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// 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 chacha20 implements the ChaCha20 and XChaCha20 encryption algorithms
// as specified in RFC 8439 and draft-irtf-cfrg-xchacha-01.
package chacha20
import (
"crypto/cipher"
"encoding/binary"
"errors"
"math/bits"
"golang.org/x/crypto/internal/subtle"
)
const (
// KeySize is the size of the key used by this cipher, in bytes.
KeySize = 32
// NonceSize is the size of the nonce used with the standard variant of this
// cipher, 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 variant of
// this cipher, in bytes.
NonceSizeX = 24
)
// Cipher is a stateful instance of ChaCha20 or XChaCha20 using a particular key
// and nonce. A *Cipher implements the cipher.Stream interface.
type Cipher struct {
// The ChaCha20 state is 16 words: 4 constant, 8 of key, 1 of counter
// (incremented after each block), and 3 of nonce.
key [8]uint32
counter uint32
nonce [3]uint32
// The last len bytes of buf are leftover key stream bytes from the previous
// XORKeyStream invocation. The size of buf depends on how many blocks are
// computed at a time.
buf [bufSize]byte
len int
// The counter-independent results of the first round are cached after they
// are computed the first time.
precompDone bool
p1, p5, p9, p13 uint32
p2, p6, p10, p14 uint32
p3, p7, p11, p15 uint32
}
var _ cipher.Stream = (*Cipher)(nil)
// NewUnauthenticatedCipher creates a new ChaCha20 stream cipher with the given
// 32 bytes key and a 12 or 24 bytes nonce. If a nonce of 24 bytes is provided,
// the XChaCha20 construction will be used. It returns an error if key or nonce
// have any other length.
//
// Note that ChaCha20, like all stream ciphers, is not authenticated and allows
// attackers to silently tamper with the plaintext. For this reason, it is more
// appropriate as a building block than as a standalone encryption mechanism.
// Instead, consider using package golang.org/x/crypto/chacha20poly1305.
func NewUnauthenticatedCipher(key, nonce []byte) (*Cipher, error) {
// This function is split into a wrapper so that the Cipher allocation will
// be inlined, and depending on how the caller uses the return value, won't
// escape to the heap.
c := &Cipher{}
return newUnauthenticatedCipher(c, key, nonce)
}
func newUnauthenticatedCipher(c *Cipher, key, nonce []byte) (*Cipher, error) {
if len(key) != KeySize {
return nil, errors.New("chacha20: wrong key size")
}
if len(nonce) == NonceSizeX {
// XChaCha20 uses the ChaCha20 core to mix 16 bytes of the nonce into a
// derived key, allowing it to operate on a nonce of 24 bytes. See
// draft-irtf-cfrg-xchacha-01, Section 2.3.
key, _ = HChaCha20(key, nonce[0:16])
cNonce := make([]byte, NonceSize)
copy(cNonce[4:12], nonce[16:24])
nonce = cNonce
} else if len(nonce) != NonceSize {
return nil, errors.New("chacha20: wrong nonce size")
}
c.key = [8]uint32{
binary.LittleEndian.Uint32(key[0:4]),
binary.LittleEndian.Uint32(key[4:8]),
binary.LittleEndian.Uint32(key[8:12]),
binary.LittleEndian.Uint32(key[12:16]),
binary.LittleEndian.Uint32(key[16:20]),
binary.LittleEndian.Uint32(key[20:24]),
binary.LittleEndian.Uint32(key[24:28]),
binary.LittleEndian.Uint32(key[28:32]),
}
c.nonce = [3]uint32{
binary.LittleEndian.Uint32(nonce[0:4]),
binary.LittleEndian.Uint32(nonce[4:8]),
binary.LittleEndian.Uint32(nonce[8:12]),
}
return c, nil
}
// The constant first 4 words of the ChaCha20 state.
const (
j0 uint32 = 0x61707865 // expa
j1 uint32 = 0x3320646e // nd 3
j2 uint32 = 0x79622d32 // 2-by
j3 uint32 = 0x6b206574 // te k
)
const blockSize = 64
// quarterRound is the core of ChaCha20. It shuffles the bits of 4 state words.
