Add documentation for the RAND_DRBG API

The RAND_DRBG API was added in PR #5462 and modified by PR #5547.
This commit adds the corresponding documention.

Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5461)

20 files changed, 1173 insertions, 240 deletions

diff --git a/CHANGES b/CHANGES
index 462394ca3a..4eaed6fecc 100644
--- a/CHANGES
+++ b/CHANGES
@@ -9,6 +9,10 @@
 
  Changes between 1.1.0g and 1.1.1 [xx XXX xxxx]
 
+  *) Added new public header file <openssl/rand_drbg.h> and documentation
+     for the RAND_DRBG API. See manual page RAND_DRBG(7) for an overview.
+     [Matthias St. Pierre]
+
   *) QNX support removed (cannot find contributors to get their approval
      for the license change).
      [Rich Salz]
diff --git a/INSTALL b/INSTALL
index 007f6a38a5..71d6b8883f 100644
--- a/INSTALL
+++ b/INSTALL
@@ -1,4 +1,3 @@
-
  OPENSSL INSTALLATION
  --------------------
 
@@ -209,12 +208,40 @@
                    without a path). This flag must be provided if the
                    zlib-dynamic option is not also used. If zlib-dynamic is used
                    then this flag is optional and a default value ("ZLIB1") is
-                   used if not provided. 
+                   used if not provided.
                    On VMS: this is the filename of the zlib library (with or
                    without a path). This flag is optional and if not provided
                    then "GNV$LIBZSHR", "GNV$LIBZSHR32" or "GNV$LIBZSHR64" is
                    used by default depending on the pointer size chosen.
 
+
+  --with-rand-seed=seed1[,seed2,...]
+                   A comma separated list of seeding methods which will be tried
+                   by OpenSSL in order to obtain random input (a.k.a "entropy")
+                   for seeding its cryptographically secure random number
+                   generator (CSPRNG). The current seeding methods are:
+
+                   os:         Use a trusted operating system entropy source.
+                               This is the default method if such an entropy
+                               source exists.
+                   getrandom:  Use the L<getrandom(2)> system call if available.
+                   devrandom:  Use the the first device from the DEVRANDOM list
+                               which can be opened to read random bytes. The
+                               DEVRANDOM preprocessor constant expands to
+                               "/dev/urandom","/dev/random","/dev/srandom" on
+                               most unix-ish operating systems.
+                   egd:        Check for an entropy generating daemon.
+                   rdcpu:      Use the RDSEED or RDRAND command if provided by
+                               the CPU.
+                   librandom:  Use librandom (not implemented yet).
+                   none:       Disable automatic seeding. This is the default
+                               on some operating systems where no suitable
+                               entropy source exists, or no support for it is
+                               implemented yet.
+
+                   For more information, see the section 'Note on random number
+                   generation' at the end of this document.
+
   no-afalgeng
                    Don't build the AFALG engine. This option will be forced if
                    on a platform that does not support AFALG.
@@ -810,7 +837,7 @@
        $ nmake TESTS='test_rsa test_dsa' test           # Windows
 
      And of course, you can combine (Unix example shown):
-       
+
        $ make VERBOSE=1 TESTS='test_rsa test_dsa' test
 
      You can find the list of available tests like this:
@@ -883,7 +910,7 @@
                         command symbols.
          [.SYSTEST]     Contains the installation verification procedure.
          [.HTML]        Contains the HTML rendition of the manual pages.
-                        
+
 
      Additionally, install will add the following directories under
      OPENSSLDIR (the directory given with --openssldir or its default)
@@ -1164,10 +1191,22 @@
 
  Availability of cryptographically secure random numbers is required for
  secret key generation. OpenSSL provides several options to seed the
- internal PRNG. If not properly seeded, the internal PRNG will refuse
+ internal CSPRNG. If not properly seeded, the internal CSPRNG will refuse
  to deliver random bytes and a "PRNG not seeded error" will occur.
- On systems without /dev/urandom (or similar) device, it may be necessary
- to install additional support software to obtain a random seed.
- Please check out the manual pages for RAND_add(), RAND_bytes(), RAND_egd(),
- and the FAQ for more information.
 
