[Date Prev][Date Next]   [Thread Prev][Thread Next]   [Thread Index] [Date Index] [Author Index]

[dm-devel] [PATCH 2/2] dm-crypt: Add TCW IV mode for old CBC TCRYPT containers.



The dmcrypt already can activate TCRYPT (TrueCrypt compatible) containers
in LRW or XTS block encryption mode.

TCRYPT containers prior to version 4.1 used CBC mode with some
additional tweaks.

This patch adds support for these containers.

For now, there is no support for chained ciphers, this TCW mode support
only containers encrypted with one cipher
(Tested with AES, Twofish, Serpentm CAST5 and TripleDES).

While TCRYPT CBC mode is legacy and is known to be vulnerable
to some watermarking attacks (e.g. revealing of hidden disk
existence) it can be still useful to mount old containers
without using 3rd party software or for independent forensic
analysis of such containers.

(Both userspace and kernel code is independent implementation
based on format documentation and completely avoids use of original
source code.)

Encryption uses CBC mode with special IV generated from
additional key, xored with sector number.

There is also second key used for "whitening" of sectors.
Whitening key is xored with sector number and mixed using
CRC32 and resulting value is applied to whole sector.
(Detailed calculation is in Truecrypt documentation for version < 4.1
and will be also described on dmcrypt site.)

IV and whitening key is concatenated with encryption key,
so kernel receives all these keys as K|IV_key|Whitening_key
in one string.
Length of IV key is always the same as IV of selected cipher
and length of whitening key is fixed to TCW_WHITENING_SIZE,
so key string can be split properly.

The experimental support for activation of these containers
is already present in git devel brach of cryptsetup.

Signed-off-by: Milan Broz <gmazyland gmail com>
---
 drivers/md/dm-crypt.c | 178 +++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 177 insertions(+), 1 deletion(-)

diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
index 878bda7..0b3923d 100644
--- a/drivers/md/dm-crypt.c
+++ b/drivers/md/dm-crypt.c
@@ -2,6 +2,7 @@
  * Copyright (C) 2003 Christophe Saout <christophe saout de>
  * Copyright (C) 2004 Clemens Fruhwirth <clemens endorphin org>
  * Copyright (C) 2006-2009 Red Hat, Inc. All rights reserved.
+ * Copyright (C) 2013 Milan Broz <gmazyland gmail com>
  *
  * This file is released under the GPL.
  */
@@ -98,6 +99,13 @@ struct iv_lmk_private {
 	u8 *seed;
 };
 
+#define TCW_WHITENING_SIZE 16
+struct iv_tcw_private {
+	struct crypto_shash *crc32_tfm;
+	u8 *iv_seed;
+	u8 *whitening;
+};
+
 /*
  * Crypt: maps a linear range of a block device
  * and encrypts / decrypts at the same time.
@@ -139,6 +147,7 @@ struct crypt_config {
 		struct iv_essiv_private essiv;
 		struct iv_benbi_private benbi;
 		struct iv_lmk_private lmk;
+		struct iv_tcw_private tcw;
 	} iv_gen_private;
 	sector_t iv_offset;
 	unsigned int iv_size;
@@ -231,6 +240,16 @@ static struct crypto_ablkcipher *any_tfm(struct crypt_config *cc)
  *         version 3: the same as version 2 with additional IV seed
  *                   (it uses 65 keys, last key is used as IV seed)
  *
+ * tcw:  Compatible implementation of the block chaining mode used
+ *       by the TrueCrypt device encryption system (prior to version 4.1).
+ *       For more info see http://www.truecrypt.org.
+ *       It operates on full 512 byte sectors and uses CBC
+ *       with an IV derived from initial key and the sector number.
+ *       In addition, whitening value is applied on every sector, whitening
+ *       is calculated from initial key, sector number and mixed using CRC32.
+ *       Note that this encryption scheme is vulnerable to watermarking attacks
+ *       and should be used for old compatible containers access only.
+ *
  * plumb: unimplemented, see:
  * http://article.gmane.org/gmane.linux.kernel.device-mapper.dm-crypt/454
  */
@@ -609,6 +628,150 @@ static int crypt_iv_lmk_post(struct crypt_config *cc, u8 *iv,
 	return r;
 }
 
