = append(f.prev, f.info...)
input = append(input, f.counter)
f.expander.Reset()
f.expander.Write(input)
f.prev = f.expander.Sum(f.prev[:0])
f.counter++
// Copy the new batch into p
f.cache = f.prev
n = copy(p, f.cache)
p = p[n:]
}
// Save leftovers for next run
f.cache = f.cache[n:]
return need, nil
}
// New returns a new HKDF using the given hash, the secret keying material to expand
// and optional salt and info fields.
func New(hash func() hash.Hash, secret, salt, info []byte) io.Reader {
if salt == nil {
salt = make([]byte, hash().Size())
}
extractor := hmac.New(hash, salt)
extractor.Write(secret)
prk := extractor.Sum(nil)
return &hkdf{hmac.New(hash, prk), extractor.Size(), info, 1, nil, nil}
}
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