+ The seeding method can be configured using the --with-rand-seed option,
+ which can be used to specify a comma separated list of seed methods.
+ However in most cases OpenSSL will choose a suitable default method,
+ so it is not necessary to explicitely provide this option. Note also
+ that not all methods are available on all platforms.
+
+ I) On operating systems which provide a suitable randomness source (in
+ form  of a system call or system device), OpenSSL will use the optimal
+ available  method to seed the CSPRNG from the operating system's
+ randomness sources. This corresponds to the option --with-rand-seed=os.
+
+ II) On systems without such a suitable randomness source, automatic seeding
+ and reseeding is disabled (--with-rand-seed=none) and it may be necessary
+ to install additional support software to obtain a random seed and reseed
+ the CSPRNG manually.  Please check out the manual pages for RAND_add(),
+ RAND_bytes(), RAND_egd(), and the FAQ for more information.
diff --git a/crypto/rand/drbg_lib.c b/crypto/rand/drbg_lib.c
index 723f6307fd..e1b3ddb57f 100644
--- a/crypto/rand/drbg_lib.c
+++ b/crypto/rand/drbg_lib.c
@@ -17,8 +17,9 @@
 #include "internal/cryptlib_int.h"
 
 /*
- * Support framework for NIST SP 800-90A DRBG, AES-CTR mode.
- * The RAND_DRBG is OpenSSL's pointer to an instance of the DRBG.
+ * Support framework for NIST SP 800-90A DRBG
+ *
+ * See manual page RAND_DRBG(7) for a general overview.
  *
  * The OpenSSL model is to have new and free functions, and that new
  * does all initialization.  That is not the NIST model, which has
@@ -29,80 +30,46 @@
  */
 
 /*
- * THE THREE SHARED DRBGs
- *
- * There are three shared DRBGs (master, public and private), which are
- * accessed concurrently by all threads.
+ * The three shared DRBG instances
  *
- * THE MASTER DRBG
+ * There are three shared DRBG instances: <master>, <public>, and <private>.
+ */
+
+/*
+ * The <master> DRBG
  *
  * Not used directly by the application, only for reseeding the two other
  * DRBGs. It reseeds itself by pulling either randomness from os entropy
- * sources or by consuming randomnes which was added by RAND_add()
- */
-static RAND_DRBG *drbg_master;
-/*+
- * DRBG HIERARCHY
- *
- * In addition there are DRBGs, which are not shared, but used only by a
- * single thread at every time, for example the DRBGs which are owned by
- * an SSL context. All DRBGs are organized in a hierarchical fashion
- * with the <master> DRBG as root.
+ * sources or by consuming randomness which was added by RAND_add().
  *
- * This gives the following overall picture:
- *
- *                  <os entropy sources>
- *                         |
- *    RAND_add() ==>    <master>          \
- *                       /   \            | shared DRBGs (with locking)
- *                 <public>  <private>    /
- *                     |
- *                   <ssl>  owned by an SSL context
- *
- * AUTOMATIC RESEEDING
- *
- * Before satisfying a generate request, a DRBG reseeds itself automatically,
- * if one of the following two conditions holds:
- *
- * - the number of generate requests since the last reseeding exceeds a
- *   certain threshold, the so called |reseed_interval|. This behaviour
- *   can be disabled by setting the |reseed_interval| to 0.
- *
- * - the time elapsed since the last reseeding exceeds a certain time
- *   interval, the so called |reseed_time_interval|. This behaviour
- *   can be disabled by setting the |reseed_time_interval| to 0.
- *
- * MANUAL RESEEDING
+ * The <master> DRBG is a global instance which is accessed concurrently by
+ * all threads. The necessary locking is managed automatically by its child
+ * DRBG instances during reseeding.
+ */
+static RAND_DRBG *master_drbg;
+/*
+ * The <public> DRBG
  *
- * For the three shared DRBGs (and only for these) there is another way to
- * reseed them manually by calling RAND_seed() (or RAND_add() with a positive
- * |randomness| argument). This will immediately reseed the <master> DRBG.
- * The <public> and <private> DRBG will detect this on their next generate
- * call and reseed, pulling randomness from <master>.
+ * Used by default for generating random bytes using RAND_bytes().
  *
- * LOCKING
+ * The <public> DRBG is thread-local, i.e., there is one instance per thread.
+ */
+static CRYPTO_THREAD_LOCAL public_drbg;
+/*
+ * The <private> DRBG
  *
- * The three shared DRBGs are intended to be used concurrently, so they
- * support locking. The RAND methods take the locks automatically, so using
- * the RAND api (in particular RAND_bytes() and RAND_priv_bytes()) is
- * thread-safe. Note however that accessing the shared DRBGs directly via
- * the RAND_DRBG interface is *not* thread-safe.
+ * Used by default for generating private keys using RAND_priv_bytes()
  *
- * All other DRBG instances don't support locking, because they are
- * intendended to be used by a single thread. Instead of accessing a single
- * DRBG instance concurrently from different threads, it is recommended to
- * instantiate a separate DRBG instance per thread. Using the same shared
- * DRBG (preferrably the public DRBG) as parent of DRBG instances on
- * different threads is safe.
+ * The <private> DRBG is thread-local, i.e., there is one instance per thread.
  */
+static CRYPTO_THREAD_LOCAL private_drbg;
+
 
