crypto: import an implementation of the XTS cipher mode

The XTS (XEX with tweaked-codebook and ciphertext stealing)
cipher mode is commonly used in full disk encryption. There
is unfortunately no implementation of it in either libgcrypt
or nettle, so we need to provide our own.

The libtomcrypt project provides a repository of crypto
algorithms under a choice of either "public domain" or
the "what the fuck public license".

So this impl is taken from the libtomcrypt GIT repo and
adapted to be compatible with the way we need to call
ciphers provided by nettle/gcrypt.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

2 files changed, 231 insertions, 0 deletions

diff --git a/crypto/Makefile.objs b/crypto/Makefile.objs
index 454f9db..e6590dc 100644
--- a/crypto/Makefile.objs
+++ b/crypto/Makefile.objs
@@ -18,6 +18,7 @@ crypto-obj-y += ivgen-essiv.o
 crypto-obj-y += ivgen-plain.o
 crypto-obj-y += ivgen-plain64.o
 crypto-obj-y += afsplit.o
+crypto-obj-y += xts.o
 
 # Let the userspace emulators avoid linking gnutls/etc
 crypto-aes-obj-y = aes.o
diff --git a/crypto/xts.c b/crypto/xts.c
new file mode 100644
index 0000000..9521234
--- /dev/null
+++ b/crypto/xts.c
@@ -0,0 +1,230 @@
+/*
+ * QEMU Crypto XTS cipher mode
+ *
+ * Copyright (c) 2015-2016 Red Hat, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser 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
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ *
+ * This code is originally derived from public domain / WTFPL code in
+ * LibTomCrypt crytographic library http://libtom.org. The XTS code
+ * was donated by Elliptic Semiconductor Inc (www.ellipticsemi.com)
+ * to the LibTom Projects
+ *
+ */
+
+#include "qemu/osdep.h"
+#include "crypto/xts.h"
+
+static void xts_mult_x(uint8_t *I)
+{
+    int x;
+    uint8_t t, tt;
+
+    for (x = t = 0; x < 16; x++) {
+        tt = I[x] >> 7;
+        I[x] = ((I[x] << 1) | t) & 0xFF;
+        t = tt;
+    }
+    if (tt) {
+        I[0] ^= 0x87;
+    }
+}
+
+
+/**
+ * xts_tweak_uncrypt:
+ * @param ctxt: the cipher context
+ * @param func: the cipher function
+ * @src: buffer providing the cipher text of XTS_BLOCK_SIZE bytes
+ * @dst: buffer to output the plain text of XTS_BLOCK_SIZE bytes
+ * @iv: the initialization vector tweak of XTS_BLOCK_SIZE bytes
+ *
+ * Decrypt data with a tweak
+ */
+static void xts_tweak_decrypt(const void *ctx,
+                              xts_cipher_func *func,
+                              const uint8_t *src,
+                              uint8_t *dst,
+                              uint8_t *iv)
+{
+    unsigned long x;
+
+    /* tweak encrypt block i */
+    for (x = 0; x < XTS_BLOCK_SIZE; x++) {
+        dst[x] = src[x] ^ iv[x];
+    }
+
+    func(ctx, XTS_BLOCK_SIZE, dst, dst);
+
+    for (x = 0; x < XTS_BLOCK_SIZE; x++) {
+        dst[x] = dst[x] ^ iv[x];
+    }
+
+    /* LFSR the tweak */
+    xts_mult_x(iv);
+}
+
+
+void xts_decrypt(const void *datactx,
+                 const void *tweakctx,
+                 xts_cipher_func *encfunc,
+                 xts_cipher_func *decfunc,
+                 uint8_t *iv,
+                 size_t length,
+                 uint8_t *dst,
+                 const uint8_t *src)
+{
+    uint8_t PP[XTS_BLOCK_SIZE], CC[XTS_BLOCK_SIZE], T[XTS_BLOCK_SIZE];
+    unsigned long i, m, mo, lim;
+
+    /* get number of blocks */
+    m = length >> 4;
+    mo = length & 15;
+
+    /* must have at least one full block */
+    g_assert(m != 0);
+
+    if (mo == 0) {
