Avoid two memory allocations in each RAND_DRBG_bytes
[openssl.git] / crypto / rand / drbg_lib.c
index 16ac03b8b63f45532e95aa169664abb4d1636c14..8e372e593eed6b008c7d8f881809a483038edfea 100644 (file)
@@ -72,9 +72,24 @@ static const char ossl_pers_string[] = "OpenSSL NIST SP 800-90A DRBG";
 static CRYPTO_ONCE rand_drbg_init = CRYPTO_ONCE_STATIC_INIT;
 
 
-
-static int rand_drbg_type = RAND_DRBG_TYPE;
-static unsigned int rand_drbg_flags = RAND_DRBG_FLAGS;
+#define RAND_DRBG_TYPE_FLAGS    ( \
+    RAND_DRBG_FLAG_MASTER | RAND_DRBG_FLAG_PUBLIC | RAND_DRBG_FLAG_PRIVATE )
+
+#define RAND_DRBG_TYPE_MASTER                     0
+#define RAND_DRBG_TYPE_PUBLIC                     1
+#define RAND_DRBG_TYPE_PRIVATE                    2
+
+/* Defaults */
+static int rand_drbg_type[3] = {
+    RAND_DRBG_TYPE, /* Master */
+    RAND_DRBG_TYPE, /* Public */
+    RAND_DRBG_TYPE  /* Private */
+};
+static unsigned int rand_drbg_flags[3] = {
+    RAND_DRBG_FLAGS | RAND_DRBG_FLAG_MASTER, /* Master */
+    RAND_DRBG_FLAGS | RAND_DRBG_FLAG_PUBLIC, /* Public */
+    RAND_DRBG_FLAGS | RAND_DRBG_FLAG_PRIVATE /* Private */
+};
 
 static unsigned int master_reseed_interval = MASTER_RESEED_INTERVAL;
 static unsigned int slave_reseed_interval  = SLAVE_RESEED_INTERVAL;
@@ -82,13 +97,50 @@ static unsigned int slave_reseed_interval  = SLAVE_RESEED_INTERVAL;
 static time_t master_reseed_time_interval = MASTER_RESEED_TIME_INTERVAL;
 static time_t slave_reseed_time_interval  = SLAVE_RESEED_TIME_INTERVAL;
 
-static RAND_DRBG *drbg_setup(RAND_DRBG *parent);
+/* A logical OR of all used DRBG flag bits (currently there is only one) */
+static const unsigned int rand_drbg_used_flags =
+    RAND_DRBG_FLAG_CTR_NO_DF | RAND_DRBG_FLAG_HMAC | RAND_DRBG_TYPE_FLAGS;
+
+
+static RAND_DRBG *drbg_setup(RAND_DRBG *parent, int drbg_type);
 
 static RAND_DRBG *rand_drbg_new(int secure,
                                 int type,
                                 unsigned int flags,
                                 RAND_DRBG *parent);
 
+static int is_ctr(int type)
+{
+    switch (type) {
+    case NID_aes_128_ctr:
+    case NID_aes_192_ctr:
+    case NID_aes_256_ctr:
+        return 1;
+    default:
+        return 0;
+    }
+}
+
+static int is_digest(int type)
+{
+    switch (type) {
+    case NID_sha1:
+    case NID_sha224:
+    case NID_sha256:
+    case NID_sha384:
+    case NID_sha512:
+    case NID_sha512_224:
+    case NID_sha512_256:
+    case NID_sha3_224:
+    case NID_sha3_256:
+    case NID_sha3_384:
+    case NID_sha3_512:
+        return 1;
+    default:
+        return 0;
+    }
+}
+
 /*
  * Set/initialize |drbg| to be of type |type|, with optional |flags|.
  *
@@ -101,26 +153,35 @@ int RAND_DRBG_set(RAND_DRBG *drbg, int type, unsigned int flags)
     int ret = 1;
 
     if (type == 0 && flags == 0) {
-        type = rand_drbg_type;
-        flags = rand_drbg_flags;
+        type = rand_drbg_type[RAND_DRBG_TYPE_MASTER];
+        flags = rand_drbg_flags[RAND_DRBG_TYPE_MASTER];
+    }
+
+    /* If set is called multiple times - clear the old one */
+    if (drbg->type != 0 && (type != drbg->type || flags != drbg->flags)) {
+        drbg->meth->uninstantiate(drbg);
+        rand_pool_free(drbg->adin_pool);
+        drbg->adin_pool = NULL;
     }
 
     drbg->state = DRBG_UNINITIALISED;
     drbg->flags = flags;
     drbg->type = type;
 