 
 /* NIST SP 800-90A DRBG recommends the use of a personalization string. */
 static const char ossl_pers_string[] = "OpenSSL NIST SP 800-90A DRBG";
 
 static CRYPTO_ONCE rand_drbg_init = CRYPTO_ONCE_STATIC_INIT;
-static CRYPTO_THREAD_LOCAL private_drbg_thread_local_key;
-static CRYPTO_THREAD_LOCAL public_drbg_thread_local_key;
 
 
 
@@ -683,40 +650,10 @@ err:
 /*
  * Set the RAND_DRBG callbacks for obtaining entropy and nonce.
  *
- * In the following, the signature and the semantics of the
- * get_entropy() and cleanup_entropy() callbacks are explained.
- *
- * GET_ENTROPY
- *
- *     size_t get_entropy(RAND_DRBG *ctx,
- *                        unsigned char **pout,
- *                        int entropy,
- *                        size_t min_len, size_t max_len);
- *
- * This is a request to allocate and fill a buffer of size
- * |min_len| <= size <= |max_len| (in bytes) which contains
- * at least |entropy| bits of randomness. The buffer's address is
- * to be returned in |*pout| and the number of collected
- * randomness bytes (which may be less than the allocated size
- * of the buffer) as return value.
+ * Setting the callbacks is allowed only if the drbg has not been
+ * initialized yet. Otherwise, the operation will fail.
  *
- * If the callback fails to acquire at least |entropy| bits of
- * randomness, it shall return a buffer length of 0.
- *
- * CLEANUP_ENTROPY
- *
- *     void cleanup_entropy(RAND_DRBG *ctx,
- *                          unsigned char *out, size_t outlen);
- *
- * A request to clear and free the buffer allocated by get_entropy().
- * The values |out| and |outlen| are expected to be the random buffer's
- * address and length, as returned by the get_entropy() callback.
- *
- * GET_NONCE, CLEANUP_NONCE
- *
- * Signature and semantics of the get_nonce() and cleanup_nonce()
- * callbacks are analogous to get_entropy() and cleanup_entropy().
- * Currently, the nonce is used only for the known answer tests.
+ * Returns 1 on success, 0 on failure.
  */
 int RAND_DRBG_set_callbacks(RAND_DRBG *drbg,
                             RAND_DRBG_get_entropy_fn get_entropy,
@@ -936,12 +873,12 @@ DEFINE_RUN_ONCE_STATIC(do_rand_drbg_init)
 
     ossl_init_thread_start(OPENSSL_INIT_THREAD_RAND);
 
-    drbg_master = drbg_setup(NULL);
+    master_drbg = drbg_setup(NULL);
 
-    ret &= CRYPTO_THREAD_init_local(&private_drbg_thread_local_key, NULL);
-    ret &= CRYPTO_THREAD_init_local(&public_drbg_thread_local_key, NULL);
+    ret &= CRYPTO_THREAD_init_local(&private_drbg, NULL);
+    ret &= CRYPTO_THREAD_init_local(&public_drbg, NULL);
 
