Initial twofish support

Rev: src/symmetric/generate_q.c:1.1
Rev: src/symmetric/include/twofish.h:1.1
Rev: src/symmetric/twofish.c:1.1

1 files changed, 574 insertions, 0 deletions

diff --git a/twofish.c b/twofish.c
new file mode 100644
index 00000000..ea029730
--- /dev/null
+++ b/twofish.c
@@ -0,0 +1,574 @@
+/*
+ * twofish - An implementation of the twofish cipher.
+ * Copyright (C) 1999 Ruud de Rooij <ruud@debian.org>
+ *
+ * Modifications for lsh, integrated testing
+ * Copyright (C) 1999 J.H.M. Dassen (Ray) <jdassen@wi.LeidenUniv.nl>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Library General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Library General Public License for more details.
+ *
+ * You should have received a copy of the GNU Library General Public
+ * License along with this library; if not, write to the Free
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ */
+
+/* ------------------------------------------------------------------------- */
+
+#include "twofish.h"
+
+#include <stdlib.h>
+#include <string.h>
+#include <assert.h>
+
+UNUSED static char cvs_id[] =
+"$Id$";
+
+/* ------------------------------------------------------------------------- */
+
+/* Type definitions for byte and word.  word refers to a 32-bit unsigned
+ * value.
+ */
+
+typedef UINT8 byte;
+typedef UINT32 word;
+
+/* Bitwise rotations on 32-bit words.  These are defined as macros that
+ * evaluate their argument twice, so do not apply to any expressions with
+ * side effects.
+ */
+
+#define rol1(x) (((x) << 1) | (((x) & 0x80000000) >> 31))
+#define rol8(x) (((x) << 8) | (((x) & 0xFF000000) >> 24))
+#define rol9(x) (((x) << 9) | (((x) & 0xFF800000) >> 23))
+#define ror1(x) (((x) >> 1) | (((x) & 0x00000001) << 31))
+
+/* void bytes_to_words(word * dest, const byte * src, int n);
+ * void words_to_bytes(byte * dest, const byte * src, int n);
+ *
+ * Copy n*4 bytes to n words and vice versa.
+ */
+
+#if defined(__i386__)
+
+/* In the i386 case, these are simply memcpy's since the memory layout
+ * of an array of bytes and an array of words is identical.
+ */
+
+#define bytes_to_words(dest,src,n) memcpy(dest,src,(n)*4)
+#define words_to_bytes(dest,src,n) memcpy(dest,src,(n)*4)
+
+#else
+
+/* These versions are independent of endianness and word size. */
+
+static void
+bytes_to_words(word *dest, const byte *src, int n)
+{
+    while (n-- > 0) {
+        *dest++ = src[0] | src[1] << 8 | src[2] << 16 | src[3] << 24;
+        src += 4;
+    }
+}
+
+static void
+words_to_bytes(byte *dest, const word *src, int n)
+{
+    while (n-- > 0) {
+        *dest++ = *src;
+        *dest++ = *src >> 8;
+        *dest++ = *src >> 16;
+        *dest++ = *src >> 24;
+        src++;
+    }
+}
+
+#endif
+
+/* ------------------------------------------------------------------------- */
+
+/* The permutations q0 and q1.  These are fixed permutations on 8-bit values.
+ * The permutations have been computed using the program generate_q
+ * which is distributed along with this file.