// It's executed 4 times for each of the 20 ChaCha20 rounds, operating on all 16
// words each round, in columnar or diagonal groups of 4 at a time.
func quarterRound(a, b, c, d uint32) (uint32, uint32, uint32, uint32) {
a += b
d ^= a
d = bits.RotateLeft32(d, 16)
c += d
b ^= c
b = bits.RotateLeft32(b, 12)
a += b
d ^= a
d = bits.RotateLeft32(d, 8)
c += d
b ^= c
b = bits.RotateLeft32(b, 7)
return a, b, c, d
}
// XORKeyStream XORs each byte in the given slice with a byte from the
// cipher's key stream. Dst and src must overlap entirely or not at all.
//
// If len(dst) < len(src), XORKeyStream will panic. It is acceptable
// to pass a dst bigger than src, and in that case, XORKeyStream will
// only update dst[:len(src)] and will not touch the rest of dst.
//
// Multiple calls to XORKeyStream behave as if the concatenation of
// the src buffers was passed in a single run. That is, Cipher
// maintains state and does not reset at each XORKeyStream call.
func (s *Cipher) XORKeyStream(dst, src []byte) {
if len(src) == 0 {
return
}
if len(dst) < len(src) {
panic("chacha20: output smaller than input")
}
dst = dst[:len(src)]
if subtle.InexactOverlap(dst, src) {
panic("chacha20: invalid buffer overlap")
}
// First, drain any remaining key stream from a previous XORKeyStream.
if s.len != 0 {
keyStream := s.buf[bufSize-s.len:]
if len(src) < len(keyStream) {
keyStream = keyStream[:len(src)]
}
_ = src[len(keyStream)-1] // bounds check elimination hint
for i, b := range keyStream {
dst[i] = src[i] ^ b
}
s.len -= len(keyStream)
src = src[len(keyStream):]
dst = dst[len(keyStream):]
}
const blocksPerBuf = bufSize / blockSize
numBufs := (uint64(len(src)) + bufSize - 1) / bufSize
if uint64(s.counter)+numBufs*blocksPerBuf >= 1<<32 {
panic("chacha20: counter overflow")
}
// xorKeyStreamBlocks implementations expect input lengths that are a
// multiple of bufSize. Platform-specific ones process multiple blocks at a
// time, so have bufSizes that are a multiple of blockSize.
rem := len(src) % bufSize
full := len(src) - rem
if full > 0 {
s.xorKeyStreamBlocks(dst[:full], src[:full])
}
// If we have a partial (multi-)block, pad it for xorKeyStreamBlocks, and
// keep the leftover keystream for the next XORKeyStream invocation.
if rem > 0 {
s.buf = [bufSize]byte{}
copy(s.buf[:], src[full:])
s.xorKeyStreamBlocks(s.buf[:], s.buf[:])
s.len = bufSize - copy(dst[full:], s.buf[:])
}
}
func (s *Cipher) xorKeyStreamBlocksGeneric(dst, src []byte) {
if len(dst) != len(src) || len(dst)%blockSize != 0 {
panic("chacha20: internal error: wrong dst and/or src length")
}
// To generate each block of key stream, the initial cipher state
// (represented below) is passed through 20 rounds of shuffling,
// alternatively applying quarterRounds by columns (like 1, 5, 9, 13)
// or by diagonals (like 1, 6, 11, 12).
//
// 0:cccccccc 1:cccccccc 2:cccccccc 3:cccccccc
// 4:kkkkkkkk 5:kkkkkkkk 6:kkkkkkkk 7:kkkkkkkk
// 8:kkkkkkkk 9:kkkkkkkk 10:kkkkkkkk 11:kkkkkkkk
// 12:bbbbbbbb 13:nnnnnnnn 14:nnnnnnnn 15:nnnnnnnn
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