-    switch (type) {
-    default:
-        RANDerr(RAND_F_RAND_DRBG_SET, RAND_R_UNSUPPORTED_DRBG_TYPE);
-        return 0;
-    case 0:
+    if (type == 0) {
         /* Uninitialized; that's okay. */
+        drbg->meth = NULL;
         return 1;
-    case NID_aes_128_ctr:
-    case NID_aes_192_ctr:
-    case NID_aes_256_ctr:
+    } else if (is_ctr(type)) {
         ret = drbg_ctr_init(drbg);
-        break;
+    } else if (is_digest(type)) {
+        if (flags & RAND_DRBG_FLAG_HMAC)
+            ret = drbg_hmac_init(drbg);
+        else
+            ret = drbg_hash_init(drbg);
+    } else {
+        RANDerr(RAND_F_RAND_DRBG_SET, RAND_R_UNSUPPORTED_DRBG_TYPE);
+        return 0;
     }
 
     if (ret == 0)
@@ -135,27 +196,31 @@ int RAND_DRBG_set(RAND_DRBG *drbg, int type, unsigned int flags)
  */
 int RAND_DRBG_set_defaults(int type, unsigned int flags)
 {
-    int ret = 1;
-
-    switch (type) {
-    default:
+    int all;
+    if (!(is_digest(type) || is_ctr(type))) {
         RANDerr(RAND_F_RAND_DRBG_SET_DEFAULTS, RAND_R_UNSUPPORTED_DRBG_TYPE);
         return 0;
-    case NID_aes_128_ctr:
-    case NID_aes_192_ctr:
-    case NID_aes_256_ctr:
-        break;
     }
 
-    if ((flags & ~RAND_DRBG_USED_FLAGS) != 0) {
+    if ((flags & ~rand_drbg_used_flags) != 0) {
         RANDerr(RAND_F_RAND_DRBG_SET_DEFAULTS, RAND_R_UNSUPPORTED_DRBG_FLAGS);
         return 0;
     }
 
-    rand_drbg_type  = type;
-    rand_drbg_flags = flags;
-
-    return ret;
+    all = ((flags & RAND_DRBG_TYPE_FLAGS) == 0);
+    if (all || (flags & RAND_DRBG_FLAG_MASTER) != 0) {
+        rand_drbg_type[RAND_DRBG_TYPE_MASTER] = type;
+        rand_drbg_flags[RAND_DRBG_TYPE_MASTER] = flags | RAND_DRBG_FLAG_MASTER;
+    }
+    if (all || (flags & RAND_DRBG_FLAG_PUBLIC) != 0) {
+        rand_drbg_type[RAND_DRBG_TYPE_PUBLIC]  = type;
+        rand_drbg_flags[RAND_DRBG_TYPE_PUBLIC] = flags | RAND_DRBG_FLAG_PUBLIC;
+    }
+    if (all || (flags & RAND_DRBG_FLAG_PRIVATE) != 0) {
+        rand_drbg_type[RAND_DRBG_TYPE_PRIVATE] = type;
+        rand_drbg_flags[RAND_DRBG_TYPE_PRIVATE] = flags | RAND_DRBG_FLAG_PRIVATE;
+    }
+    return 1;
 }
 
 
@@ -224,7 +289,7 @@ static RAND_DRBG *rand_drbg_new(int secure,
 
     return drbg;
 
-err:
+ err:
     if (drbg->secure)
         OPENSSL_secure_free(drbg);
     else
@@ -253,6 +318,7 @@ void RAND_DRBG_free(RAND_DRBG *drbg)
 
     if (drbg->meth != NULL)
         drbg->meth->uninstantiate(drbg);
+    rand_pool_free(drbg->adin_pool);
     CRYPTO_THREAD_lock_free(drbg->lock);
     CRYPTO_free_ex_data(CRYPTO_EX_INDEX_DRBG, drbg, &drbg->ex_data);
 
