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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 by xorKeyStreamBlocks.
buf [bufSize]byte
len int
// overflow is set when the counter overflowed, no more blocks can be
// generated, and the next XORKeyStream call should panic.
overflow bool
// 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")
}
key, nonce = key[:KeySize], nonce[:NonceSize] // bounds check elimination hint
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
}
// SetCounter sets the Cipher counter. The next invocation of XORKeyStream will
// behave as if (64 * counter) bytes had been encrypted so far.
//
// To prevent accidental counter reuse, SetCounter panics if counter is less
// than the current value.
//
// Note that the execution time of XORKeyStream is not independent of the
// counter value.
func (s *Cipher) SetCounter(counter uint32) {
// Internally, s may buffer multiple blocks, which complicates this
// implementation slightly. When checking whether the counter has rolled
// back, we must use both s.counter and s.len to determine how many blocks
// we have already output.
outputCounter := s.counter - uint32(s.len)/blockSize
if s.overflow || counter < outputCounter {
panic("chacha20: SetCounter attempted to rollback counter")
}
// In the general case, we set the new counter value and reset s.len to 0,
// causing the next call to XORKeyStream to refill the buffer. However, if
// we're advancing within the existing buffer, we can save work by simply
// setting s.len.
if counter < s.counter {
s.len = int(s.counter-counter) * blockSize
} else {
s.counter = counter
s.len = 0
}
}
// 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)
dst, src = dst[len(keyStream):], src[len(keyStream):]
}
if len(src) == 0 {
return
}
// If we'd need to let the counter overflow and keep generating output,
// panic immediately. If instead we'd only reach the last block, remember
// not to generate any more output after the buffer is drained.
numBlocks := (uint64(len(src)) + blockSize - 1) / blockSize
if s.overflow || uint64(s.counter)+numBlocks > 1<<32 {
panic("chacha20: counter overflow")
} else if uint64(s.counter)+numBlocks == 1<<32 {
s.overflow = true
}
// 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.
full := len(src) - len(src)%bufSize
if full > 0 {
s.xorKeyStreamBlocks(dst[:full], src[:full])
}
dst, src = dst[full:], src[full:]
// If using a multi-block xorKeyStreamBlocks would overflow, use the generic
// one that does one block at a time.
const blocksPerBuf = bufSize / blockSize
if uint64(s.counter)+blocksPerBuf > 1<<32 {
s.buf = [bufSize]byte{}
numBlocks := (len(src) + blockSize - 1) / blockSize
buf := s.buf[bufSize-numBlocks*blockSize:]
copy(buf, src)
s.xorKeyStreamBlocksGeneric(buf, buf)
s.len = len(buf) - copy(dst, buf)
return
}
// If we have a partial (multi-)block, pad it for xorKeyStreamBlocks, and
// keep the leftover keystream for the next XORKeyStream invocation.
if len(src) > 0 {
s.buf = [bufSize]byte{}
copy(s.buf[:], src)
s.xorKeyStreamBlocks(s.buf[:], s.buf[:])
s.len = bufSize - copy(dst, 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
//
// c=constant k=key b=blockcount n=nonce
var (
c0, c1, c2, c3 = j0, j1, j2, j3
c4, c5, c6, c7 = s.key[0], s.key[1], s.key[2], s.key[3]
c8, c9, c10, c11 = s.key[4], s.key[5], s.key[6], s.key[7]
_, c13, c14, c15 = s.counter, s.nonce[0], s.nonce[1], s.nonce[2]
)
// Three quarters of the first round don't depend on the counter, so we can
// calculate them here, and reuse them for multiple blocks in the loop, and
// for future XORKeyStream invocations.
if !s.precompDone {
s.p1, s.p5, s.p9, s.p13 = quarterRound(c1, c5, c9, c13)
s.p2, s.p6, s.p10, s.p14 = quarterRound(c2, c6, c10, c14)
s.p3, s.p7, s.p11, s.p15 = quarterRound(c3, c7, c11, c15)
s.precompDone = true
}
// A condition of len(src) > 0 would be sufficient, but this also
// acts as a bounds check elimination hint.
for len(src) >= 64 && len(dst) >= 64 {
// The remainder of the first column round.
fcr0, fcr4, fcr8, fcr12 := quarterRound(c0, c4, c8, s.counter)
// The second diagonal round.
x0, x5, x10, x15 := quarterRound(fcr0, s.p5, s.p10, s.p15)
x1, x6, x11, x12 := quarterRound(s.p1, s.p6, s.p11, fcr12)
x2, x7, x8, x13 := quarterRound(s.p2, s.p7, fcr8, s.p13)
x3, x4, x9, x14 := quarterRound(s.p3, fcr4, s.p9, s.p14)
// The remaining 18 rounds.
for i := 0; i < 9; i++ {
// Column round.