+ */
+
+static byte q0[] = { 0xA9, 0x67, 0xB3, 0xE8, 0x04, 0xFD, 0xA3, 0x76,
+                     0x9A, 0x92, 0x80, 0x78, 0xE4, 0xDD, 0xD1, 0x38,
+                     0x0D, 0xC6, 0x35, 0x98, 0x18, 0xF7, 0xEC, 0x6C,
+                     0x43, 0x75, 0x37, 0x26, 0xFA, 0x13, 0x94, 0x48,
+                     0xF2, 0xD0, 0x8B, 0x30, 0x84, 0x54, 0xDF, 0x23,
+                     0x19, 0x5B, 0x3D, 0x59, 0xF3, 0xAE, 0xA2, 0x82,
+                     0x63, 0x01, 0x83, 0x2E, 0xD9, 0x51, 0x9B, 0x7C,
+                     0xA6, 0xEB, 0xA5, 0xBE, 0x16, 0x0C, 0xE3, 0x61,
+                     0xC0, 0x8C, 0x3A, 0xF5, 0x73, 0x2C, 0x25, 0x0B,
+                     0xBB, 0x4E, 0x89, 0x6B, 0x53, 0x6A, 0xB4, 0xF1,
+                     0xE1, 0xE6, 0xBD, 0x45, 0xE2, 0xF4, 0xB6, 0x66,
+                     0xCC, 0x95, 0x03, 0x56, 0xD4, 0x1C, 0x1E, 0xD7,
+                     0xFB, 0xC3, 0x8E, 0xB5, 0xE9, 0xCF, 0xBF, 0xBA,
+                     0xEA, 0x77, 0x39, 0xAF, 0x33, 0xC9, 0x62, 0x71,
+                     0x81, 0x79, 0x09, 0xAD, 0x24, 0xCD, 0xF9, 0xD8,
+                     0xE5, 0xC5, 0xB9, 0x4D, 0x44, 0x08, 0x86, 0xE7,
+                     0xA1, 0x1D, 0xAA, 0xED, 0x06, 0x70, 0xB2, 0xD2,
+                     0x41, 0x7B, 0xA0, 0x11, 0x31, 0xC2, 0x27, 0x90,
+                     0x20, 0xF6, 0x60, 0xFF, 0x96, 0x5C, 0xB1, 0xAB,
+                     0x9E, 0x9C, 0x52, 0x1B, 0x5F, 0x93, 0x0A, 0xEF,
+                     0x91, 0x85, 0x49, 0xEE, 0x2D, 0x4F, 0x8F, 0x3B,
+                     0x47, 0x87, 0x6D, 0x46, 0xD6, 0x3E, 0x69, 0x64,
+                     0x2A, 0xCE, 0xCB, 0x2F, 0xFC, 0x97, 0x05, 0x7A,
+                     0xAC, 0x7F, 0xD5, 0x1A, 0x4B, 0x0E, 0xA7, 0x5A,
+                     0x28, 0x14, 0x3F, 0x29, 0x88, 0x3C, 0x4C, 0x02,
+                     0xB8, 0xDA, 0xB0, 0x17, 0x55, 0x1F, 0x8A, 0x7D,
+                     0x57, 0xC7, 0x8D, 0x74, 0xB7, 0xC4, 0x9F, 0x72,
+                     0x7E, 0x15, 0x22, 0x12, 0x58, 0x07, 0x99, 0x34,
+                     0x6E, 0x50, 0xDE, 0x68, 0x65, 0xBC, 0xDB, 0xF8,
+                     0xC8, 0xA8, 0x2B, 0x40, 0xDC, 0xFE, 0x32, 0xA4,
+                     0xCA, 0x10, 0x21, 0xF0, 0xD3, 0x5D, 0x0F, 0x00,
+                     0x6F, 0x9D, 0x36, 0x42, 0x4A, 0x5E, 0xC1, 0xE0, };
+
+static byte q1[] = { 0x75, 0xF3, 0xC6, 0xF4, 0xDB, 0x7B, 0xFB, 0xC8,
+                     0x4A, 0xD3, 0xE6, 0x6B, 0x45, 0x7D, 0xE8, 0x4B,
+                     0xD6, 0x32, 0xD8, 0xFD, 0x37, 0x71, 0xF1, 0xE1,
+                     0x30, 0x0F, 0xF8, 0x1B, 0x87, 0xFA, 0x06, 0x3F,
