struct entropy_store;
struct entropy_store {
- /* mostly-read data: */
+ /* read-only data: */
struct poolinfo *poolinfo;
__u32 *pool;
const char *name;
struct entropy_store *pull;
/* read-write data: */
- spinlock_t lock ____cacheline_aligned_in_smp;
+ spinlock_t lock;
unsigned add_ptr;
int entropy_count;
int input_rotate;
static void extract_buf(struct entropy_store *r, __u8 *out)
{
int i;
- __u32 data[16], buf[5 + SHA_WORKSPACE_WORDS];
+ __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);
- sha_init(buf);
/*
- * 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(buf, (__u8 *)(r->pool + i), buf + 5);
- /* feed back portion of the resulting hash */
- add_entropy_words(r, &buf[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, &buf[i % 5], 1, data);
- sha_transform(buf, (__u8 *)data, buf + 5);
+ 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.
*/
-
- buf[0] ^= buf[3];
- buf[1] ^= buf[4];
- buf[2] ^= rol32(buf[2], 16);
- memcpy(out, buf, EXTRACT_SIZE);
- memset(buf, 0, sizeof(buf));
+ hash[0] ^= hash[3];
+ hash[1] ^= hash[4];
+ hash[2] ^= rol32(hash[2], 16);
+ memcpy(out, hash, EXTRACT_SIZE);
+ memset(hash, 0, sizeof(hash));
}
static ssize_t extract_entropy(struct entropy_store *r, void *buf,