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chacha.go
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/
chacha.go
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// Copyright (c) 2016 Andreas Auernhammer. All rights reserved.
// Use of this source code is governed by a license that can be
// found in the LICENSE file.
// Package chacha implements some low-level functions of the
// ChaCha cipher family.
package chacha // import "github.com/aead/chacha20/chacha"
import (
"encoding/binary"
"errors"
"math"
)
const (
// NonceSize is the size of the ChaCha20 nonce in bytes.
NonceSize = 8
// INonceSize is the size of the IETF-ChaCha20 nonce in bytes.
INonceSize = 12
// XNonceSize is the size of the XChaCha20 nonce in bytes.
XNonceSize = 24
// KeySize is the size of the key in bytes.
KeySize = 32
)
var (
useSSE2 bool
useSSSE3 bool
useAVX bool
useAVX2 bool
)
var (
errKeySize = errors.New("chacha20/chacha: bad key length")
errInvalidNonce = errors.New("chacha20/chacha: bad nonce length")
)
func setup(state *[64]byte, nonce, key []byte) (err error) {
if len(key) != KeySize {
err = errKeySize
return
}
var Nonce [16]byte
switch len(nonce) {
case NonceSize:
copy(Nonce[8:], nonce)
initialize(state, key, &Nonce)
case INonceSize:
copy(Nonce[4:], nonce)
initialize(state, key, &Nonce)
case XNonceSize:
var tmpKey [32]byte
var hNonce [16]byte
copy(hNonce[:], nonce[:16])
copy(tmpKey[:], key)
HChaCha20(&tmpKey, &hNonce, &tmpKey)
copy(Nonce[8:], nonce[16:])
initialize(state, tmpKey[:], &Nonce)
// BUG(aead): A "good" compiler will remove this (optimizations)
// But using the provided key instead of tmpKey,
// will change the key (-> probably confuses users)
for i := range tmpKey {
tmpKey[i] = 0
}
default:
err = errInvalidNonce
}
return
}
// XORKeyStream crypts bytes from src to dst using the given nonce and key.
// The length of the nonce determinds the version of ChaCha20:
// - NonceSize: ChaCha20/r with a 64 bit nonce and a 2^64 * 64 byte period.
// - INonceSize: ChaCha20/r as defined in RFC 7539 and a 2^32 * 64 byte period.
// - XNonceSize: XChaCha20/r with a 192 bit nonce and a 2^64 * 64 byte period.
// The rounds argument specifies the number of rounds performed for keystream
// generation - valid values are 8, 12 or 20. The src and dst may be the same slice
// but otherwise should not overlap. If len(dst) < len(src) this function panics.
// If the nonce is neither 64, 96 nor 192 bits long, this function panics.
func XORKeyStream(dst, src, nonce, key []byte, rounds int) {
if rounds != 20 && rounds != 12 && rounds != 8 {
panic("chacha20/chacha: bad number of rounds")
}
if len(dst) < len(src) {
panic("chacha20/chacha: dst buffer is to small")
}
if len(nonce) == INonceSize && uint64(len(src)) > (1<<38) {
panic("chacha20/chacha: src is too large")
}
var block, state [64]byte
if err := setup(&state, nonce, key); err != nil {
panic(err)
}
xorKeyStream(dst, src, &block, &state, rounds)
}
// Cipher implements ChaCha20/r (XChaCha20/r) for a given number of rounds r.
type Cipher struct {
state, block [64]byte
off int
rounds int // 20 for ChaCha20
noncesize int
}
// NewCipher returns a new *chacha.Cipher implementing the ChaCha20/r or XChaCha20/r
// (r = 8, 12 or 20) stream cipher. The nonce must be unique for one key for all time.
// The length of the nonce determinds the version of ChaCha20:
// - NonceSize: ChaCha20/r with a 64 bit nonce and a 2^64 * 64 byte period.
// - INonceSize: ChaCha20/r as defined in RFC 7539 and a 2^32 * 64 byte period.
// - XNonceSize: XChaCha20/r with a 192 bit nonce and a 2^64 * 64 byte period.
// If the nonce is neither 64, 96 nor 192 bits long, a non-nil error is returned.
func NewCipher(nonce, key []byte, rounds int) (*Cipher, error) {
if rounds != 20 && rounds != 12 && rounds != 8 {
panic("chacha20/chacha: bad number of rounds")
}
c := new(Cipher)
if err := setup(&(c.state), nonce, key); err != nil {
return nil, err
}
c.rounds = rounds
if len(nonce) == INonceSize {
c.noncesize = INonceSize
} else {
c.noncesize = NonceSize
}
return c, nil
}
// XORKeyStream crypts bytes from src to dst. Src and dst may be the same slice
// but otherwise should not overlap. If len(dst) < len(src) the function panics.
func (c *Cipher) XORKeyStream(dst, src []byte) {
if len(dst) < len(src) {
panic("chacha20/chacha: dst buffer is to small")
}
if c.off > 0 {
n := len(c.block[c.off:])
if len(src) <= n {
for i, v := range src {
dst[i] = v ^ c.block[c.off]
c.off++
}
if c.off == 64 {
c.off = 0
}
return
}
for i, v := range c.block[c.off:] {
dst[i] = src[i] ^ v
}
src = src[n:]
dst = dst[n:]
c.off = 0
}
// check for counter overflow
blocksToXOR := len(src) / 64
if len(src)%64 != 0 {
blocksToXOR++
}
var overflow bool
if c.noncesize == INonceSize {
overflow = binary.LittleEndian.Uint32(c.state[48:]) > math.MaxUint32-uint32(blocksToXOR)
} else {
overflow = binary.LittleEndian.Uint64(c.state[48:]) > math.MaxUint64-uint64(blocksToXOR)
}
if overflow {
panic("chacha20/chacha: counter overflow")
}
c.off += xorKeyStream(dst, src, &(c.block), &(c.state), c.rounds)
}
// SetCounter skips ctr * 64 byte blocks. SetCounter(0) resets the cipher.
// This function always skips the unused keystream of the current 64 byte block.
func (c *Cipher) SetCounter(ctr uint64) {
if c.noncesize == INonceSize {
binary.LittleEndian.PutUint32(c.state[48:], uint32(ctr))
} else {
binary.LittleEndian.PutUint64(c.state[48:], ctr)
}
c.off = 0
}
// HChaCha20 generates 32 pseudo-random bytes from a 128 bit nonce and a 256 bit secret key.
// It can be used as a key-derivation-function (KDF).
func HChaCha20(out *[32]byte, nonce *[16]byte, key *[32]byte) { hChaCha20(out, nonce, key) }