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random: make backtracking attacks harder 

At each extraction, we change (poolbits / 16) + 32 bits in the pool,
or 96 bits in the case of the secondary pools. Thus, a brute-force
backtracking attack on the pool state is less difficult than breaking
the hash. In certain cases, this difficulty may be is reduced to 2^64
iterations.

Instead, hash the entire pool in one go, then feedback the whole hash
(160 bits) in one go. This will make backtracking at least as hard as
inverting the hash.

Signed-off-by: Matt Mackall <mpm@selenic.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Matt Mackall 2008-04-29 01:03:00 -07:00 committed by Linus Torvalds
parent ffd8d3fa58
commit 1c0ad3d492
1 changed files with 18 additions and 20 deletions
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38
drivers/char/random.c
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@ -767,37 +767,35 @@ static void extract_buf(struct entropy_store *r, __u8 *out)
int i;
__u32 extract[16], hash[5], workspace[SHA_WORKSPACE_WORDS];
/* Generate a hash across the pool, 16 words (512 bits) at a time */
sha_init(hash);
/*
* As we hash the pool, we mix intermediate values of
* the hash back into the pool. This eliminates
* backtracking attacks (where the attacker knows
* the state of the pool plus the current outputs, and
* attempts to find previous ouputs), unless the hash
* function can be inverted.
*/
for (i = 0; i < r->poolinfo->poolwords; i += 16) {
/* hash blocks of 16 words = 512 bits */
for (i = 0; i < r->poolinfo->poolwords; i += 16)
sha_transform(hash, (__u8 *)(r->pool + i), workspace);
/* feed back portion of the resulting hash */
add_entropy_words(r, &hash[i % 5], 1);
}
/*
* To avoid duplicates, we atomically extract a
* portion of the pool while mixing, and hash one
* final time.
* We mix the hash back into the pool to prevent backtracking
* attacks (where the attacker knows the state of the pool
* plus the current outputs, and attempts to find previous
* ouputs), unless the hash function can be inverted. By
* mixing at least a SHA1 worth of hash data back, we make
* brute-forcing the feedback as hard as brute-forcing the
* hash.
*/
__add_entropy_words(r, hash, 5, extract);
/*
* To avoid duplicates, we atomically extract a portion of the
* pool while mixing, and hash one final time.
*/
__add_entropy_words(r, &hash[i % 5], 1, extract);
sha_transform(hash, (__u8 *)extract, workspace);
memset(extract, 0, sizeof(extract));
memset(workspace, 0, sizeof(workspace));
/*
* In case the hash function has some recognizable
* output pattern, we fold it in half.
* In case the hash function has some recognizable output
* pattern, we fold it in half. Thus, we always feed back
* twice as much data as we output.
*/
hash[0] ^= hash[3];
hash[1] ^= hash[4];
hash[2] ^= rol32(hash[2], 16);