-    if (drbg_master == NULL || ret == 0)
+    if (master_drbg == NULL || ret == 0)
         return 0;
 
     return 1;
@@ -950,21 +887,21 @@ DEFINE_RUN_ONCE_STATIC(do_rand_drbg_init)
 /* Clean up the global DRBGs before exit */
 void rand_drbg_cleanup_int(void)
 {
-    RAND_DRBG_free(drbg_master);
-    drbg_master = NULL;
+    RAND_DRBG_free(master_drbg);
+    master_drbg = NULL;
 
-    CRYPTO_THREAD_cleanup_local(&private_drbg_thread_local_key);
-    CRYPTO_THREAD_cleanup_local(&public_drbg_thread_local_key);
+    CRYPTO_THREAD_cleanup_local(&private_drbg);
+    CRYPTO_THREAD_cleanup_local(&public_drbg);
 }
 
 void drbg_delete_thread_state()
 {
     RAND_DRBG *drbg;
 
-    drbg = CRYPTO_THREAD_get_local(&public_drbg_thread_local_key);
+    drbg = CRYPTO_THREAD_get_local(&public_drbg);
     RAND_DRBG_free(drbg);
 
-    drbg = CRYPTO_THREAD_get_local(&private_drbg_thread_local_key);
+    drbg = CRYPTO_THREAD_get_local(&private_drbg);
     RAND_DRBG_free(drbg);
 }
 
@@ -1044,7 +981,7 @@ RAND_DRBG *RAND_DRBG_get0_master(void)
     if (!RUN_ONCE(&rand_drbg_init, do_rand_drbg_init))
         return NULL;
 
-    return drbg_master;
+    return master_drbg;
 }
 
 /*
@@ -1058,11 +995,11 @@ RAND_DRBG *RAND_DRBG_get0_public(void)
     if (!RUN_ONCE(&rand_drbg_init, do_rand_drbg_init))
         return NULL;
 
-    drbg = CRYPTO_THREAD_get_local(&public_drbg_thread_local_key);
+    drbg = CRYPTO_THREAD_get_local(&public_drbg);
     if (drbg == NULL) {
         ossl_init_thread_start(OPENSSL_INIT_THREAD_RAND);
-        drbg = drbg_setup(drbg_master);
-        CRYPTO_THREAD_set_local(&public_drbg_thread_local_key, drbg);
+        drbg = drbg_setup(master_drbg);
+        CRYPTO_THREAD_set_local(&public_drbg, drbg);
     }
     return drbg;
 }
@@ -1078,11 +1015,11 @@ RAND_DRBG *RAND_DRBG_get0_private(void)
     if (!RUN_ONCE(&rand_drbg_init, do_rand_drbg_init))
         return NULL;
 
-    drbg = CRYPTO_THREAD_get_local(&private_drbg_thread_local_key);
+    drbg = CRYPTO_THREAD_get_local(&private_drbg);
     if (drbg == NULL) {
         ossl_init_thread_start(OPENSSL_INIT_THREAD_RAND);
-        drbg = drbg_setup(drbg_master);
-        CRYPTO_THREAD_set_local(&private_drbg_thread_local_key, drbg);
+        drbg = drbg_setup(master_drbg);
+        CRYPTO_THREAD_set_local(&private_drbg, drbg);
     }
     return drbg;
 }
diff --git a/doc/man3/CRYPTO_get_ex_new_index.pod b/doc/man3/CRYPTO_get_ex_new_index.pod
index 1a4ad52de0..4d5a2b93a0 100644
--- a/doc/man3/CRYPTO_get_ex_new_index.pod
+++ b/doc/man3/CRYPTO_get_ex_new_index.pod
@@ -40,20 +40,22 @@ Several OpenSSL structures can have application-specific data attached to them,
 known as "exdata."
 The specific structures are:
 
-    SSL
-    SSL_CTX
-    SSL_SESSION
-    X509
-    X509_STORE
-    X509_STORE_CTX
+    APP
+    BIO
     DH
+    DRBG
     DSA
     EC_KEY
-    RSA
     ENGINE
+    RSA
+    SSL
+    SSL_CTX
+    SSL_SESSION
     UI
     UI_METHOD
-    BIO
+    X509
+    X509_STORE
+    X509_STORE_CTX
 
 Each is identified by an B<CRYPTO_EX_INDEX_xxx> define in the B<crypto.h>
 header file.  In addition, B<CRYPTO_EX_INDEX_APP> is reserved for
@@ -155,7 +157,7 @@ dup_func() should return 0 for failure and 1 for success.
 
