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// 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.
package hkdf
import (
"crypto/hmac"
"errors"
"hash"
"io"
)
// 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 Extract(hash func() hash.Hash, secret, salt []byte) []byte {
if salt == nil {
salt = make([]byte, hash().Size())
}
extractor := hmac.New(hash, salt)
extractor.Write(secret)
return extractor.Sum(nil)
}
type hkdf struct {
expander hash.Hash
size int
info []byte
counter byte
prev []byte
buf []byte
}
func (f *hkdf) Read(p []byte) (int, error) {
// Check whether enough data can be generated
need := len(p)
remains := len(f.buf) + int(255-f.counter+1)*f.size
if remains < need {
return 0, errors.New("hkdf: entropy limit reached")
}
// Read any leftover from the buffer
n := copy(p, f.buf)
p = p[n:]
// Fill the rest of the buffer
for len(p) > 0 {
if f.counter > 1 {
f.expander.Reset()
}
f.expander.Write(f.prev)
f.expander.Write(f.info)
f.expander.Write([]byte{f.counter})
f.prev = f.expander.Sum(f.prev[:0])
f.counter++
// Copy the new batch into p
f.buf = f.prev
n = copy(p, f.buf)
p = p[n:]
}
// Save leftovers for next run
f.buf = f.buf[n:]
return need, nil
}
// 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 Expand(hash func() hash.Hash, pseudorandomKey, info []byte) io.Reader {
expander := hmac.New(hash, pseudorandomKey)
return &hkdf{expander, expander.Size(), info, 1, nil, nil}
}
// 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.
func New(hash func() hash.Hash, secret, salt, info []byte) io.Reader {
prk := Extract(hash, secret, salt)
return Expand(hash, prk, info)
}
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