392 lines
13 KiB
Go
392 lines
13 KiB
Go
// Copyright 2016 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package chacha20 implements the ChaCha20 and XChaCha20 encryption algorithms
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// as specified in RFC 8439 and draft-irtf-cfrg-xchacha-01.
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package chacha20
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import (
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"crypto/cipher"
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"encoding/binary"
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"errors"
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"math/bits"
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"golang.org/x/crypto/internal/subtle"
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)
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const (
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// KeySize is the size of the key used by this cipher, in bytes.
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KeySize = 32
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// NonceSize is the size of the nonce used with the standard variant of this
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// cipher, in bytes.
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//
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// Note that this is too short to be safely generated at random if the same
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// key is reused more than 2³² times.
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NonceSize = 12
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// NonceSizeX is the size of the nonce used with the XChaCha20 variant of
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// this cipher, in bytes.
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NonceSizeX = 24
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)
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// Cipher is a stateful instance of ChaCha20 or XChaCha20 using a particular key
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// and nonce. A *Cipher implements the cipher.Stream interface.
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type Cipher struct {
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// The ChaCha20 state is 16 words: 4 constant, 8 of key, 1 of counter
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// (incremented after each block), and 3 of nonce.
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key [8]uint32
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counter uint32
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nonce [3]uint32
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// The last len bytes of buf are leftover key stream bytes from the previous
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// XORKeyStream invocation. The size of buf depends on how many blocks are
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// computed at a time.
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buf [bufSize]byte
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len int
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// The counter-independent results of the first round are cached after they
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// are computed the first time.
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precompDone bool
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p1, p5, p9, p13 uint32
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p2, p6, p10, p14 uint32
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p3, p7, p11, p15 uint32
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}
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var _ cipher.Stream = (*Cipher)(nil)
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// NewUnauthenticatedCipher creates a new ChaCha20 stream cipher with the given
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// 32 bytes key and a 12 or 24 bytes nonce. If a nonce of 24 bytes is provided,
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// the XChaCha20 construction will be used. It returns an error if key or nonce
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// have any other length.
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//
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// Note that ChaCha20, like all stream ciphers, is not authenticated and allows
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// attackers to silently tamper with the plaintext. For this reason, it is more
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// appropriate as a building block than as a standalone encryption mechanism.
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// Instead, consider using package golang.org/x/crypto/chacha20poly1305.
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func NewUnauthenticatedCipher(key, nonce []byte) (*Cipher, error) {
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// This function is split into a wrapper so that the Cipher allocation will
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// be inlined, and depending on how the caller uses the return value, won't
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// escape to the heap.
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c := &Cipher{}
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return newUnauthenticatedCipher(c, key, nonce)
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}
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func newUnauthenticatedCipher(c *Cipher, key, nonce []byte) (*Cipher, error) {
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if len(key) != KeySize {
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return nil, errors.New("chacha20: wrong key size")
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}
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if len(nonce) == NonceSizeX {
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// XChaCha20 uses the ChaCha20 core to mix 16 bytes of the nonce into a
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// derived key, allowing it to operate on a nonce of 24 bytes. See
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// draft-irtf-cfrg-xchacha-01, Section 2.3.
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key, _ = HChaCha20(key, nonce[0:16])
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cNonce := make([]byte, NonceSize)
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copy(cNonce[4:12], nonce[16:24])
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nonce = cNonce
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} else if len(nonce) != NonceSize {
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return nil, errors.New("chacha20: wrong nonce size")
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}
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c.key = [8]uint32{
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binary.LittleEndian.Uint32(key[0:4]),
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binary.LittleEndian.Uint32(key[4:8]),
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binary.LittleEndian.Uint32(key[8:12]),
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binary.LittleEndian.Uint32(key[12:16]),
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binary.LittleEndian.Uint32(key[16:20]),
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binary.LittleEndian.Uint32(key[20:24]),
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binary.LittleEndian.Uint32(key[24:28]),
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binary.LittleEndian.Uint32(key[28:32]),
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}
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c.nonce = [3]uint32{
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binary.LittleEndian.Uint32(nonce[0:4]),
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binary.LittleEndian.Uint32(nonce[4:8]),
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binary.LittleEndian.Uint32(nonce[8:12]),
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}
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return c, nil
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}
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// The constant first 4 words of the ChaCha20 state.
