diff options
| -rw-r--r-- | drivers/char/random.c | 36 |
1 files changed, 17 insertions, 19 deletions
diff --git a/drivers/char/random.c b/drivers/char/random.c index d125a4b792d..e52f64cbef0 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -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); + for (i = 0; i < r->poolinfo->poolwords; i += 16) + sha_transform(hash, (__u8 *)(r->pool + i), workspace); + /* - * 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. + * 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. */ - for (i = 0; i < r->poolinfo->poolwords; i += 16) { - /* hash blocks of 16 words = 512 bits */ - sha_transform(hash, (__u8 *)(r->pool + i), workspace); - /* feed back portion of the resulting hash */ - add_entropy_words(r, &hash[i % 5], 1); - } + __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. + * 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); |