 =head1 COPYRIGHT
 
-Copyright 2015-2017 The OpenSSL Project Authors. All Rights Reserved.
+Copyright 2015-2018 The OpenSSL Project Authors. All Rights Reserved.
 
 Licensed under the OpenSSL license (the "License").  You may not use
 this file except in compliance with the License.  You can obtain a copy
diff --git a/doc/man3/RAND_DRBG_generate.pod b/doc/man3/RAND_DRBG_generate.pod
new file mode 100644
index 0000000000..b39ee93f51
--- /dev/null
+++ b/doc/man3/RAND_DRBG_generate.pod
@@ -0,0 +1,88 @@
+=pod
+
+=head1 NAME
+
+RAND_DRBG_generate,
+RAND_DRBG_bytes
+- generate random bytes using the given drbg instance
+
+=head1 SYNOPSIS
+
+ #include <openssl/rand_drbg.h>
+
+ int RAND_DRBG_generate(RAND_DRBG *drbg,
+                        unsigned char *out, size_t outlen,
+                        int prediction_resistance,
+                        const unsigned char *adin, size_t adinlen);
+
+ int RAND_DRBG_bytes(RAND_DRBG *drbg,
+                     unsigned char *out, size_t outlen);
+
+
+=head1 DESCRIPTION
+
+RAND_DRBG_generate() generates B<outlen> random bytes using the given
+DRBG instance B<drbg> and stores them in the buffer at B<out>.
+
+Before generating the output, the DRBG instance checks whether the maximum
+number of generate requests (I<reseed interval>) or the maximum timespan
+(I<reseed time interval>) since its last seeding have been reached.
+If this is the case, the DRBG reseeds automatically.
+Additionally, an immediate reseeding can be requested by setting the
+B<prediction_resistance> flag to 1. See NOTES section for more details.
+
+The caller can optionally provide additional data to be used for reseeding
+by passing a pointer B<adin> to a buffer of length B<adinlen>.
+This additional data is mixed into the internal state of the random
+generator but does not contribute to the entropy count.
+The additional data can be omitted by setting B<adin> to NULL and
+B<adinlen> to 0;
+
+RAND_DRBG_bytes() generates B<outlen> random bytes using the given
+DRBG instance B<drbg> and stores them in the buffer at B<out>.
+This function is a wrapper around the RAND_DRBG_generate() call,
+which collects some additional data from low entropy sources
+(e.g., a high resolution timer) and calls
+RAND_DRBG_generate(drbg, out, outlen, 0, adin, adinlen).
+
+
+=head1 RETURN VALUES
+
+RAND_DRBG_generate() and RAND_DRBG_bytes() return 1 on success,
+and 0 on failure.
+
+=head1 NOTES
+
+The I<reseed interval> and I<reseed time interval> of the B<drbg> are set to
+reasonable default values, which in general do not have to be adjusted.
+If necessary, they can be changed using L<RAND_DRBG_set_reseed_interval(3)>
+and L<RAND_DRBG_set_reseed_time_interval(3)>, respectively.
+
+A request for prediction resistance can only be satisfied by pulling fresh
+entropy from one of the approved entropy sources listed in section 5.5.2 of
+[NIST SP 800-90C].
+Since the default DRBG implementation does not have access to such an approved
+entropy source, a request for prediction resistance will always fail.
+In other words, prediction resistance is currently not supported yet by the DRBG.
+
+=head1 HISTORY
+
+The RAND_DRBG functions were added in OpenSSL 1.1.1.
+
+=head1 SEE ALSO
+
+L<RAND_bytes(3)>,
+L<RAND_DRBG_set_reseed_interval(3)>,
+L<RAND_DRBG_set_reseed_time_interval(3)>,
+L<RAND_DRBG(7)>
+
+=head1 COPYRIGHT
+
+Copyright 2017-2018 The OpenSSL Project Authors. All Rights Reserved.
+
+Licensed under the OpenSSL license (the "License").  You may not use