@@ -285,8 +351,7 @@ int RAND_DRBG_instantiate(RAND_DRBG *drbg,
         goto end;
     }
 
-    if (drbg->meth == NULL)
-    {
+    if (drbg->meth == NULL) {
         RANDerr(RAND_F_RAND_DRBG_INSTANTIATE,
                 RAND_R_NO_DRBG_IMPLEMENTATION_SELECTED);
         goto end;
@@ -313,11 +378,18 @@ int RAND_DRBG_instantiate(RAND_DRBG *drbg,
         max_entropylen += drbg->max_noncelen;
     }
 
+    drbg->reseed_next_counter = tsan_load(&drbg->reseed_prop_counter);
+    if (drbg->reseed_next_counter) {
+        drbg->reseed_next_counter++;
+        if(!drbg->reseed_next_counter)
+            drbg->reseed_next_counter = 1;
+    }
+
     if (drbg->get_entropy != NULL)
         entropylen = drbg->get_entropy(drbg, &entropy, min_entropy,
                                        min_entropylen, max_entropylen, 0);
     if (entropylen < min_entropylen
-        || entropylen > max_entropylen) {
+            || entropylen > max_entropylen) {
         RANDerr(RAND_F_RAND_DRBG_INSTANTIATE, RAND_R_ERROR_RETRIEVING_ENTROPY);
         goto end;
     }
@@ -338,29 +410,15 @@ int RAND_DRBG_instantiate(RAND_DRBG *drbg,
     }
 
     drbg->state = DRBG_READY;
-    drbg->generate_counter = 0;
+    drbg->reseed_gen_counter = 1;
     drbg->reseed_time = time(NULL);
-    if (drbg->reseed_counter > 0) {
-        if (drbg->parent == NULL)
-            drbg->reseed_counter++;
-        else
-            drbg->reseed_counter = drbg->parent->reseed_counter;
-    }
+    tsan_store(&drbg->reseed_prop_counter, drbg->reseed_next_counter);
 
-end:
+ end:
     if (entropy != NULL && drbg->cleanup_entropy != NULL)
         drbg->cleanup_entropy(drbg, entropy, entropylen);
-    if (nonce != NULL && drbg->cleanup_nonce!= NULL )
+    if (nonce != NULL && drbg->cleanup_nonce != NULL)
         drbg->cleanup_nonce(drbg, nonce, noncelen);
-    if (drbg->pool != NULL) {
-        if (drbg->state == DRBG_READY) {
-            RANDerr(RAND_F_RAND_DRBG_INSTANTIATE,
-                    RAND_R_ERROR_ENTROPY_POOL_WAS_IGNORED);
-            drbg->state = DRBG_ERROR;
-        }
-        rand_pool_free(drbg->pool);
-        drbg->pool = NULL;
-    }
     if (drbg->state == DRBG_READY)
         return 1;
     return 0;
@@ -375,8 +433,8 @@ end:
  */
 int RAND_DRBG_uninstantiate(RAND_DRBG *drbg)
 {
-    if (drbg->meth == NULL)
-    {
+    int index = -1, type, flags;
+    if (drbg->meth == NULL) {
         RANDerr(RAND_F_RAND_DRBG_UNINSTANTIATE,
                 RAND_R_NO_DRBG_IMPLEMENTATION_SELECTED);
         return 0;
@@ -387,7 +445,23 @@ int RAND_DRBG_uninstantiate(RAND_DRBG *drbg)
      * initial values.
      */
     drbg->meth->uninstantiate(drbg);
-    return RAND_DRBG_set(drbg, drbg->type, drbg->flags);
+
+    /* The reset uses the default values for type and flags */
+    if (drbg->flags & RAND_DRBG_FLAG_MASTER)
+        index = RAND_DRBG_TYPE_MASTER;
+    else if (drbg->flags & RAND_DRBG_FLAG_PRIVATE)
+        index = RAND_DRBG_TYPE_PRIVATE;
+    else if (drbg->flags & RAND_DRBG_FLAG_PUBLIC)
+        index = RAND_DRBG_TYPE_PUBLIC;
+
+    if (index != -1) {
+        flags = rand_drbg_flags[index];
+        type = rand_drbg_type[index];
+    } else {
+        flags = drbg->flags;
+        type = drbg->type;
+    }
+    return RAND_DRBG_set(drbg, type, flags);
 }
 