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const (
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j0 uint32 = 0x61707865 // expa
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j1 uint32 = 0x3320646e // nd 3
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j2 uint32 = 0x79622d32 // 2-by
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j3 uint32 = 0x6b206574 // te k
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)
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const blockSize = 64
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// quarterRound is the core of ChaCha20. It shuffles the bits of 4 state words.
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// It's executed 4 times for each of the 20 ChaCha20 rounds, operating on all 16
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// words each round, in columnar or diagonal groups of 4 at a time.
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func quarterRound(a, b, c, d uint32) (uint32, uint32, uint32, uint32) {
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a += b
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d ^= a
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d = bits.RotateLeft32(d, 16)
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c += d
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b ^= c
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b = bits.RotateLeft32(b, 12)
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a += b
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d ^= a
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d = bits.RotateLeft32(d, 8)
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c += d
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b ^= c
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b = bits.RotateLeft32(b, 7)
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return a, b, c, d
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}
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// SetCounter sets the Cipher counter. The next invocation of XORKeyStream will
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// behave as if (64 * counter) bytes had been encrypted so far.
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//
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// To prevent accidental counter reuse, SetCounter panics if counter is
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// less than the current value.
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func (s *Cipher) SetCounter(counter uint32) {
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// Internally, s may buffer multiple blocks, which complicates this
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// implementation slightly. When checking whether the counter has rolled
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// back, we must use both s.counter and s.len to determine how many blocks
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// we have already output.
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outputCounter := s.counter - uint32(s.len)/blockSize
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if counter < outputCounter {
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panic("chacha20: SetCounter attempted to rollback counter")
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}
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// In the general case, we set the new counter value and reset s.len to 0,
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// causing the next call to XORKeyStream to refill the buffer. However, if
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// we're advancing within the existing buffer, we can save work by simply
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// setting s.len.
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if counter < s.counter {
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s.len = int(s.counter-counter) * blockSize
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} else {
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s.counter = counter
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s.len = 0
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}
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}
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// XORKeyStream XORs each byte in the given slice with a byte from the
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// cipher's key stream. Dst and src must overlap entirely or not at all.
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//
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// If len(dst) < len(src), XORKeyStream will panic. It is acceptable
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// to pass a dst bigger than src, and in that case, XORKeyStream will
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// only update dst[:len(src)] and will not touch the rest of dst.
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//
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// Multiple calls to XORKeyStream behave as if the concatenation of
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// the src buffers was passed in a single run. That is, Cipher
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// maintains state and does not reset at each XORKeyStream call.
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func (s *Cipher) XORKeyStream(dst, src []byte) {
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if len(src) == 0 {
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return
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}
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if len(dst) < len(src) {
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panic("chacha20: output smaller than input")
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}
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dst = dst[:len(src)]
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if subtle.InexactOverlap(dst, src) {
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panic("chacha20: invalid buffer overlap")
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}
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// First, drain any remaining key stream from a previous XORKeyStream.
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if s.len != 0 {
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keyStream := s.buf[bufSize-s.len:]
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if len(src) < len(keyStream) {
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keyStream = keyStream[:len(src)]
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}
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_ = src[len(keyStream)-1] // bounds check elimination hint
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for i, b := range keyStream {
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dst[i] = src[i] ^ b
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}
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s.len -= len(keyStream)
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src = src[len(keyStream):]
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dst = dst[len(keyStream):]
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}
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const blocksPerBuf = bufSize / blockSize
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numBufs := (uint64(len(src)) + bufSize - 1) / bufSize
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if uint64(s.counter)+numBufs*blocksPerBuf >= 1<<32 {
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panic("chacha20: counter overflow")
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}
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// xorKeyStreamBlocks implementations expect input lengths that are a
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// multiple of bufSize. Platform-specific ones process multiple blocks at a
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// time, so have bufSizes that are a multiple of blockSize.