+this file except in compliance with the License.  You can obtain a copy
+in the file LICENSE in the source distribution or at
+L<https://www.openssl.org/source/license.html>.
+
+=cut
diff --git a/doc/man3/RAND_DRBG_get0_master.pod b/doc/man3/RAND_DRBG_get0_master.pod
new file mode 100644
index 0000000000..c958bf20ec
--- /dev/null
+++ b/doc/man3/RAND_DRBG_get0_master.pod
@@ -0,0 +1,80 @@
+=pod
+
+=head1 NAME
+
+RAND_DRBG_get0_master,
+RAND_DRBG_get0_public,
+RAND_DRBG_get0_private
+- get access to the global RAND_DRBG instances
+
+=head1 SYNOPSIS
+
+ #include <openssl/rand_drbg.h>
+
+ RAND_DRBG *RAND_DRBG_get0_master(void);
+ RAND_DRBG *RAND_DRBG_get0_public(void);
+ RAND_DRBG *RAND_DRBG_get0_private(void);
+
+
+=head1 DESCRIPTION
+
+The default RAND API implementation (RAND_OpenSSL()) utilizes three
+shared DRBG instances which are accessed via the RAND API:
+
+The <public> and <private> DRBG are thread-local instances, which are used
+by RAND_bytes() and RAND_priv_bytes(), respectively.
+The <master> DRBG is a global instance, which is not intended to be used
+directly, but is used internally to reseed the other two instances.
+
+These functions here provide access to the shared DRBG instances.
+
+=head1 RETURN VALUES
+
+RAND_DRBG_get0_master() returns a pointer to the <master> DRBG instance.
+
+RAND_DRBG_get0_public() returns a pointer to the <public> DRBG instance.
+
+RAND_DRBG_get0_private() returns a pointer to the <private> DRBG instance.
+
+
+=head1 NOTES
+
+It is not thread-safe to access the <master> DRBG instance.
+The <public> and <private> DRBG instance can be accessed safely, because
+they are thread-local. Note however, that changes to these two instances
+apply only to the current thread.
+
+For that reason it is recommended not to change the settings of these
+three instances directly.
+Instead, an application should change the default settings for new DRBG instances
+at initialization time, before creating additional threads.
+
+During initialization, it is possible to change the reseed interval
+and reseed time interval.
+It is also possible to exchange the reseeding callbacks entirely.
+
+
+=head1 HISTORY
+
+The RAND_DRBG functions were added in OpenSSL 1.1.1.
+
+=head1 SEE ALSO
+
+L<RAND_DRBG_set_callbacks(3)>,
+L<RAND_DRBG_set_reseed_defaults(3)>,
+L<RAND_DRBG_set_reseed_interval(3)>,
+L<RAND_DRBG_set_reseed_time_interval(3)>,
+L<RAND_DRBG_set_callbacks(3)>,
+L<RAND_DRBG_generate(3)>,
+L<RAND_DRBG(7)>
+
+=head1 COPYRIGHT
+
+Copyright 2017-2018 The OpenSSL Project Authors. All Rights Reserved.
+
+Licensed under the OpenSSL license (the "License").  You may not use
+this file except in compliance with the License.  You can obtain a copy
+in the file LICENSE in the source distribution or at
+L<https://www.openssl.org/source/license.html>.
+
+=cut
diff --git a/doc/man3/RAND_DRBG_new.pod b/doc/man3/RAND_DRBG_new.pod
new file mode 100644
index 0000000000..dcd7a94419
--- /dev/null
+++ b/doc/man3/RAND_DRBG_new.pod
@@ -0,0 +1,127 @@
+=pod
+
+=head1 NAME
+
+RAND_DRBG_new,
+RAND_DRBG_secure_new,
+RAND_DRBG_set,
+RAND_DRBG_set_defaults,
+RAND_DRBG_instantiate,
+RAND_DRBG_uninstantiate,
+RAND_DRBG_free
+- initialize and cleanup a RAND_DRBG instance
+
+=head1 SYNOPSIS
+
+ #include <openssl/rand_drbg.h>
+
+
+ RAND_DRBG *RAND_DRBG_new(int type,
+                          unsigned int flags,
+                          RAND_DRBG *parent);
+
+ RAND_DRBG *RAND_DRBG_secure_new(int type,
+                                 unsigned int flags,