 /*
@@ -413,21 +487,29 @@ int RAND_DRBG_reseed(RAND_DRBG *drbg,
         return 0;
     }
 
-    if (adin == NULL)
+    if (adin == NULL) {
         adinlen = 0;
-    else if (adinlen > drbg->max_adinlen) {
+    else if (adinlen > drbg->max_adinlen) {
         RANDerr(RAND_F_RAND_DRBG_RESEED, RAND_R_ADDITIONAL_INPUT_TOO_LONG);
         return 0;
     }
 
     drbg->state = DRBG_ERROR;
+
+    drbg->reseed_next_counter = tsan_load(&drbg->reseed_prop_counter);
+    if (drbg->reseed_next_counter) {
+        drbg->reseed_next_counter++;
+        if(!drbg->reseed_next_counter)
+            drbg->reseed_next_counter = 1;
+    }
+
     if (drbg->get_entropy != NULL)
         entropylen = drbg->get_entropy(drbg, &entropy, drbg->strength,
                                        drbg->min_entropylen,
                                        drbg->max_entropylen,
                                        prediction_resistance);
     if (entropylen < drbg->min_entropylen
-        || entropylen > drbg->max_entropylen) {
+            || entropylen > drbg->max_entropylen) {
         RANDerr(RAND_F_RAND_DRBG_RESEED, RAND_R_ERROR_RETRIEVING_ENTROPY);
         goto end;
     }
@@ -436,16 +518,11 @@ int RAND_DRBG_reseed(RAND_DRBG *drbg,
         goto end;
 
     drbg->state = DRBG_READY;
-    drbg->generate_counter = 0;
+    drbg->reseed_gen_counter = 1;
     drbg->reseed_time = time(NULL);
-    if (drbg->reseed_counter > 0) {
-        if (drbg->parent == NULL)
-            drbg->reseed_counter++;
-        else
-            drbg->reseed_counter = drbg->parent->reseed_counter;
-    }
+    tsan_store(&drbg->reseed_prop_counter, drbg->reseed_next_counter);
 
-end:
+ end:
     if (entropy != NULL && drbg->cleanup_entropy != NULL)
         drbg->cleanup_entropy(drbg, entropy, entropylen);
     if (drbg->state == DRBG_READY)
@@ -479,8 +556,10 @@ int rand_drbg_restart(RAND_DRBG *drbg,
 
     if (drbg->pool != NULL) {
         RANDerr(RAND_F_RAND_DRBG_RESTART, ERR_R_INTERNAL_ERROR);
+        drbg->state = DRBG_ERROR;
         rand_pool_free(drbg->pool);
         drbg->pool = NULL;
+        return 0;
     }
 
     if (buffer != NULL) {
@@ -488,24 +567,25 @@ int rand_drbg_restart(RAND_DRBG *drbg,
             if (drbg->max_entropylen < len) {
                 RANDerr(RAND_F_RAND_DRBG_RESTART,
                     RAND_R_ENTROPY_INPUT_TOO_LONG);
+                drbg->state = DRBG_ERROR;
                 return 0;
             }
 
             if (entropy > 8 * len) {
                 RANDerr(RAND_F_RAND_DRBG_RESTART, RAND_R_ENTROPY_OUT_OF_RANGE);
+                drbg->state = DRBG_ERROR;
                 return 0;
             }
 
             /* will be picked up by the rand_drbg_get_entropy() callback */
-            drbg->pool = rand_pool_new(entropy, len, len);
+            drbg->pool = rand_pool_attach(buffer, len, entropy);
             if (drbg->pool == NULL)
                 return 0;
-
-            rand_pool_add(drbg->pool, buffer, len, entropy);
         } else {
             if (drbg->max_adinlen < len) {
                 RANDerr(RAND_F_RAND_DRBG_RESTART,
                         RAND_R_ADDITIONAL_INPUT_TOO_LONG);
+                drbg->state = DRBG_ERROR;
                 return 0;
             }
             adin = buffer;
@@ -545,14 +625,8 @@ int rand_drbg_restart(RAND_DRBG *drbg,
         }
     }
 
