// 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.

// Package xtea implements XTEA encryption, as defined in Needham and Wheeler's
// 1997 technical report, "Tea extensions."
package xtea

// For details, see http://www.cix.co.uk/~klockstone/xtea.pdf

import (
	"os"
	"strconv"
)

// The XTEA block size in bytes.
const BlockSize = 8

// A Cipher is an instance of an XTEA cipher using a particular key.
// table contains a series of precalculated values that are used each round.
type Cipher struct {
	table [64]uint32
}

type KeySizeError int

func (k KeySizeError) String() string {
	return "crypto/xtea: invalid key size " + strconv.Itoa(int(k))
}

// NewCipher creates and returns a new Cipher.
// The key argument should be the XTEA key.
// XTEA only supports 128 bit (16 byte) keys.
func NewCipher(key []byte) (*Cipher, os.Error) {
	k := len(key)
	switch k {
	default:
		return nil, KeySizeError(k)
	case 16:
		break
	}

	c := new(Cipher)
	initCipher(c, key)

	return c, nil
}

// BlockSize returns the XTEA block size, 8 bytes.
// It is necessary to satisfy the Cipher interface in the
// package "crypto/cipher".
func (c *Cipher) BlockSize() int { return BlockSize }

// Encrypt encrypts the 8 byte buffer src using the key and stores the result in dst.
// Note that for amounts of data larger than a block,
// it is not safe to just call Encrypt on successive blocks;
// instead, use an encryption mode like CBC (see crypto/cipher/cbc.go).
func (c *Cipher) Encrypt(dst, src []byte) { encryptBlock(c, dst, src) }

// Decrypt decrypts the 8 byte buffer src using the key k and stores the result in dst.
func (c *Cipher) Decrypt(dst, src []byte) { decryptBlock(c, dst, src) }

// Reset zeros the table, so that it will no longer appear in the process's memory.
func (c *Cipher) Reset() {
	for i := 0; i < len(c.table); i++ {
		c.table[i] = 0
	}
}

// initCipher initializes the cipher context by creating a look up table
// of precalculated values that are based on the key.
func initCipher(c *Cipher, key []byte) {
	// Load the key into four uint32s
	var k [4]uint32
	for i := 0; i < len(k); i++ {
		j := i << 2 // Multiply by 4
		k[i] = uint32(key[j+0])<<24 | uint32(key[j+1])<<16 | uint32(key[j+2])<<8 | uint32(key[j+3])
	}

	// Precalculate the table
	const delta = 0x9E3779B9
	var sum uint32 = 0

	// Two rounds of XTEA applied per loop
	for i := 0; i < numRounds; {
		c.table[i] = sum + k[sum&3]
		i++
		sum += delta
		c.table[i] = sum + k[(sum>>11)&3]
		i++
	}
}