+                                 RAND_DRBG *parent);
+
+ int RAND_DRBG_set(RAND_DRBG *drbg,
+                   int type, unsigned int flags);
+
+ int RAND_DRBG_set_defaults(int type, unsigned int flags);
+
+ int RAND_DRBG_instantiate(RAND_DRBG *drbg,
+                           const unsigned char *pers, size_t perslen);
+
+ int RAND_DRBG_uninstantiate(RAND_DRBG *drbg);
+
+ void RAND_DRBG_free(RAND_DRBG *drbg);
+
+
+=head1 DESCRIPTION
+
+RAND_DRBG_new() and RAND_DRBG_secure_new()
+create a new DRBG instance of the given B<type>, allocated from the heap resp.
+the secure heap
+(using OPENSSL_zalloc() resp. OPENSSL_secure_zalloc()).
+
+RAND_DRBG_set() initializes the B<drbg> with the given B<type> and B<flags>.
+
+RAND_DRBG_set_defaults() sets the default B<type> and B<flags> for new DRBG
+instances.
+
+Currently, all DRBG types are based on AES-CTR, so B<type> can be one of the
+following values: NID_aes_128_ctr, NID_aes_192_ctr, NID_aes_256_ctr.
+Before the DRBG can be used to generate random bits, it is necessary to set
+its type and to instantiate it.
+
+The optional B<flags> argument specifies a set of bit flags which can be
+joined using the | operator. Currently, the only flag is
+RAND_DRBG_FLAG_CTR_NO_DF, which disables the use of a the derivation function
+ctr_df. For an explanation, see [NIST SP 800-90A Rev. 1].
+
+If a B<parent> instance is specified then this will be used instead of
+the default entropy source for reseeding the B<drbg>. It is said that the
+B<drbg> is I<chained> to its B<parent>.
+For more information, see the NOTES section.
+
+
+RAND_DRBG_instantiate()
+seeds the B<drbg> instance using random input from trusted entropy sources.
+Optionally, a personalization string B<pers> of length B<perslen> can be
+specified.
+To omit the personalization string, set B<pers>=NULL and B<perslen>=0;
+
+RAND_DRBG_uninstantiate()
+clears the internal state of the B<drbg> and puts it back in the
+uninstantiated state.
+
+=head1 RETURN VALUES
+
+
+RAND_DRBG_new() and RAND_DRBG_secure_new() return a pointer to a DRBG
+instance allocated on the heap, resp. secure heap.
+
+RAND_DRBG_set(),
+RAND_DRBG_instantiate(), and
+RAND_DRBG_uninstantiate()
+return 1 on success, and 0 on failure.
+
+RAND_DRBG_free() does not return a value.
+
+=head1 NOTES
+
+The DRBG design supports I<chaining>, which means that a DRBG instance can
+use another B<parent> DRBG instance instead of the default entropy source
+to obtain fresh random input for reseeding, provided that B<parent> DRBG
+instance was properly instantiated, either from a trusted entropy source,
+or from yet another parent DRBG instance.
+For a detailed description of the reseeding process, see L<RAND_DRBG(7)>.
+
+The default DRBG type and flags are applied only during creation of a DRBG
+instance.
+To ensure that they are applied to the global and thread-local DRBG instances
+(<master>, resp. <public> and <private>), it is necessary to call
+RAND_DRBG_set_defaults() before creating any thread and before calling any
+cryptographic routines that obtain random data directly or indirectly.
+
+=head1 HISTORY
+
+The RAND_DRBG functions were added in OpenSSL 1.1.1.
+
+=head1 SEE ALSO
+
+L<OPENSSL_zalloc(3)>,
+L<OPENSSL_secure_zalloc(3)>,
+L<RAND_DRBG_generate(3)>,
+L<RAND_DRBG(7)>
+
+=head1 COPYRIGHT
+
+Copyright 2017-2018 The OpenSSL Project Authors. All Rights Reserved.
+
+Licensed under the OpenSSL license (the "License").  You may not use
