--- /dev/null
+// 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"
+ "encoding/binary"
+ "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
+ binary.LittleEndian.PutUint64(tweak[:8], sectorNum)
+
+ c.k2.Encrypt(tweak[:], tweak[:])
+
+ for len(plaintext) > 0 {
+ for j := range tweak {
+ ciphertext[j] = plaintext[j] ^ tweak[j]
+ }
+ c.k1.Encrypt(ciphertext, ciphertext)
+ for j := range tweak {
+ ciphertext[j] ^= tweak[j]
+ }
+ plaintext = plaintext[blockSize:]
+ ciphertext = ciphertext[blockSize:]
+
+ 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
+ binary.LittleEndian.PutUint64(tweak[:8], sectorNum)
+
+ c.k2.Encrypt(tweak[:], tweak[:])
+
+ for len(ciphertext) > 0 {
+ for j := range tweak {
+ plaintext[j] = ciphertext[j] ^ tweak[j]
+ }
+ c.k1.Decrypt(plaintext, plaintext)
+ for j := range tweak {
+ plaintext[j] ^= tweak[j]
+ }
+ plaintext = plaintext[blockSize:]
+ ciphertext = ciphertext[blockSize:]
+
+ 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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