Messages in this thread Patch in this message | | | Subject | [PATCH 06 of 12] random: make backtracking attacks harder | Date | Thu, 17 Jan 2008 20:33:48 -0600 | From | Matt Mackall <> |
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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>
diff -r 42aa9f950f97 -r 9569d3011032 drivers/char/random.c --- a/drivers/char/random.c Thu Jan 17 20:25:23 2008 -0600 +++ b/drivers/char/random.c Thu Jan 17 20:25:23 2008 -0600 @@ -768,37 +768,35 @@ 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[i % 5], 1, extract); + __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. + */ 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);
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