-    /* check whether a given entropy pool was cleared properly during reseed */
-    if (drbg->pool != NULL) {
-        drbg->state = DRBG_ERROR;
-        RANDerr(RAND_F_RAND_DRBG_RESTART, ERR_R_INTERNAL_ERROR);
-        rand_pool_free(drbg->pool);
-        drbg->pool = NULL;
-        return 0;
-    }
+    rand_pool_free(drbg->pool);
+    drbg->pool = NULL;
 
     return drbg->state == DRBG_READY;
 }
@@ -602,7 +676,7 @@ int RAND_DRBG_generate(RAND_DRBG *drbg, unsigned char *out, size_t outlen,
     }
 
     if (drbg->reseed_interval > 0) {
-        if (drbg->generate_counter >= drbg->reseed_interval)
+        if (drbg->reseed_gen_counter > drbg->reseed_interval)
             reseed_required = 1;
     }
     if (drbg->reseed_time_interval > 0) {
@@ -611,8 +685,11 @@ int RAND_DRBG_generate(RAND_DRBG *drbg, unsigned char *out, size_t outlen,
             || now - drbg->reseed_time >= drbg->reseed_time_interval)
             reseed_required = 1;
     }
-    if (drbg->reseed_counter > 0 && drbg->parent != NULL) {
-        if (drbg->reseed_counter != drbg->parent->reseed_counter)
+    if (drbg->parent != NULL) {
+        unsigned int reseed_counter = tsan_load(&drbg->reseed_prop_counter);
+        if (reseed_counter > 0
+                && tsan_load(&drbg->parent->reseed_prop_counter)
+                   != reseed_counter)
             reseed_required = 1;
     }
 
@@ -631,7 +708,7 @@ int RAND_DRBG_generate(RAND_DRBG *drbg, unsigned char *out, size_t outlen,
         return 0;
     }
 
-    drbg->generate_counter++;
+    drbg->reseed_gen_counter++;
 
     return 1;
 }
@@ -649,9 +726,18 @@ int RAND_DRBG_bytes(RAND_DRBG *drbg, unsigned char *out, size_t outlen)
     unsigned char *additional = NULL;
     size_t additional_len;
     size_t chunk;
-    size_t ret;
+    size_t ret = 0;
+
+    if (drbg->adin_pool == NULL) {
+        if (drbg->type == 0)
+            goto err;
+        drbg->adin_pool = rand_pool_new(0, 0, drbg->max_adinlen);
+        if (drbg->adin_pool == NULL)
+            goto err;
+    }
 
-    additional_len = rand_drbg_get_additional_data(&additional, drbg->max_adinlen);
+    additional_len = rand_drbg_get_additional_data(drbg->adin_pool,
+                                                   &additional);
 
     for ( ; outlen > 0; outlen -= chunk, out += chunk) {
         chunk = outlen;
@@ -663,9 +749,9 @@ int RAND_DRBG_bytes(RAND_DRBG *drbg, unsigned char *out, size_t outlen)
     }
     ret = 1;
 
-err:
-    if (additional_len != 0)
-        OPENSSL_secure_clear_free(additional, additional_len);
+ err:
+    if (additional != NULL)
+        rand_drbg_cleanup_additional_data(drbg->adin_pool, additional);
 
     return ret;
 }
@@ -684,7 +770,8 @@ int RAND_DRBG_set_callbacks(RAND_DRBG *drbg,
                             RAND_DRBG_get_nonce_fn get_nonce,
                             RAND_DRBG_cleanup_nonce_fn cleanup_nonce)
 {
-    if (drbg->state != DRBG_UNINITIALISED)
+    if (drbg->state != DRBG_UNINITIALISED
+            || drbg->parent != NULL)
         return 0;
     drbg->get_entropy = get_entropy;
     drbg->cleanup_entropy = cleanup_entropy;
@@ -848,11 +935,12 @@ void *RAND_DRBG_get_ex_data(const RAND_DRBG *drbg, int idx)
  *
  * Returns a pointer to the new DRBG instance on success, NULL on failure.
  */
-static RAND_DRBG *drbg_setup(RAND_DRBG *parent)
+static RAND_DRBG *drbg_setup(RAND_DRBG *parent, int drbg_type)
 {
     RAND_DRBG *drbg;
 