+                     0x5E, 0xBA, 0xAE, 0x5B, 0x8A, 0x00, 0xBC, 0x9D,
+                     0x6D, 0xC1, 0xB1, 0x0E, 0x80, 0x5D, 0xD2, 0xD5,
+                     0xA0, 0x84, 0x07, 0x14, 0xB5, 0x90, 0x2C, 0xA3,
+                     0xB2, 0x73, 0x4C, 0x54, 0x92, 0x74, 0x36, 0x51,
+                     0x38, 0xB0, 0xBD, 0x5A, 0xFC, 0x60, 0x62, 0x96,
+                     0x6C, 0x42, 0xF7, 0x10, 0x7C, 0x28, 0x27, 0x8C,
+                     0x13, 0x95, 0x9C, 0xC7, 0x24, 0x46, 0x3B, 0x70,
+                     0xCA, 0xE3, 0x85, 0xCB, 0x11, 0xD0, 0x93, 0xB8,
+                     0xA6, 0x83, 0x20, 0xFF, 0x9F, 0x77, 0xC3, 0xCC,
+                     0x03, 0x6F, 0x08, 0xBF, 0x40, 0xE7, 0x2B, 0xE2,
+                     0x79, 0x0C, 0xAA, 0x82, 0x41, 0x3A, 0xEA, 0xB9,
+                     0xE4, 0x9A, 0xA4, 0x97, 0x7E, 0xDA, 0x7A, 0x17,
+                     0x66, 0x94, 0xA1, 0x1D, 0x3D, 0xF0, 0xDE, 0xB3,
+                     0x0B, 0x72, 0xA7, 0x1C, 0xEF, 0xD1, 0x53, 0x3E,
+                     0x8F, 0x33, 0x26, 0x5F, 0xEC, 0x76, 0x2A, 0x49,
+                     0x81, 0x88, 0xEE, 0x21, 0xC4, 0x1A, 0xEB, 0xD9,
+                     0xC5, 0x39, 0x99, 0xCD, 0xAD, 0x31, 0x8B, 0x01,
+                     0x18, 0x23, 0xDD, 0x1F, 0x4E, 0x2D, 0xF9, 0x48,
+                     0x4F, 0xF2, 0x65, 0x8E, 0x78, 0x5C, 0x58, 0x19,
+                     0x8D, 0xE5, 0x98, 0x57, 0x67, 0x7F, 0x05, 0x64,
+                     0xAF, 0x63, 0xB6, 0xFE, 0xF5, 0xB7, 0x3C, 0xA5,
+                     0xCE, 0xE9, 0x68, 0x44, 0xE0, 0x4D, 0x43, 0x69,
+                     0x29, 0x2E, 0xAC, 0x15, 0x59, 0xA8, 0x0A, 0x9E,
+                     0x6E, 0x47, 0xDF, 0x34, 0x35, 0x6A, 0xCF, 0xDC,
+                     0x22, 0xC9, 0xC0, 0x9B, 0x89, 0xD4, 0xED, 0xAB,
+                     0x12, 0xA2, 0x0D, 0x52, 0xBB, 0x02, 0x2F, 0xA9,
+                     0xD7, 0x61, 0x1E, 0xB4, 0x50, 0x04, 0xF6, 0xC2,
+                     0x16, 0x25, 0x86, 0x56, 0x55, 0x09, 0xBE, 0x91, };
+
+/* ------------------------------------------------------------------------- */
+
+/* byte gf_multiply(byte p, byte a, byte b)
+ *
+ * Multiplication in GF(2^8).
+ *
+ * This function multiplies a times b in the Galois Field GF(2^8) with
+ * primitive polynomial p.
+ * The representation of the polynomials a, b, and p uses bits with
+ * values 2^i to represent the terms x^i.  The polynomial p contains an
+ * implicit term x^8.
+ *
+ * Note that addition and subtraction in GF(2^8) is simply the XOR
+ * operation.
