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

import (
	
	
	
	
)

// 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 ( func() hash.Hash, ,  []byte) []byte {
	if  == nil {
		 = make([]byte, ().Size())
	}
	 := hmac.New(, )
	.Write()
	return .Sum(nil)
}

type hkdf struct {
	expander hash.Hash
	size     int

	info    []byte
	counter byte

	prev []byte
	buf  []byte
}

func ( *hkdf) ( []byte) (int, error) {
	// Check whether enough data can be generated
	 := len()
	 := len(.buf) + int(255-.counter+1)*.size
	if  <  {
		return 0, errors.New("hkdf: entropy limit reached")
	}
	// Read any leftover from the buffer
	 := copy(, .buf)
	 = [:]

	// Fill the rest of the buffer
	for len() > 0 {
		.expander.Reset()
		.expander.Write(.prev)
		.expander.Write(.info)
		.expander.Write([]byte{.counter})
		.prev = .expander.Sum(.prev[:0])
		.counter++

		// Copy the new batch into p
		.buf = .prev
		 = copy(, .buf)
		 = [:]
	}
	// Save leftovers for next run
	.buf = .buf[:]

	return , 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 ( func() hash.Hash, ,  []byte) io.Reader {
	 := hmac.New(, )
	return &hkdf{, .Size(), , 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 ( func() hash.Hash, , ,  []byte) io.Reader {
	 := Extract(, , )
	return Expand(, , )
}