-    drbg = RAND_DRBG_secure_new(rand_drbg_type, rand_drbg_flags, parent);
+    drbg = RAND_DRBG_secure_new(rand_drbg_type[drbg_type],
+                                rand_drbg_flags[drbg_type], parent);
     if (drbg == NULL)
         return NULL;
 
@@ -861,7 +949,7 @@ static RAND_DRBG *drbg_setup(RAND_DRBG *parent)
         goto err;
 
     /* enable seed propagation */
-    drbg->reseed_counter = 1;
+    tsan_store(&drbg->reseed_prop_counter, 1);
 
     /*
      * Ignore instantiation error to support just-in-time instantiation.
@@ -885,8 +973,6 @@ err:
  */
 DEFINE_RUN_ONCE_STATIC(do_rand_drbg_init)
 {
-    int ret = 1;
-
     /*
      * ensure that libcrypto is initialized, otherwise the
      * DRBG locks are not cleaned up properly
@@ -894,37 +980,47 @@ DEFINE_RUN_ONCE_STATIC(do_rand_drbg_init)
     if (!OPENSSL_init_crypto(0, NULL))
         return 0;
 
-    ossl_init_thread_start(OPENSSL_INIT_THREAD_RAND);
-
-    master_drbg = drbg_setup(NULL);
+    if (!CRYPTO_THREAD_init_local(&private_drbg, NULL))
+        return 0;
 
-    ret &= CRYPTO_THREAD_init_local(&private_drbg, NULL);
-    ret &= CRYPTO_THREAD_init_local(&public_drbg, NULL);
+    if (!CRYPTO_THREAD_init_local(&public_drbg, NULL))
+        goto err1;
 
-    if (master_drbg == NULL || ret == 0)
-        return 0;
+    master_drbg = drbg_setup(NULL, RAND_DRBG_TYPE_MASTER);
+    if (master_drbg == NULL)
+        goto err2;
 
     return 1;
+
+err2:
+    CRYPTO_THREAD_cleanup_local(&public_drbg);
+err1:
+    CRYPTO_THREAD_cleanup_local(&private_drbg);
+    return 0;
 }
 
 /* Clean up the global DRBGs before exit */
 void rand_drbg_cleanup_int(void)
 {
-    RAND_DRBG_free(master_drbg);
-    master_drbg = NULL;
+    if (master_drbg != NULL) {
+        RAND_DRBG_free(master_drbg);
+        master_drbg = NULL;
 
-    CRYPTO_THREAD_cleanup_local(&private_drbg);
-    CRYPTO_THREAD_cleanup_local(&public_drbg);
+        CRYPTO_THREAD_cleanup_local(&private_drbg);
+        CRYPTO_THREAD_cleanup_local(&public_drbg);
+    }
 }
 
-void drbg_delete_thread_state()
+void drbg_delete_thread_state(void)
 {
     RAND_DRBG *drbg;
 
     drbg = CRYPTO_THREAD_get_local(&public_drbg);
+    CRYPTO_THREAD_set_local(&public_drbg, NULL);
     RAND_DRBG_free(drbg);
 
     drbg = CRYPTO_THREAD_get_local(&private_drbg);
+    CRYPTO_THREAD_set_local(&private_drbg, NULL);
     RAND_DRBG_free(drbg);
 }
 
@@ -942,11 +1038,53 @@ static int drbg_bytes(unsigned char *out, int count)
     return ret;
 }
 
+/*
+ * Calculates the minimum length of a full entropy buffer
+ * which is necessary to seed (i.e. instantiate) the DRBG
+ * successfully.
+ *
+ * NOTE: There is a copy of this function in drbgtest.c.
+ *       If you change anything here, you need to update
+ *       the copy accordingly.
+ */
+static size_t rand_drbg_seedlen(RAND_DRBG *drbg)
+{
+    /*
+     * If no os entropy source is available then RAND_seed(buffer, bufsize)
+     * is expected to succeed if and only if the buffer length satisfies
+     * the following requirements, which follow from the calculations
+     * in RAND_DRBG_instantiate().
+     */
+    size_t min_entropy = drbg->strength;
+    size_t min_entropylen = drbg->min_entropylen;
+
+    /*
+     * Extra entropy for the random nonce in the absence of a
+     * get_nonce callback, see comment in RAND_DRBG_instantiate().
+     */
+    if (drbg->min_noncelen > 0 && drbg->get_nonce == NULL) {
+        min_entropy += drbg->strength / 2;
+        min_entropylen += drbg->min_noncelen;
+    }
+
+    /*
+     * Convert entropy requirement from bits to bytes
+     * (dividing by 8 without rounding upwards, because
+     * all entropy requirements are divisible by 8).
+     */
+    min_entropy >>= 3;
+
+    /* Return a value that satisfies both requirements */
+    return min_entropy > min_entropylen ? min_entropy : min_entropylen;
+}
+
 /* Implements the default OpenSSL RAND_add() method */
 static int drbg_add(const void *buf, int num, double randomness)
 {
     int ret = 0;
     RAND_DRBG *drbg = RAND_DRBG_get0_master();
+    size_t buflen;
+    size_t seedlen;
 