+ */
+
+static byte
+gf_multiply(byte p, byte a, byte b)
+{
+    word shift  = b;
+    byte result = 0;
+    while (a) {
+        if (a & 1) result ^= shift;
+        a = a >> 1;
+        shift = shift << 1;
+        if (shift & 0x100) shift ^= p;
+    }
+    return result;
+}
+
+/* ------------------------------------------------------------------------- */
+
+/* The matrix RS as specified in section 4.3 the twofish paper. */
+
+static byte rs_matrix[4][8] = {
+    { 0x01, 0xA4, 0x55, 0x87, 0x5A, 0x58, 0xDB, 0x9E },
+    { 0xA4, 0x56, 0x82, 0xF3, 0x1E, 0xC6, 0x68, 0xE5 },
+    { 0x02, 0xA1, 0xFC, 0xC1, 0x47, 0xAE, 0x3D, 0x19 },
+    { 0xA4, 0x55, 0x87, 0x5A, 0x58, 0xDB, 0x9E, 0x03 } };
+
+/* word compute_s(word m1, word m2);
+ *
+ * Computes the value RS * M, where M is a byte vector composed of the
+ * bytes of m1 and m2.  Arithmetic is done in GF(2^8) with primitive
+ * polynomial x^8 + x^6 + x^3 + x^2 + 1.
+ *
+ * This function is used to compute the sub-keys S which are in turn used
+ * to generate the S-boxes.
+ */
+
+static word
+compute_s(word m1, word m2)
+{
+    word s = 0;
+    int i;
+    for (i = 0; i < 4; i++)
+        s |=  ((  gf_multiply(0x4D, m1,       rs_matrix[i][0])
+                ^ gf_multiply(0x4D, m1 >> 8,  rs_matrix[i][1])
+                ^ gf_multiply(0x4D, m1 >> 16, rs_matrix[i][2])
+                ^ gf_multiply(0x4D, m1 >> 24, rs_matrix[i][3])
+                ^ gf_multiply(0x4D, m2,       rs_matrix[i][4])
+                ^ gf_multiply(0x4D, m2 >> 8,  rs_matrix[i][5])
+                ^ gf_multiply(0x4D, m2 >> 16, rs_matrix[i][6])
+                ^ gf_multiply(0x4D, m2 >> 24, rs_matrix[i][7])) << (i*8));
+    return s;
+}
+
+/* ------------------------------------------------------------------------- */
+
+/* This table describes which q S-boxes are used for each byte in each stage
+ * of the function h, cf. figure 2 of the twofish paper.
+ */
+
+static byte * q_table[4][5] = { { q1, q1, q0, q0, q1 },
+                                { q0, q1, q1, q0, q0 },
+                                { q0, q0, q0, q1, q1 },
+                                { q1, q0, q1, q1, q0 } };
+
+/* The matrix MDS as specified in section 4.3.2 of the twofish paper. */
+
+static byte mds_matrix[4][4] = { { 0x01, 0xEF, 0x5B, 0x5B },
+                                { 0x5B, 0xEF, 0xEF, 0x01 },
+                                { 0xEF, 0x5B, 0x01, 0xEF },
+                                { 0xEF, 0x01, 0xEF, 0x5B } };
+
+/* word h_byte(int k, int i, byte x, byte l0, byte l1, byte l2, byte l3);
+ *
+ * Perform the h function (section 4.3.2) on one byte.  It consists of
+ * repeated applications of the q permutation, followed by a XOR with
+ * part of a sub-key.  Finally, the value is multiplied by one column of
+ * the MDS matrix.  To obtain the result for a full word, the results of
+ * h for the individual bytes are XORed.
+ *
+ * k is the key size (/ 64 bits), i is the byte number (0 = LSB), x is the
+ * actual byte to apply the function to; l0, l1, l2, and l3 are the
+ * appropriate bytes from the subkey.  Note that only l0..lk are used.
+ */
+
+static word
+h_byte(int k, int i, byte x, byte l0, byte l1, byte l2, byte l3)
+{
+    byte y = q_table[i][4][l0 ^
+                 q_table[i][3][l1 ^
+                     q_table[i][2][k == 2 ? x : l2 ^
+                         q_table[i][1][k == 3 ? x : l3 ^ q_table[i][0][x]]]]];
+
+    return ((word)gf_multiply(0x69, mds_matrix[0][i], y))       |
+           ((word)gf_multiply(0x69, mds_matrix[1][i], y) << 8)  |
+           ((word)gf_multiply(0x69, mds_matrix[2][i], y) << 16) |
+           ((word)gf_multiply(0x69, mds_matrix[3][i], y) << 24);
+}
+
+/* word h(int k, byte x, word l0, word l1, word l2, word l3);
+ *
+ * Perform the function h on a word.  See the description of h_byte() above.