+this file except in compliance with the License.  You can obtain a copy
+in the file LICENSE in the source distribution or at
+L<https://www.openssl.org/source/license.html>.
+
+=cut
diff --git a/doc/man3/RAND_DRBG_reseed.pod b/doc/man3/RAND_DRBG_reseed.pod
new file mode 100644
index 0000000000..da3a40be44
--- /dev/null
+++ b/doc/man3/RAND_DRBG_reseed.pod
@@ -0,0 +1,111 @@
+=pod
+
+=head1 NAME
+
+RAND_DRBG_reseed,
+RAND_DRBG_set_reseed_interval,
+RAND_DRBG_set_reseed_time_interval,
+RAND_DRBG_set_reseed_defaults
+- reseed a RAND_DRBG instance
+
+=head1 SYNOPSIS
+
+ #include <openssl/rand_drbg.h>
+
+ int RAND_DRBG_reseed(RAND_DRBG *drbg,
+                      const unsigned char *adin, size_t adinlen);
+
+ int RAND_DRBG_set_reseed_interval(RAND_DRBG *drbg,
+                                   unsigned int interval);
+
+ int RAND_DRBG_set_reseed_time_interval(RAND_DRBG *drbg,
+                                        time_t interval);
+
+ int RAND_DRBG_set_reseed_defaults(
+                                   unsigned int master_reseed_interval,
+                                   unsigned int slave_reseed_interval,
+                                   time_t master_reseed_time_interval,
+                                   time_t slave_reseed_time_interval
+                                   );
+
+
+=head1 DESCRIPTION
+
+RAND_DRBG_reseed()
+reseeds the given B<drbg>, obtaining entropy input from its entropy source
+and mixing in the specified additional data provided in the buffer B<adin>
+of length B<adinlen>.
+The additional data can be omitted by setting B<adin> to NULL and B<adinlen>
+to 0.
+
+RAND_DRBG_set_reseed_interval()
+sets the reseed interval of the B<drbg>, which is the maximum allowed number
+of generate requests between consecutive reseedings.
+If B<interval> > 0, then the B<drbg> will reseed automatically whenever the
+number of generate requests since its last seeding exceeds the given reseed
+interval.
+If B<interval> == 0, then this feature is disabled.
+
+
+RAND_DRBG_set_reseed_time_interval()
+sets the reseed time interval of the B<drbg>, which is the maximum allowed
+number of seconds between consecutive reseedings.
+If B<interval> > 0, then the B<drbg> will reseed automatically whenever the
+elapsed time since its last reseeding exceeds the given reseed time interval.
+If B<interval> == 0, then this feature is disabled.
+
+RAND_DRBG_set_reseed_defaults() sets the default values for the reseed interval
+(B<master_reseed_interval> and B<slave_reseed_interval>)
+and the reseed time interval
+(B<master_reseed_time_interval> and B<slave_reseed_tme_interval>)
+of DRBG instances.
+The default values are set independently for master DRBG instances (which don't
+have a parent) and slave DRBG instances (which are chained to a parent DRBG).
+
+=head1 RETURN VALUES
+
+RAND_DRBG_reseed(),
+RAND_DRBG_set_reseed_interval(), and
+RAND_DRBG_set_reseed_time_interval(),
+return 1 on success, 0 on failure.
+
+
+=head1 NOTES
+
+The default OpenSSL random generator is already set up for automatic reseeding,
+so in general it is not necessary to reseed it explicitly, or to modify
+its reseeding thresholds.
+
+Normally, the entropy input for seeding a DRBG is either obtained from a
+trusted os entropy source or from a parent DRBG instance, which was seeded
+(directly or indirectly) from a trusted os entropy source.
+In exceptional cases it is possible to replace the reseeding mechanism entirely
+by providing application defined callbacks using RAND_DRBG_set_callbacks().