+        lim = m;
+    } else {
+        lim = m - 1;
+    }
+
+    /* encrypt the iv */
+    encfunc(tweakctx, XTS_BLOCK_SIZE, T, iv);
+
+    for (i = 0; i < lim; i++) {
+        xts_tweak_decrypt(datactx, decfunc, src, dst, T);
+
+        src += XTS_BLOCK_SIZE;
+        dst += XTS_BLOCK_SIZE;
+    }
+
+    /* if length is not a multiple of XTS_BLOCK_SIZE then */
+    if (mo > 0) {
+        memcpy(CC, T, XTS_BLOCK_SIZE);
+        xts_mult_x(CC);
+
+        /* PP = tweak decrypt block m-1 */
+        xts_tweak_decrypt(datactx, decfunc, src, PP, CC);
+
+        /* Pm = first length % XTS_BLOCK_SIZE bytes of PP */
+        for (i = 0; i < mo; i++) {
+            CC[i] = src[XTS_BLOCK_SIZE + i];
+            dst[XTS_BLOCK_SIZE + i] = PP[i];
+        }
+        for (; i < XTS_BLOCK_SIZE; i++) {
+            CC[i] = PP[i];
+        }
+
+        /* Pm-1 = Tweak uncrypt CC */
+        xts_tweak_decrypt(datactx, decfunc, CC, dst, T);
+    }
+
+    /* Decrypt the iv back */
+    decfunc(tweakctx, XTS_BLOCK_SIZE, iv, T);
+}
+
+
+/**
+ * xts_tweak_crypt:
+ * @param ctxt: the cipher context
+ * @param func: the cipher function
+ * @src: buffer providing the plain text of XTS_BLOCK_SIZE bytes
+ * @dst: buffer to output the cipher text of XTS_BLOCK_SIZE bytes
+ * @iv: the initialization vector tweak of XTS_BLOCK_SIZE bytes
+ *
+ * Encrypt data with a tweak
+ */
+static void xts_tweak_encrypt(const void *ctx,
+                              xts_cipher_func *func,
+                              const uint8_t *src,
+                              uint8_t *dst,
+                              uint8_t *iv)
+{
+    unsigned long x;
+
+    /* tweak encrypt block i */
+    for (x = 0; x < XTS_BLOCK_SIZE; x++) {
+        dst[x] = src[x] ^ iv[x];
+    }
+
+    func(ctx, XTS_BLOCK_SIZE, dst, dst);
+
+    for (x = 0; x < XTS_BLOCK_SIZE; x++) {
+        dst[x] = dst[x] ^ iv[x];
+    }
+
+    /* LFSR the tweak */
+    xts_mult_x(iv);
+}
+
+
+void xts_encrypt(const void *datactx,
+                 const void *tweakctx,
+                 xts_cipher_func *encfunc,
+                 xts_cipher_func *decfunc,
+                 uint8_t *iv,
+                 size_t length,
+                 uint8_t *dst,
+                 const uint8_t *src)
+{
+    uint8_t PP[XTS_BLOCK_SIZE], CC[XTS_BLOCK_SIZE], T[XTS_BLOCK_SIZE];
+    unsigned long i, m, mo, lim;
+
+    /* get number of blocks */
+    m = length >> 4;
+    mo = length & 15;
+
+    /* must have at least one full block */
+    g_assert(m != 0);
+
+    if (mo == 0) {
+        lim = m;
+    } else {
+        lim = m - 1;
+    }
+
+    /* encrypt the iv */
+    encfunc(tweakctx, XTS_BLOCK_SIZE, T, iv);
+
+    for (i = 0; i < lim; i++) {
+        xts_tweak_encrypt(datactx, encfunc, src, dst, T);
+
+        dst += XTS_BLOCK_SIZE;
+        src += XTS_BLOCK_SIZE;
+    }
+
+    /* if length is not a multiple of XTS_BLOCK_SIZE then */
+    if (mo > 0) {
+        /* CC = tweak encrypt block m-1 */
+        xts_tweak_encrypt(datactx, encfunc, src, CC, T);
+
+        /* Cm = first length % XTS_BLOCK_SIZE bytes of CC */
+        for (i = 0; i < mo; i++) {
+            PP[i] = src[XTS_BLOCK_SIZE + i];
+            dst[XTS_BLOCK_SIZE + i] = CC[i];
+        }
+
+        for (; i < XTS_BLOCK_SIZE; i++) {
+            PP[i] = CC[i];
+        }
+
+        /* Cm-1 = Tweak encrypt PP */
+        xts_tweak_encrypt(datactx, encfunc, PP, dst, T);
+    }
+
+    /* Decrypt the iv back */
+    decfunc(tweakctx, XTS_BLOCK_SIZE, iv, T);
+}