+ */
+
+static word
+h(int k, byte x, word l0, word l1, word l2, word l3)
+{
+    return   h_byte(k, 0, x, l0,       l1,       l2,       l3)
+           ^ h_byte(k, 1, x, l0 >> 8,  l1 >> 8,  l2 >> 8,  l3 >> 8)
+           ^ h_byte(k, 2, x, l0 >> 16, l1 >> 16, l2 >> 16, l3 >> 16)
+           ^ h_byte(k, 3, x, l0 >> 24, l1 >> 24, l2 >> 24, l3 >> 24);
+}
+
+
+/*
+ * Sanity check using the test vectors from appendix 2 of the Twofish paper.
+ */
+static int
+twofish_selftest(void) {
+  byte testkey128[16] =
+       { 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+         0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 };
+  byte ciphertext128[16] =
+       {
+	 0x5D, 0x9D, 0x4E, 0xEF, 0xFA, 0x91, 0x51, 0x57,
+	 0x55, 0x24, 0xF1, 0x15, 0x81, 0x5A, 0x12, 0xE0 };
+
+  byte testkey192[24] =
+       { 0x01, 0x23, 0x45, 0x67, 0x89, 0xAB, 0xCD, 0xEF,
+         0xFE, 0xDC, 0xBA, 0x98, 0x76, 0x54, 0x32, 0x10,
+         0x00, 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77 };
+  byte ciphertext192[16] =
+       { 0xE7, 0x54, 0x49, 0x21, 0x2B, 0xEE, 0xF9, 0xF4,
+	 0xA3, 0x90, 0xBD, 0x86, 0x0A, 0x64, 0x09, 0x41 };
+
+  byte testkey256[32] =
+       { 0x01, 0x23, 0x45, 0x67, 0x89, 0xAB, 0xCD, 0xEF,
+         0xFE, 0xDC, 0xBA, 0x98, 0x76, 0x54, 0x32, 0x10,
+         0x00, 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77,
+         0x88, 0x99, 0xAA, 0xBB, 0xCC, 0xDD, 0xEE, 0xFF };
+  byte ciphertext256[16] =
+       { 0x37, 0xFE, 0x26, 0xFF, 0x1C, 0xF6, 0x61, 0x75,
+	 0xF5, 0xDD, 0xF4, 0xC3, 0x3B, 0x97, 0xA2, 0x05 };
+
+  TWOFISH_context * context;
+  int i;
+  byte plaintext[16], ciphertext[16];
+
+  context = twofish_setup(16, testkey128);
+  bzero(plaintext, 16);
+
+  for (i = 0 ; i < 50; i++) {
+     twofish_encrypt(context, plaintext, ciphertext);
+     memcpy(plaintext, ciphertext, 16);
+  }
+  if (!memcmp(ciphertext, ciphertext128, 16)) {
+    return 0;
+  }
+
+  context = twofish_setup(24, testkey192);
+  bzero(plaintext, 16);
+
+  for (i = 0 ; i < 50; i++) {
+     twofish_encrypt(context, plaintext, ciphertext);
+     memcpy(plaintext, ciphertext, 16);
+  }
+  if (!memcmp(ciphertext, ciphertext192, 16)) {
+    return 0;
+  }
+
+  context = twofish_setup(32, testkey256);
+  bzero(plaintext, 16);
+
+  for (i = 0 ; i < 50; i++) {
+     twofish_encrypt(context, plaintext, ciphertext);
+     memcpy(plaintext, ciphertext, 16);
+  }
+  if (!memcmp(ciphertext, ciphertext256, 16)) {
+    return 0;
+  }
+
+  return 1;
+}
+
+/* ------------------------------------------------------------------------- */
+
+/* API */
+
+/* Structure which contains the tables containing the subkeys and the
+ * key-dependent s-boxes.
+ */
+
+
+/* TWOFISH_context * twofish_setup(size_t keysize, const void * key);
+ *
+ * Set up internal tables required for twofish encryption and decryption.
