Fix reseeding issues of the public RAND_DRBG

Reseeding is handled very differently by the classic RAND_METHOD API
and the new RAND_DRBG api. These differences led to some problems when
the new RAND_DRBG was made the default OpenSSL RNG. In particular,
RAND_add() did not work as expected anymore. These issues are discussed
on the thread '[openssl-dev] Plea for a new public OpenSSL RNG API'
and in Pull Request #4328. This commit fixes the mentioned issues,
introducing the following changes:

- Replace the fixed size RAND_BYTES_BUFFER by a new RAND_POOL API which
  facilitates collecting entropy by the get_entropy() callback.
- Don't use RAND_poll()/RAND_add() for collecting entropy from the
  get_entropy() callback anymore. Instead, replace RAND_poll() by
  RAND_POOL_acquire_entropy().
- Add a new function rand_drbg_restart() which tries to get the DRBG
  in an instantiated state by all means, regardless of the current
  state (uninstantiated, error, ...) the DRBG is in. If the caller
  provides entropy or additional input, it will be used for reseeding.
- Restore the original documented behaviour of RAND_add() and RAND_poll()
  (namely to reseed the DRBG immediately) by a new implementation based
  on rand_drbg_restart().
- Add automatic error recovery from temporary failures of the entropy
  source to RAND_DRBG_generate() using the rand_drbg_restart() function.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Ben Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/4328)

1 files changed, 45 insertions, 0 deletions

diff --git a/include/internal/rand.h b/include/internal/rand.h
index 07f141d6cc..feda9beaa1 100644
--- a/include/internal/rand.h
+++ b/include/internal/rand.h
@@ -14,6 +14,17 @@
 #define RAND_DRBG_FLAG_CTR_USE_DF            0x2
 
 /*
+ * Default security strength (in the sense of [NIST SP 800-90Ar1])
+ * of the default OpenSSL DRBG, and the corresponding NID.
+ *
+ * Currently supported values: 128, 192, 256
+ *
+ * TODO(DRBG): would be nice to have the strength configurable
+ */
+# define RAND_DRBG_STRENGTH             128
+# define RAND_DRBG_NID                  NID_aes_128_ctr
+
+/*
  * Object lifetime functions.
  */
 RAND_DRBG *RAND_DRBG_new(int type, unsigned int flags, RAND_DRBG *parent);
@@ -64,4 +75,38 @@ int RAND_DRBG_set_callbacks(RAND_DRBG *dctx,
                             RAND_DRBG_get_nonce_fn get_nonce,
                             RAND_DRBG_cleanup_nonce_fn cleanup_nonce);
 
+/*
+ * RAND_POOL functions
+ */
+RAND_POOL *RAND_POOL_new(int entropy_requested, size_t min_len, size_t max_len);
+void RAND_POOL_free(RAND_POOL *pool);
+
+const unsigned char *RAND_POOL_buffer(RAND_POOL *pool);
+unsigned char *RAND_POOL_detach(RAND_POOL *pool);
+
+size_t RAND_POOL_entropy(RAND_POOL *pool);
+size_t RAND_POOL_length(RAND_POOL *pool);
+
+size_t RAND_POOL_entropy_available(RAND_POOL *pool);
+size_t RAND_POOL_entropy_needed(RAND_POOL *pool);
+size_t RAND_POOL_bytes_needed(RAND_POOL *pool, unsigned int entropy_per_byte);
+size_t RAND_POOL_bytes_remaining(RAND_POOL *pool);
+
+size_t RAND_POOL_add(RAND_POOL *pool,
+                     const unsigned char *buffer, size_t len, size_t entropy);
+unsigned char *RAND_POOL_add_begin(RAND_POOL *pool, size_t len);
+size_t RAND_POOL_add_end(RAND_POOL *pool, size_t len, size_t entropy);
+
+
+/*
+ * Add random bytes to the pool to acquire requested amount of entropy
+ *
+ * This function is platform specific and tries to acquire the requested
+ * amount of entropy by polling platform specific entropy sources.
+ *
+ * If the function succeeds in acquiring at least |entropy_requested| bits
+ * of entropy, the total entropy count is returned. If it fails, it returns
+ * an entropy count of 0.
+ */
+size_t RAND_POOL_acquire_entropy(RAND_POOL *pool);
 #endif