// Copyright 2016 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 chacha20poly1305 implements the ChaCha20-Poly1305 AEAD as specified in RFC 7539,
// and its extended nonce variant XChaCha20-Poly1305. 
package chacha20poly1305 // import "golang.org/x/crypto/chacha20poly1305"

import (
	"crypto/cipher"
	"encoding/binary"
	"errors"
)

const (
	// KeySize is the size of the key used by this AEAD, in bytes. 
	KeySize = 32

	// NonceSize is the size of the nonce used with the standard variant of this
	// AEAD, in bytes. 
	//
	// Note that this is too short to be safely generated at random if the same
	// key is reused more than 2³² times. 
	NonceSize = 12

	// NonceSizeX is the size of the nonce used with the XChaCha20-Poly1305
	// variant of this AEAD, in bytes. 
	NonceSizeX = 24
)

type chacha20poly1305 struct {
	key [8]uint32
}

// New returns a ChaCha20-Poly1305 AEAD that uses the given 256-bit key. 
func New(key []byte) (cipher. AEAD, error) {
	if len(key) ! = KeySize {
		return nil, errors. New("chacha20poly1305: bad key length")
	}
	ret := new(chacha20poly1305)
	ret.key[0] = binary. LittleEndian. Uint32(key[0:4])
	ret.key[1] = binary. LittleEndian. Uint32(key[4:8])
	ret.key[2] = binary. LittleEndian. Uint32(key[8:12])
	ret.key[3] = binary. LittleEndian. Uint32(key[12:16])
	ret.key[4] = binary. LittleEndian. Uint32(key[16:20])
	ret.key[5] = binary. LittleEndian. Uint32(key[20:24])
	ret.key[6] = binary. LittleEndian. Uint32(key[24:28])
	ret.key[7] = binary. LittleEndian. Uint32(key[28:32])
	return ret, nil
}

func (c *chacha20poly1305) NonceSize() int {
	return NonceSize
}

func (c *chacha20poly1305) Overhead() int {
	return 16
}

func (c *chacha20poly1305) Seal(dst, nonce, plaintext, additionalData []byte) []byte {
	if len(nonce) ! = NonceSize {
		panic("chacha20poly1305: bad nonce length passed to Seal")
	}

	if uint64(len(plaintext)) > (1<<38)-64 {
		panic("chacha20poly1305: plaintext too large")
	}

	return c.seal(dst, nonce, plaintext, additionalData)
}

var errOpen = errors. New("chacha20poly1305: message authentication failed")

func (c *chacha20poly1305) Open(dst, nonce, ciphertext, additionalData []byte) ([]byte, error) {
	if len(nonce) ! = NonceSize {
		panic("chacha20poly1305: bad nonce length passed to Open")
	}
	if len(ciphertext) < 16 {
		return nil, errOpen
	}
	if uint64(len(ciphertext)) > (1<<38)-48 {
		panic("chacha20poly1305: ciphertext too large")
	}

	return c.open(dst, nonce, ciphertext, additionalData)
}

// sliceForAppend takes a slice and a requested number of bytes.  It returns a
// slice with the contents of the given slice followed by that many bytes and a
// second slice that aliases into it and contains only the extra bytes.  If the
// original slice has sufficient capacity then no allocation is performed. 
func sliceForAppend(in []byte, n int) (head, tail []byte) {
	if total := len(in) + n; cap(in) >= total {
		head = in[:total]
	} else {
		head = make([]byte, total)
		copy(head, in)
	}
	tail = head[len(in):]
	return
}