+ *
+ * The key size is specified in bytes.  Key sizes up to 32 bytes are
+ * supported.  Larger key sizes are silently truncated.  The function
+ * returns a pointer which must be passed as the first argument to
+ * twofish_encrypt() and twofish_decrypt().  When no more encryption or
+ * decryption with this key is to be performed, the storage for the tables
+ * can be reclaimed with the free() function.
+ * If no memory is available to store the tables, twofish_setup()
+ * returns NULL.
+ */
+
+TWOFISH_context *
+twofish_setup(size_t keysize, const void *key)
+{
+    TWOFISH_context * context = malloc(sizeof (TWOFISH_context));
+    byte key_copy[32];
+    word m[8], s[4], t;
+    int i, j, k;
+
+#ifndef NDEBUG
+    static int initialized = 0;
+
+    if (!initialized) {
+         initialized = 1;
+         assert(twofish_selftest());
+    }
+#endif
+
+    if (!context)
+        return NULL;
+
+    /* Extend or truncate key as necessary */
+
+    memset(key_copy, 0, 32);
+    if (keysize > 32) keysize =32;
+    memcpy(key_copy, key, keysize);
+
+    bytes_to_words(m, key_copy, keysize/4);
+
+    if (keysize <= 16)
+        k = 2;
+    else if (keysize <= 24)
+        k = 3;
+    else
+        k = 4;
+
+    /* Compute sub-keys */
+
+    for (i = 0; i < 20; i++) {
+        t = h(k, 2*i+1, m[1], m[3], m[5], m[7]);
+        t = rol8(t);
+        t += (context->keys[2*i] =
+                   t + h(k, 2*i, m[0], m[2], m[4], m[6]));
+        t = rol9(t);
+        context->keys[2*i+1] = t;
+    }
+
+    /* Compute key-dependent S-boxes */
+
+    for (i = 0; i < k; i++)
+        s[k-1-i] = compute_s(m[2*i], m[2*i+1]);
+
+    for (i = 0; i < 4; i++)
+        for (j = 0; j < 256; j++)
+            context->s_box[i][j] = h_byte(k, i, j, s[0] >> (i*8),
+                                                   s[1] >> (i*8),
+                                                   s[2] >> (i*8),
+                                                   s[3] >> (i*8));
+    return context;
+}
+
+/* void twofish_encrypt(void * context,
+ *                      const void * plaintext,
+ *                      void * ciphertext);
+ *
+ * Encrypt 16 bytes of data with the twofish algorithm.
+ *
+ * Before this function can be used, twofish_setup() must be used in order to
+ * set up various tables required for the encryption algorithm.
+ * The first argument is the handle returned from twofish_setup().
+ * This function always encrypts 16 bytes of plaintext to 16 bytes of
+ * ciphertext.  The memory areas of the plaintext and the ciphertext can
+ * overlap.
+ */
+
+void
+twofish_encrypt(void *context, const void *plaintext, void *ciphertext)
+{
+    word words[4];
+    word r0, r1, r2, r3, t0, t1;
+    int i;
+    word * keys        = ((TWOFISH_context *)context)->keys;
+    word (*s_box)[256] = ((TWOFISH_context *)context)->s_box;
+
+    bytes_to_words(words, plaintext, 4);
+
+    r0 = words[0] ^ keys[0];
+    r1 = words[1] ^ keys[1];
+    r2 = words[2] ^ keys[2];
+    r3 = words[3] ^ keys[3];
+
+    for (i = 0; i < 8; i++) {
+        t1 =   s_box[1][r1 & 0xFF]
+             ^ s_box[2][(r1 >> 8) & 0xFF]