+static void crypt_iv_tcw_dtr(struct crypt_config *cc)
+{
+	struct iv_tcw_private *tcw = &cc->iv_gen_private.tcw;
+
+	kzfree(tcw->iv_seed);
+	tcw->iv_seed = NULL;
+	kzfree(tcw->whitening);
+	tcw->whitening = NULL;
+
+	if (tcw->crc32_tfm && !IS_ERR(tcw->crc32_tfm))
+		crypto_free_shash(tcw->crc32_tfm);
+	tcw->crc32_tfm = NULL;
+}
+
+static int crypt_iv_tcw_ctr(struct crypt_config *cc, struct dm_target *ti,
+			    const char *opts)
+{
+	struct iv_tcw_private *tcw = &cc->iv_gen_private.tcw;
+
+	if (cc->key_size <= (cc->iv_size + TCW_WHITENING_SIZE)) {
+		ti->error = "Wrong key size for TCW";
+		return -EINVAL;
+	}
+
+	tcw->crc32_tfm = crypto_alloc_shash("crc32", 0, 0);
+	if (IS_ERR(tcw->crc32_tfm)) {
+		ti->error = "Error initializing CRC32 in TCW";
+		return PTR_ERR(tcw->crc32_tfm);
+	}
+
+	tcw->iv_seed = kzalloc(cc->iv_size, GFP_KERNEL);
+	tcw->whitening = kzalloc(TCW_WHITENING_SIZE, GFP_KERNEL);
+	if (!tcw->iv_seed || !tcw->whitening) {
+		crypt_iv_tcw_dtr(cc);
+		ti->error = "Error kmallocing seed storage in TCW";
+		return -ENOMEM;
+	}
+
+	return 0;
+}
+
+static int crypt_iv_tcw_init(struct crypt_config *cc)
+{
+	struct iv_tcw_private *tcw = &cc->iv_gen_private.tcw;
+	int key_offset = cc->key_size - cc->iv_size - TCW_WHITENING_SIZE;
+
+	memcpy(tcw->iv_seed, &cc->key[key_offset], cc->iv_size);
+	memcpy(tcw->whitening, &cc->key[key_offset + cc->iv_size],
+	       TCW_WHITENING_SIZE);
+
+	return 0;
+}
+
+static int crypt_iv_tcw_wipe(struct crypt_config *cc)
+{
+	struct iv_tcw_private *tcw = &cc->iv_gen_private.tcw;
+
+	memset(tcw->iv_seed, 0, cc->iv_size);
+	memset(tcw->whitening, 0, TCW_WHITENING_SIZE);
+
+	return 0;
+}
+
+static int crypt_iv_tcw_whitening(struct crypt_config *cc,
+				  struct dm_crypt_request *dmreq,
+				  u8 *data)
+{
+	struct iv_tcw_private *tcw = &cc->iv_gen_private.tcw;
+	u64 sector = cpu_to_le64((u64)dmreq->iv_sector);
+	u8 buf[TCW_WHITENING_SIZE];
+	struct {
+		struct shash_desc desc;
+		char ctx[crypto_shash_descsize(tcw->crc32_tfm)];
+	} sdesc;
+	int i, r;
+
+	/* xor whitening with sector number */
+	memcpy(buf, tcw->whitening, TCW_WHITENING_SIZE);
+	crypto_xor(buf, (u8*)&sector, 8);
+	crypto_xor(&buf[8], (u8*)&sector, 8);
+
+	/* calculate crc32 for every 32bit part and xor it */
+	sdesc.desc.tfm = tcw->crc32_tfm;
+	sdesc.desc.flags = CRYPTO_TFM_REQ_MAY_SLEEP;
+	for (i = 0; i < 4; i++) {
+		if ((r = crypto_shash_init(&sdesc.desc)))
+			goto out;
+		if ((r = crypto_shash_update(&sdesc.desc, &buf[i * 4], 4)))
+			goto out;
+		if ((r = crypto_shash_final(&sdesc.desc, &buf[i * 4])))
+			goto out;
+	}
+	crypto_xor(&buf[0], &buf[12], 4);
+	crypto_xor(&buf[4], &buf[8], 4);
+
+	/* apply whitening (8 bytes) to whole sector */
+	for (i = 0; i < ((1 << SECTOR_SHIFT) / 8); i++)
+		crypto_xor(data + i * 8, buf, 8);
+out:
+	memset(buf, 0, sizeof(buf));
+	return r;
+}
+
+static int crypt_iv_tcw_gen(struct crypt_config *cc, u8 *iv,
+			    struct dm_crypt_request *dmreq)
+{
+	struct iv_tcw_private *tcw = &cc->iv_gen_private.tcw;
+	u64 sector = cpu_to_le64((u64)dmreq->iv_sector);
+	u8 *src;
+	int r = 0;
+
+	/* Remove whitening from ciphertext */
+	if (bio_data_dir(dmreq->ctx->bio_in) != WRITE) {
+		src = kmap_atomic(sg_page(&dmreq->sg_in));
+		r = crypt_iv_tcw_whitening(cc, dmreq, src + dmreq->sg_in.offset);
+		kunmap_atomic(src);
+	}
+
+	/* Calculate IV */
+	memcpy(iv, tcw->iv_seed, cc->iv_size);
+	crypto_xor(iv, (u8*)&sector, 8);
+	if (cc->iv_size > 8)
+		crypto_xor(&iv[8], (u8*)&sector, cc->iv_size - 8);
+
+	return r;
+}
+
+static int crypt_iv_tcw_post(struct crypt_config *cc, u8 *iv,
+			     struct dm_crypt_request *dmreq)
+{
+	u8 *dst;
+	int r;
+
+	if (bio_data_dir(dmreq->ctx->bio_in) != WRITE)
+		return 0;
+
+	/* Apply whitening on ciphertext */
+	dst = kmap_atomic(sg_page(&dmreq->sg_out));
+	r = crypt_iv_tcw_whitening(cc, dmreq, dst + dmreq->sg_out.offset);
+	kunmap_atomic(dst);
+
+	return r;
+}
+
 static struct crypt_iv_operations crypt_iv_plain_ops = {
 	.generator = crypt_iv_plain_gen
 };
@@ -644,6 +807,15 @@ static struct crypt_iv_operations crypt_iv_lmk_ops = {
 	.post	   = crypt_iv_lmk_post
 };
 