     if (drbg == NULL)
         return 0;
@@ -954,20 +1092,49 @@ static int drbg_add(const void *buf, int num, double randomness)
     if (num < 0 || randomness < 0.0)
         return 0;
 
-    if (randomness > (double)drbg->max_entropylen) {
+    rand_drbg_lock(drbg);
+    seedlen = rand_drbg_seedlen(drbg);
+
+    buflen = (size_t)num;
+
+    if (buflen < seedlen || randomness < (double) seedlen) {
+#if defined(OPENSSL_RAND_SEED_NONE)
+        /*
+         * If no os entropy source is available, a reseeding will fail
+         * inevitably. So we use a trick to mix the buffer contents into
+         * the DRBG state without forcing a reseeding: we generate a
+         * dummy random byte, using the buffer content as additional data.
+         * Note: This won't work with RAND_DRBG_FLAG_CTR_NO_DF.
+         */
+        unsigned char dummy[1];
+
+        ret = RAND_DRBG_generate(drbg, dummy, sizeof(dummy), 0, buf, buflen);
+        rand_drbg_unlock(drbg);
+        return ret;
+#else
+        /*
+         * If an os entropy source is avaible then we declare the buffer content
+         * as additional data by setting randomness to zero and trigger a regular
+         * reseeding.
+         */
+        randomness = 0.0;
+#endif
+    }
+
+
+    if (randomness > (double)seedlen) {
         /*
          * The purpose of this check is to bound |randomness| by a
          * relatively small value in order to prevent an integer
          * overflow when multiplying by 8 in the rand_drbg_restart()
-         * call below.
+         * call below. Note that randomness is measured in bytes,
+         * not bits, so this value corresponds to eight times the
+         * security strength.
          */
-        return 0;
+        randomness = (double)seedlen;
     }
 
-    rand_drbg_lock(drbg);
-    ret = rand_drbg_restart(drbg, buf,
-                            (size_t)(unsigned int)num,
-                            (size_t)(8*randomness));
+    ret = rand_drbg_restart(drbg, buf, buflen, (size_t)(8 * randomness));
     rand_drbg_unlock(drbg);
 
     return ret;
@@ -1020,8 +1187,9 @@ RAND_DRBG *RAND_DRBG_get0_public(void)
 
     drbg = CRYPTO_THREAD_get_local(&public_drbg);
     if (drbg == NULL) {
-        ossl_init_thread_start(OPENSSL_INIT_THREAD_RAND);
-        drbg = drbg_setup(master_drbg);
+        if (!ossl_init_thread_start(OPENSSL_INIT_THREAD_RAND))
+            return NULL;
+        drbg = drbg_setup(master_drbg, RAND_DRBG_TYPE_PUBLIC);
         CRYPTO_THREAD_set_local(&public_drbg, drbg);
     }
     return drbg;
@@ -1040,8 +1208,9 @@ RAND_DRBG *RAND_DRBG_get0_private(void)
 
     drbg = CRYPTO_THREAD_get_local(&private_drbg);
     if (drbg == NULL) {
-        ossl_init_thread_start(OPENSSL_INIT_THREAD_RAND);
-        drbg = drbg_setup(master_drbg);
+        if (!ossl_init_thread_start(OPENSSL_INIT_THREAD_RAND))
+            return NULL;
+        drbg = drbg_setup(master_drbg, RAND_DRBG_TYPE_PRIVATE);
         CRYPTO_THREAD_set_local(&private_drbg, drbg);
     }
     return drbg;