x0, x4, x8, x12 = quarterRound(x0, x4, x8, x12)
x1, x5, x9, x13 = quarterRound(x1, x5, x9, x13)
x2, x6, x10, x14 = quarterRound(x2, x6, x10, x14)
x3, x7, x11, x15 = quarterRound(x3, x7, x11, x15)
// Diagonal round.
x0, x5, x10, x15 = quarterRound(x0, x5, x10, x15)
x1, x6, x11, x12 = quarterRound(x1, x6, x11, x12)
x2, x7, x8, x13 = quarterRound(x2, x7, x8, x13)
x3, x4, x9, x14 = quarterRound(x3, x4, x9, x14)
}
// Add back the initial state to generate the key stream, then
// XOR the key stream with the source and write out the result.
addXor(dst[0:4], src[0:4], x0, c0)
addXor(dst[4:8], src[4:8], x1, c1)
addXor(dst[8:12], src[8:12], x2, c2)
addXor(dst[12:16], src[12:16], x3, c3)
addXor(dst[16:20], src[16:20], x4, c4)
addXor(dst[20:24], src[20:24], x5, c5)
addXor(dst[24:28], src[24:28], x6, c6)
addXor(dst[28:32], src[28:32], x7, c7)
addXor(dst[32:36], src[32:36], x8, c8)
addXor(dst[36:40], src[36:40], x9, c9)
addXor(dst[40:44], src[40:44], x10, c10)
addXor(dst[44:48], src[44:48], x11, c11)
addXor(dst[48:52], src[48:52], x12, s.counter)
addXor(dst[52:56], src[52:56], x13, c13)
addXor(dst[56:60], src[56:60], x14, c14)
addXor(dst[60:64], src[60:64], x15, c15)
s.counter += 1
src, dst = src[blockSize:], dst[blockSize:]
}
}
// HChaCha20 uses the ChaCha20 core to generate a derived key from a 32 bytes
// key and a 16 bytes nonce. It returns an error if key or nonce have any other
// length. It is used as part of the XChaCha20 construction.
func HChaCha20(key, nonce []byte) ([]byte, error) {
// This function is split into a wrapper so that the slice allocation will
// be inlined, and depending on how the caller uses the return value, won't
// escape to the heap.
out := make([]byte, 32)
return hChaCha20(out, key, nonce)
}
func hChaCha20(out, key, nonce []byte) ([]byte, error) {
if len(key) != KeySize {
return nil, errors.New("chacha20: wrong HChaCha20 key size")
}
if len(nonce) != 16 {
return nil, errors.New("chacha20: wrong HChaCha20 nonce size")
}
x0, x1, x2, x3 := j0, j1, j2, j3
x4 := binary.LittleEndian.Uint32(key[0:4])
x5 := binary.LittleEndian.Uint32(key[4:8])
x6 := binary.LittleEndian.Uint32(key[8:12])
x7 := binary.LittleEndian.Uint32(key[12:16])
x8 := binary.LittleEndian.Uint32(key[16:20])
x9 := binary.LittleEndian.Uint32(key[20:24])
x10 := binary.LittleEndian.Uint32(key[24:28])
x11 := binary.LittleEndian.Uint32(key[28:32])
x12 := binary.LittleEndian.Uint32(nonce[0:4])
x13 := binary.LittleEndian.Uint32(nonce[4:8])
x14 := binary.LittleEndian.Uint32(nonce[8:12])
x15 := binary.LittleEndian.Uint32(nonce[12:16])
for i := 0; i < 10; i++ {
// Diagonal round.
x0, x4, x8, x12 = quarterRound(x0, x4, x8, x12)
x1, x5, x9, x13 = quarterRound(x1, x5, x9, x13)
x2, x6, x10, x14 = quarterRound(x2, x6, x10, x14)
x3, x7, x11, x15 = quarterRound(x3, x7, x11, x15)
// Column round.
x0, x5, x10, x15 = quarterRound(x0, x5, x10, x15)
x1, x6, x11, x12 = quarterRound(x1, x6, x11, x12)
x2, x7, x8, x13 = quarterRound(x2, x7, x8, x13)
x3, x4, x9, x14 = quarterRound(x3, x4, x9, x14)
}
_ = out[31] // bounds check elimination hint
binary.LittleEndian.PutUint32(out[0:4], x0)
binary.LittleEndian.PutUint32(out[4:8], x1)
binary.LittleEndian.PutUint32(out[8:12], x2)
binary.LittleEndian.PutUint32(out[12:16], x3)
binary.LittleEndian.PutUint32(out[16:20], x12)
binary.LittleEndian.PutUint32(out[20:24], x13)
binary.LittleEndian.PutUint32(out[24:28], x14)
binary.LittleEndian.PutUint32(out[28:32], x15)
return out, nil
}