+static struct crypt_iv_operations crypt_iv_tcw_ops = {
+	.ctr	   = crypt_iv_tcw_ctr,
+	.dtr	   = crypt_iv_tcw_dtr,
+	.init	   = crypt_iv_tcw_init,
+	.wipe	   = crypt_iv_tcw_wipe,
+	.generator = crypt_iv_tcw_gen,
+	.post	   = crypt_iv_tcw_post
+};
+
 static void crypt_convert_init(struct crypt_config *cc,
 			       struct convert_context *ctx,
 			       struct bio *bio_out, struct bio *bio_in,
@@ -1499,6 +1671,10 @@ static int crypt_ctr_cipher(struct dm_target *ti,
 			cc->key_parts++;
 			cc->key_extra_size = cc->key_size / cc->key_parts;
 		}
+	} else if (strcmp(ivmode, "tcw") == 0) {
+		cc->iv_gen_ops = &crypt_iv_tcw_ops;
+		cc->key_parts += 2; /* IV + whitening */
+		cc->key_extra_size = cc->iv_size + TCW_WHITENING_SIZE;
 	} else {
 		ret = -EINVAL;
 		ti->error = "Invalid IV mode";
@@ -1824,7 +2000,7 @@ static int crypt_iterate_devices(struct dm_target *ti,
 
 static struct target_type crypt_target = {
 	.name   = "crypt",
-	.version = {1, 12, 1},
+	.version = {1, 13, 0},
 	.module = THIS_MODULE,
 	.ctr    = crypt_ctr,
 	.dtr    = crypt_dtr,
-- 
1.8.4.rc3


[Date Prev][Date Next]   [Thread Prev][Thread Next]   [Thread Index] [Date Index] [Author Index]