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rem := len(src) % bufSize
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full := len(src) - rem
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if full > 0 {
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s.xorKeyStreamBlocks(dst[:full], src[:full])
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}
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// If we have a partial (multi-)block, pad it for xorKeyStreamBlocks, and
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// keep the leftover keystream for the next XORKeyStream invocation.
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if rem > 0 {
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s.buf = [bufSize]byte{}
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copy(s.buf[:], src[full:])
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s.xorKeyStreamBlocks(s.buf[:], s.buf[:])
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s.len = bufSize - copy(dst[full:], s.buf[:])
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}
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}
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func (s *Cipher) xorKeyStreamBlocksGeneric(dst, src []byte) {
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if len(dst) != len(src) || len(dst)%blockSize != 0 {
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panic("chacha20: internal error: wrong dst and/or src length")
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}
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// To generate each block of key stream, the initial cipher state
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// (represented below) is passed through 20 rounds of shuffling,
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// alternatively applying quarterRounds by columns (like 1, 5, 9, 13)
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// or by diagonals (like 1, 6, 11, 12).
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//
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// 0:cccccccc 1:cccccccc 2:cccccccc 3:cccccccc
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// 4:kkkkkkkk 5:kkkkkkkk 6:kkkkkkkk 7:kkkkkkkk
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// 8:kkkkkkkk 9:kkkkkkkk 10:kkkkkkkk 11:kkkkkkkk
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// 12:bbbbbbbb 13:nnnnnnnn 14:nnnnnnnn 15:nnnnnnnn
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//
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// c=constant k=key b=blockcount n=nonce
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var (
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c0, c1, c2, c3 = j0, j1, j2, j3
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c4, c5, c6, c7 = s.key[0], s.key[1], s.key[2], s.key[3]
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c8, c9, c10, c11 = s.key[4], s.key[5], s.key[6], s.key[7]
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_, c13, c14, c15 = s.counter, s.nonce[0], s.nonce[1], s.nonce[2]
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)
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// Three quarters of the first round don't depend on the counter, so we can
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// calculate them here, and reuse them for multiple blocks in the loop, and
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// for future XORKeyStream invocations.
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if !s.precompDone {
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s.p1, s.p5, s.p9, s.p13 = quarterRound(c1, c5, c9, c13)
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s.p2, s.p6, s.p10, s.p14 = quarterRound(c2, c6, c10, c14)
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s.p3, s.p7, s.p11, s.p15 = quarterRound(c3, c7, c11, c15)
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s.precompDone = true
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}
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for i := 0; i < len(src); i += blockSize {
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// The remainder of the first column round.
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fcr0, fcr4, fcr8, fcr12 := quarterRound(c0, c4, c8, s.counter)
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// The second diagonal round.
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x0, x5, x10, x15 := quarterRound(fcr0, s.p5, s.p10, s.p15)
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x1, x6, x11, x12 := quarterRound(s.p1, s.p6, s.p11, fcr12)
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x2, x7, x8, x13 := quarterRound(s.p2, s.p7, fcr8, s.p13)
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x3, x4, x9, x14 := quarterRound(s.p3, fcr4, s.p9, s.p14)
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// The remaining 18 rounds.
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for i := 0; i < 9; i++ {
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// Column round.
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x0, x4, x8, x12 = quarterRound(x0, x4, x8, x12)
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x1, x5, x9, x13 = quarterRound(x1, x5, x9, x13)
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x2, x6, x10, x14 = quarterRound(x2, x6, x10, x14)
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x3, x7, x11, x15 = quarterRound(x3, x7, x11, x15)
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// Diagonal round.
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x0, x5, x10, x15 = quarterRound(x0, x5, x10, x15)
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x1, x6, x11, x12 = quarterRound(x1, x6, x11, x12)
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x2, x7, x8, x13 = quarterRound(x2, x7, x8, x13)
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x3, x4, x9, x14 = quarterRound(x3, x4, x9, x14)
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}
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// Finally, add back the initial state to generate the key stream.