+             ^ s_box[3][(r1 >> 16) & 0xFF]
+             ^ s_box[0][(r1 >> 24) & 0xFF];
+        t0 = (  s_box[0][r0 & 0xFF]
+              ^ s_box[1][(r0 >> 8) & 0xFF]
+              ^ s_box[2][(r0 >> 16) & 0xFF]
+              ^ s_box[3][(r0 >> 24) & 0xFF]) + t1;
+        r3 = (t1 + t0 + keys[4*i+9]) ^ rol1(r3);
+        r2 = (t0 + keys[4*i+8]) ^ r2;
+        r2 = ror1(r2);
+
+        t1 =   s_box[1][r3 & 0xFF]
+             ^ s_box[2][(r3 >> 8) & 0xFF]
+             ^ s_box[3][(r3 >> 16) & 0xFF]
+             ^ s_box[0][(r3 >> 24) & 0xFF];
+        t0 = (  s_box[0][r2 & 0xFF]
+              ^ s_box[1][(r2 >> 8) & 0xFF]
+              ^ s_box[2][(r2 >> 16) & 0xFF]
+              ^ s_box[3][(r2 >> 24) & 0xFF]) + t1;
+        r1 = (t1 + t0 + keys[4*i+11]) ^ rol1(r1);
+        r0 = (t0 + keys[4*i+10]) ^ r0;
+        r0 = ror1(r0);
+    }
+
+    words[0] = r2 ^ keys[4];
+    words[1] = r3 ^ keys[5];
+    words[2] = r0 ^ keys[6];
+    words[3] = r1 ^ keys[7];
+
+    words_to_bytes(ciphertext, words, 4);
+}
+
+/* void twofish_decrypt(void * context,
+ *                      const void * ciphertext,
+ *                      void * plaintext);
+ *
+ * Decrypt 16 bytes of data with the twofish algorithm.
+ *
+ * Before this function can be used, twofish_setup() must be used in order to
+ * set up various tables required for the decryption algorithm.
+ * The first argument is the handle returned from twofish_setup().
+ * This function always decrypts 16 bytes of ciphertext to 16 bytes of
+ * plaintext.  The memory areas of the plaintext and the ciphertext can
+ * overlap.
+ */
+
+void
+twofish_decrypt(void *context, const void *ciphertext, void *plaintext)
+{
+    word words[4];
+    word r0, r1, r2, r3, t0, t1;
+    int i;
+    word * keys  = ((TWOFISH_context *)context)->keys;
+    word (*s_box)[256] = ((TWOFISH_context *)context)->s_box;
+
+    bytes_to_words(words, ciphertext, 4);
+
+    r0 = words[2] ^ keys[6];
+    r1 = words[3] ^ keys[7];
+    r2 = words[0] ^ keys[4];
+    r3 = words[1] ^ keys[5];
+
+    for (i = 0; i < 8; i++) {
+        t1 =   s_box[1][r3 & 0xFF]
+             ^ s_box[2][(r3 >> 8) & 0xFF]
+             ^ s_box[3][(r3 >> 16) & 0xFF]
+             ^ s_box[0][(r3 >> 24) & 0xFF];
+        t0 = (  s_box[0][r2 & 0xFF]
+              ^ s_box[1][(r2 >> 8) & 0xFF]
+              ^ s_box[2][(r2 >> 16) & 0xFF]
+              ^ s_box[3][(r2 >> 24) & 0xFF]) + t1;
+        r1 = (t1 + t0 + keys[39-4*i]) ^ r1;
+        r1 = ror1(r1);
+        r0 = (t0 + keys[38-4*i]) ^ rol1(r0);
+
+        t1 =   s_box[1][r1 & 0xFF]
+             ^ s_box[2][(r1 >> 8) & 0xFF]
+             ^ s_box[3][(r1 >> 16) & 0xFF]
+             ^ s_box[0][(r1 >> 24) & 0xFF];
+        t0 = (  s_box[0][r0 & 0xFF]
+              ^ s_box[1][(r0 >> 8) & 0xFF]
+              ^ s_box[2][(r0 >> 16) & 0xFF]
+              ^ s_box[3][(r0 >> 24) & 0xFF]) + t1;
+        r3 = (t1 + t0 + keys[37-4*i]) ^ r3;
+        r3 = ror1(r3);
+        r2 = (t0 + keys[36-4*i]) ^ rol1(r2);
+    }
+
+    words[0] = r0 ^ keys[0];
+    words[1] = r1 ^ keys[1];
+    words[2] = r2 ^ keys[2];
+    words[3] = r3 ^ keys[3];
+
+    words_to_bytes(plaintext, words, 4);
+}