// Copyright 2009 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.

// The hmac package implements the Keyed-Hash Message Authentication Code (HMAC)
// as defined in U.S. Federal Information Processing Standards Publication 198.
// An HMAC is a cryptographic hash that uses a key to sign a message.
// The receiver verifies the hash by recomputing it using the same key.
package hmac

import (
	"crypto/md5";
	"crypto/sha1";
	"hash";
	"os";
)

// FIPS 198:
// http://csrc.nist.gov/publications/fips/fips198/fips-198a.pdf

// key is zero padded to 64 bytes
// ipad = 0x36 byte repeated to 64 bytes
// opad = 0x5c byte repeated to 64 bytes
// hmac = H([key ^ opad] H([key ^ ipad] text))

const (
	// NOTE(rsc): This constant is actually the
	// underlying hash function's block size.
	// HMAC is only conventionally used with
	// MD5 and SHA1, and both use 64-byte blocks.
	// The hash.Hash interface doesn't provide a
	// way to find out the block size.
	padSize = 64;
)

type hmac struct {
	size int;
	key []byte;
	tmp []byte;
	inner hash.Hash;
}

func (h *hmac) tmpPad(xor byte) {
	for i, k := range h.key {
		h.tmp[i] = xor ^ k;
	}
	for i := len(h.key); i < padSize; i++ {
		h.tmp[i] = xor;
	}
}

func (h *hmac) Sum() []byte {
	h.tmpPad(0x5c);
	sum := h.inner.Sum();
	for i, b := range sum {
		h.tmp[padSize + i] = b;
	}
	h.inner.Reset();
	h.inner.Write(h.tmp);
	return h.inner.Sum();
}

func (h *hmac) Write(p []byte) (n int, err os.Error) {
	return h.inner.Write(p);
}

func (h *hmac) Size() int {
	return h.size;
}

func (h *hmac) Reset() {
	h.inner.Reset();
	h.tmpPad(0x36);
	h.inner.Write(h.tmp[0:padSize]);
}

// New returns a new HMAC hash using the given hash and key.
func New(h hash.Hash, key []byte) hash.Hash {
	if len(key) > padSize {
		// If key is too big, hash it.
		h.Write(key);
		key = h.Sum();
	}
	hm := new(hmac);
	hm.inner = h;
	hm.size = h.Size();
	hm.key = make([]byte, len(key));
	for i, k := range key {
		hm.key[i] = k;
	}
	hm.tmp = make([]byte, padSize + hm.size);
	hm.Reset();
	return hm;
}

// NewMD5 returns a new HMAC-MD5 hash using the given key.
func NewMD5(key []byte) hash.Hash {
	return New(md5.New(), key);
}

// NewSHA1 returns a new HMAC-SHA1 hash using the given key.
func NewSHA1(key []byte) hash.Hash {
	return New(sha1.New(), key);
}