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Subject[PATCH/proposal] dm-crypt: add digest-based iv generation mode
Hello,

since some people keep complaining that the IV generation mechanisms
supplied in cryptoloop (and now dm-crypt) are insecure, which they
somewhat are, I just hacked a small digest based IV generation mechanism.

It simply hashes the sector number and the key and uses it as IV.

You can specify the encryption mode as "cipher-digest" like aes-md5 or
serpent-sha1 or some other combination.

Consider this as a proposal, I'm not a crypto expert. Tell me if it
contains other flaws that should be fixed.

At least the "cryptoloop-exploit" Jari Ruusu posted doesn't work anymore.


--- linux-2.6.3-mm1.orig/drivers/md/dm-crypt.c 2004-02-19 17:43:53.213856776 +0100
+++ linux-2.6.3-mm1/drivers/md/dm-crypt.c 2004-02-19 17:43:59.404915592 +0100
@@ -61,11 +61,13 @@
/*
* crypto related data
*/
- struct crypto_tfm *tfm;
+ struct crypto_tfm *cipher;
+ struct crypto_tfm *digest;
sector_t iv_offset;
int (*iv_generator)(struct crypt_config *cc, u8 *iv, sector_t sector);
int iv_size;
int key_size;
+ int digest_size;
u8 key[0];
};

@@ -102,6 +104,35 @@
return 0;
}

+static int crypt_iv_digest(struct crypt_config *cc, u8 *iv, sector_t sector)
+{
+ static DECLARE_MUTEX(tfm_mutex);
+ struct scatterlist sg[2] = {
+ {
+ .page = virt_to_page(iv),
+ .offset = offset_in_page(iv),
+ .length = sizeof(u64) / sizeof(u8)
+ }, {
+ .page = virt_to_page(cc->key),
+ .offset = offset_in_page(cc->key),
+ .length = cc->key_size
+ }
+ };
+ int i;
+
+ *(u64 *)iv = cpu_to_le64((u64)sector);
+
+ /* digests use the context in the tfm, sigh */
+ down(&tfm_mutex);
+ crypto_digest_digest(cc->digest, sg, 2, iv);
+ up(&tfm_mutex);
+
+ for(i = cc->digest_size; i < cc->iv_size; i += cc->digest_size)
+ memcpy(iv + i, iv, min(cc->digest_size, cc->iv_size - i));
+
+ return 0;
+}
+
static inline int
crypt_convert_scatterlist(struct crypt_config *cc, struct scatterlist *out,
struct scatterlist *in, unsigned int length,
@@ -116,14 +147,14 @@
return r;

if (write)
- r = crypto_cipher_encrypt_iv(cc->tfm, out, in, length, iv);
+ r = crypto_cipher_encrypt_iv(cc->cipher, out, in, length, iv);
else
- r = crypto_cipher_decrypt_iv(cc->tfm, out, in, length, iv);
+ r = crypto_cipher_decrypt_iv(cc->cipher, out, in, length, iv);
} else {
if (write)
- r = crypto_cipher_encrypt(cc->tfm, out, in, length);
+ r = crypto_cipher_encrypt(cc->cipher, out, in, length);
else
- r = crypto_cipher_decrypt(cc->tfm, out, in, length);
+ r = crypto_cipher_decrypt(cc->cipher, out, in, length);
}

return r;
@@ -436,13 +467,26 @@
return -ENOMEM;
}

+ cc->digest_size = 0;
+ cc->digest = NULL;
if (!mode || strcmp(mode, "plain") == 0)
cc->iv_generator = crypt_iv_plain;
else if (strcmp(mode, "ecb") == 0)
cc->iv_generator = NULL;
else {
- ti->error = "dm-crypt: Invalid chaining mode";
- goto bad1;
+ tfm = crypto_alloc_tfm(mode, 0);
+ if (!tfm) {
+ ti->error = "dm-crypt: Error allocating digest tfm";
+ goto bad1;
+ }
+ if (crypto_tfm_alg_type(tfm) != CRYPTO_ALG_TYPE_DIGEST) {
+ ti->error = "dm-crypt: Expected digest algorithm";
+ goto bad1;
+ }
+
+ cc->digest = tfm;
+ cc->digest_size = crypto_tfm_alg_digestsize(tfm);
+ cc->iv_generator = crypt_iv_digest;
}

if (cc->iv_generator)
@@ -455,12 +499,18 @@
ti->error = "dm-crypt: Error allocating crypto tfm";
goto bad1;
}
+ if (crypto_tfm_alg_type(tfm) != CRYPTO_ALG_TYPE_CIPHER) {
+ ti->error = "dm-crypt: Expected cipher algorithm";
+ goto bad1;
+ }

- if (tfm->crt_u.cipher.cit_decrypt_iv && tfm->crt_u.cipher.cit_encrypt_iv)
- /* at least a 32 bit sector number should fit in our buffer */
+ if (tfm->crt_u.cipher.cit_decrypt_iv &&
+ tfm->crt_u.cipher.cit_encrypt_iv) {
+ /* at least a sector number should fit in our buffer */
cc->iv_size = max(crypto_tfm_alg_ivsize(tfm),
- (unsigned int)(sizeof(u32) / sizeof(u8)));
- else {
+ (unsigned int)(sizeof(u64) / sizeof(u8)));
+ cc->iv_size = max(cc->iv_size, cc->digest_size);
+ } else {
cc->iv_size = 0;
if (cc->iv_generator) {
DMWARN("dm-crypt: Selected cipher does not support IVs");
@@ -482,7 +532,7 @@
goto bad3;
}

- cc->tfm = tfm;
+ cc->cipher = tfm;
cc->key_size = key_size;
if ((key_size == 0 && strcmp(argv[1], "-") != 0)
|| crypt_decode_key(cc->key, argv[1], key_size) < 0) {
@@ -521,6 +571,8 @@
bad2:
crypto_free_tfm(tfm);
bad1:
+ if (cc->digest)
+ crypto_free_tfm(cc->digest);
kfree(cc);
return -EINVAL;
}
@@ -532,7 +584,10 @@
mempool_destroy(cc->page_pool);
mempool_destroy(cc->io_pool);

- crypto_free_tfm(cc->tfm);
+ crypto_free_tfm(cc->cipher);
+ if (cc->digest)
+ crypto_free_tfm(cc->digest);
+
dm_put_device(ti, cc->dev);
kfree(cc);
}
@@ -680,11 +735,14 @@
break;

case STATUSTYPE_TABLE:
- cipher = crypto_tfm_alg_name(cc->tfm);
+ cipher = crypto_tfm_alg_name(cc->cipher);

- switch(cc->tfm->crt_u.cipher.cit_mode) {
+ switch(cc->cipher->crt_u.cipher.cit_mode) {
case CRYPTO_TFM_MODE_CBC:
- mode = "plain";
+ if (cc->digest)
+ mode = crypto_tfm_alg_name(cc->digest);
+ else
+ mode = "plain";
break;
case CRYPTO_TFM_MODE_ECB:
mode = "ecb";
-
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