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- // Copyright 2012 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 xts implements the XTS cipher mode as specified in IEEE P1619/D16.
- //
- // XTS mode is typically used for disk encryption, which presents a number of
- // novel problems that make more common modes inapplicable. The disk is
- // conceptually an array of sectors and we must be able to encrypt and decrypt
- // a sector in isolation. However, an attacker must not be able to transpose
- // two sectors of plaintext by transposing their ciphertext.
- //
- // XTS wraps a block cipher with Rogaway's XEX mode in order to build a
- // tweakable block cipher. This allows each sector to have a unique tweak and
- // effectively create a unique key for each sector.
- //
- // XTS does not provide any authentication. An attacker can manipulate the
- // ciphertext and randomise a block (16 bytes) of the plaintext.
- //
- // (Note: this package does not implement ciphertext-stealing so sectors must
- // be a multiple of 16 bytes.)
- package xts // import "golang.org/x/crypto/xts"
-
- import (
- "crypto/cipher"
- "errors"
- )
-
- // Cipher contains an expanded key structure. It doesn't contain mutable state
- // and therefore can be used concurrently.
- type Cipher struct {
- k1, k2 cipher.Block
- }
-
- // blockSize is the block size that the underlying cipher must have. XTS is
- // only defined for 16-byte ciphers.
- const blockSize = 16
-
- // NewCipher creates a Cipher given a function for creating the underlying
- // block cipher (which must have a block size of 16 bytes). The key must be
- // twice the length of the underlying cipher's key.
- func NewCipher(cipherFunc func([]byte) (cipher.Block, error), key []byte) (c *Cipher, err error) {
- c = new(Cipher)
- if c.k1, err = cipherFunc(key[:len(key)/2]); err != nil {
- return
- }
- c.k2, err = cipherFunc(key[len(key)/2:])
-
- if c.k1.BlockSize() != blockSize {
- err = errors.New("xts: cipher does not have a block size of 16")
- }
-
- return
- }
-
- // Encrypt encrypts a sector of plaintext and puts the result into ciphertext.
- // Plaintext and ciphertext may be the same slice but should not overlap.
- // Sectors must be a multiple of 16 bytes and less than 2²⁴ bytes.
- func (c *Cipher) Encrypt(ciphertext, plaintext []byte, sectorNum uint64) {
- if len(ciphertext) < len(plaintext) {
- panic("xts: ciphertext is smaller than plaintext")
- }
- if len(plaintext)%blockSize != 0 {
- panic("xts: plaintext is not a multiple of the block size")
- }
-
- var tweak [blockSize]byte
- for i := 0; i < 8; i++ {
- tweak[i] = byte(sectorNum)
- sectorNum >>= 8
- }
-
- c.k2.Encrypt(tweak[:], tweak[:])
-
- for i := 0; i < len(plaintext); i += blockSize {
- for j := 0; j < blockSize; j++ {
- ciphertext[i+j] = plaintext[i+j] ^ tweak[j]
- }
- c.k1.Encrypt(ciphertext[i:], ciphertext[i:])
- for j := 0; j < blockSize; j++ {
- ciphertext[i+j] ^= tweak[j]
- }
-
- mul2(&tweak)
- }
- }
-
- // Decrypt decrypts a sector of ciphertext and puts the result into plaintext.
- // Plaintext and ciphertext may be the same slice but should not overlap.
- // Sectors must be a multiple of 16 bytes and less than 2²⁴ bytes.
- func (c *Cipher) Decrypt(plaintext, ciphertext []byte, sectorNum uint64) {
- if len(plaintext) < len(ciphertext) {
- panic("xts: plaintext is smaller than ciphertext")
- }
- if len(ciphertext)%blockSize != 0 {
- panic("xts: ciphertext is not a multiple of the block size")
- }
-
- var tweak [blockSize]byte
- for i := 0; i < 8; i++ {
- tweak[i] = byte(sectorNum)
- sectorNum >>= 8
- }
-
- c.k2.Encrypt(tweak[:], tweak[:])
-
- for i := 0; i < len(plaintext); i += blockSize {
- for j := 0; j < blockSize; j++ {
- plaintext[i+j] = ciphertext[i+j] ^ tweak[j]
- }
- c.k1.Decrypt(plaintext[i:], plaintext[i:])
- for j := 0; j < blockSize; j++ {
- plaintext[i+j] ^= tweak[j]
- }
-
- mul2(&tweak)
- }
- }
-
- // mul2 multiplies tweak by 2 in GF(2¹²⁸) with an irreducible polynomial of
- // x¹²⁸ + x⁷ + x² + x + 1.
- func mul2(tweak *[blockSize]byte) {
- var carryIn byte
- for j := range tweak {
- carryOut := tweak[j] >> 7
- tweak[j] = (tweak[j] << 1) + carryIn
- carryIn = carryOut
- }
- if carryIn != 0 {
- // If we have a carry bit then we need to subtract a multiple
- // of the irreducible polynomial (x¹²⁸ + x⁷ + x² + x + 1).
- // By dropping the carry bit, we're subtracting the x^128 term
- // so all that remains is to subtract x⁷ + x² + x + 1.
- // Subtraction (and addition) in this representation is just
- // XOR.
- tweak[0] ^= 1<<7 | 1<<2 | 1<<1 | 1
- }
- }
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