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x0 += c0
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x1 += c1
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x2 += c2
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x3 += c3
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x4 += c4
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x5 += c5
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x6 += c6
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x7 += c7
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x8 += c8
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x9 += c9
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x10 += c10
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x11 += c11
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x12 += s.counter
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x13 += c13
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x14 += c14
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x15 += c15
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s.counter += 1
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if s.counter == 0 {
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panic("chacha20: internal error: counter overflow")
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}
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in, out := src[i:], dst[i:]
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in, out = in[:blockSize], out[:blockSize] // bounds check elimination hint
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// XOR the key stream with the source and write out the result.
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xor(out[0:], in[0:], x0)
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xor(out[4:], in[4:], x1)
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xor(out[8:], in[8:], x2)
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xor(out[12:], in[12:], x3)
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xor(out[16:], in[16:], x4)
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xor(out[20:], in[20:], x5)
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xor(out[24:], in[24:], x6)
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xor(out[28:], in[28:], x7)
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xor(out[32:], in[32:], x8)
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xor(out[36:], in[36:], x9)
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xor(out[40:], in[40:], x10)
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xor(out[44:], in[44:], x11)
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xor(out[48:], in[48:], x12)
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xor(out[52:], in[52:], x13)
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xor(out[56:], in[56:], x14)
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xor(out[60:], in[60:], x15)
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}
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}
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// HChaCha20 uses the ChaCha20 core to generate a derived key from a 32 bytes
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// key and a 16 bytes nonce. It returns an error if key or nonce have any other
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// length. It is used as part of the XChaCha20 construction.
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func HChaCha20(key, nonce []byte) ([]byte, error) {
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// This function is split into a wrapper so that the slice allocation will
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// be inlined, and depending on how the caller uses the return value, won't
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// escape to the heap.
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out := make([]byte, 32)
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return hChaCha20(out, key, nonce)
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}
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func hChaCha20(out, key, nonce []byte) ([]byte, error) {
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if len(key) != KeySize {
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return nil, errors.New("chacha20: wrong HChaCha20 key size")
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}
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if len(nonce) != 16 {
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return nil, errors.New("chacha20: wrong HChaCha20 nonce size")
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}
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x0, x1, x2, x3 := j0, j1, j2, j3
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x4 := binary.LittleEndian.Uint32(key[0:4])
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x5 := binary.LittleEndian.Uint32(key[4:8])
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x6 := binary.LittleEndian.Uint32(key[8:12])
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x7 := binary.LittleEndian.Uint32(key[12:16])
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x8 := binary.LittleEndian.Uint32(key[16:20])
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x9 := binary.LittleEndian.Uint32(key[20:24])
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x10 := binary.LittleEndian.Uint32(key[24:28])
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x11 := binary.LittleEndian.Uint32(key[28:32])
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x12 := binary.LittleEndian.Uint32(nonce[0:4])
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x13 := binary.LittleEndian.Uint32(nonce[4:8])
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x14 := binary.LittleEndian.Uint32(nonce[8:12])
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x15 := binary.LittleEndian.Uint32(nonce[12:16])
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for i := 0; i < 10; i++ {
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// Diagonal round.
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x0, x4, x8, x12 = quarterRound(x0, x4, x8, x12)
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x1, x5, x9, x13 = quarterRound(x1, x5, x9, x13)
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x2, x6, x10, x14 = quarterRound(x2, x6, x10, x14)
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x3, x7, x11, x15 = quarterRound(x3, x7, x11, x15)
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// Column round.
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x0, x5, x10, x15 = quarterRound(x0, x5, x10, x15)
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x1, x6, x11, x12 = quarterRound(x1, x6, x11, x12)
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x2, x7, x8, x13 = quarterRound(x2, x7, x8, x13)
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x3, x4, x9, x14 = quarterRound(x3, x4, x9, x14)
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}
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_ = out[31] // bounds check elimination hint
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binary.LittleEndian.PutUint32(out[0:4], x0)
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binary.LittleEndian.PutUint32(out[4:8], x1)
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binary.LittleEndian.PutUint32(out[8:12], x2)
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binary.LittleEndian.PutUint32(out[12:16], x3)
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binary.LittleEndian.PutUint32(out[16:20], x12)
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binary.LittleEndian.PutUint32(out[20:24], x13)
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binary.LittleEndian.PutUint32(out[24:28], x14)
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binary.LittleEndian.PutUint32(out[28:32], x15)
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return out, nil
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}
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