RAND_DRBG(7)                         OpenSSL                        RAND_DRBG(7)

       RAND_DRBG - the deterministic random bit generator

        #include <openssl/rand_drbg.h>

       The default OpenSSL RAND method is based on the RAND_DRBG class, which
       implements a deterministic random bit generator (DRBG).  A DRBG is a
       certain type of cryptographically-secure pseudo-random number generator
       (CSPRNG), which is described in [NIST SP 800-90A Rev. 1].

       While the RAND API is the 'frontend' which is intended to be used by
       application developers for obtaining random bytes, the RAND_DRBG API
       serves as the 'backend', connecting the former with the operating
       systems's entropy sources and providing access to the DRBG's
       configuration parameters.

       Unless you have very specific requirements for your random generator, it
       is in general not necessary to utilize the RAND_DRBG API directly.  The
       usual way to obtain random bytes is to use RAND_bytes(3) or
       RAND_priv_bytes(3), see also RAND(7).

   Typical Use Cases
       Typical examples for such special use cases are the following:

       • You want to use your own private DRBG instances.  Multiple DRBG
         instances which are accessed only by a single thread provide additional
         security (because their internal states are independent) and better
         scalability in multithreaded applications (because they don't need to
         be locked).

       • You need to integrate a previously unsupported entropy source.

       • You need to change the default settings of the standard OpenSSL RAND
         implementation to meet specific requirements.

       A DRBG instance can be used as the entropy source of another DRBG
       instance, provided it has itself access to a valid entropy source.  The
       DRBG instance which acts as entropy source is called the parent DRBG, the
       other instance the child DRBG.

       This is called chaining. A chained DRBG instance is created by passing a
       pointer to the parent DRBG as argument to the RAND_DRBG_new() call.  It
       is possible to create chains of more than two DRBG in a row.

       Currently, there are three shared DRBG instances, the <master>, <public>,
       and <private> DRBG.  While the <master> DRBG is a single global instance,
       the <public> and <private> DRBG are created per thread and accessed
       through thread-local storage.

       By default, the functions RAND_bytes(3) and RAND_priv_bytes(3) use the
       thread-local <public> and <private> DRBG instance, respectively.

   The <master> DRBG instance
       The <master> DRBG is not used directly by the application, only for
       reseeding the two other two DRBG instances. It reseeds itself by
       obtaining randomness either from os entropy sources or by consuming
       randomness which was added previously by RAND_add(3).

   The <public> DRBG instance
       This instance is used per default by RAND_bytes(3).

   The <private> DRBG instance
       This instance is used per default by RAND_priv_bytes(3)

       The <master> DRBG is intended to be accessed concurrently for reseeding
       by its child DRBG instances. The necessary locking is done internally.
       It is not thread-safe to access the <master> DRBG directly via the
       RAND_DRBG interface.  The <public> and <private> DRBG are thread-local,
       i.e. there is an instance of each per thread. So they can safely be
       accessed without locking via the RAND_DRBG interface.

       Pointers to these DRBG instances can be obtained using
       RAND_DRBG_get0_master(), RAND_DRBG_get0_public(), and
       RAND_DRBG_get0_private(), respectively.  Note that it is not allowed to
       store a pointer to one of the thread-local DRBG instances in a variable
       or other memory location where it will be accessed and used by multiple

       All other DRBG instances created by an application don't support locking,
       because they are intended 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 <master> DRBG as entropy source for multiple DRBG instances on
       different threads is thread-safe, because the DRBG instance will lock the
       <master> DRBG automatically for obtaining random input.

       The following picture gives an overview over how the DRBG instances work
       together and are being used.

                      | os entropy sources |
                               v           +-----------------------------+
             RAND_add() ==> <master>     <-| shared DRBG (with locking)  |
                             /   \         +-----------------------------+
                            /     \              +---------------------------+
                     <public>     <private>   <- | per-thread DRBG instances |
                        |             |          +---------------------------+
                        v             v
                      RAND_bytes()   RAND_priv_bytes()
                           |               ^
                           |               |
           +------------------+      +------------------------------------+
           | general purpose  |      | used for secrets like session keys |
           | random generator |      | and private keys for certificates  |
           +------------------+      +------------------------------------+

       The usual way to obtain random bytes is to call RAND_bytes(...) or
       RAND_priv_bytes(...). These calls are roughly equivalent to calling
       RAND_DRBG_bytes(<public>, ...) and RAND_DRBG_bytes(<private>, ...),
       respectively. The method RAND_DRBG_bytes(3) is a convenience method
       wrapping the RAND_DRBG_generate(3) function, which serves the actual
       request for random data.

       A DRBG instance seeds itself automatically, pulling random input from its
       entropy source. The entropy source can be either a trusted operating
       system entropy source, or another DRBG with access to such a source.

       Automatic reseeding occurs after a predefined number of generate
       requests.  The selection of the trusted entropy sources is configured at
       build time using the --with-rand-seed option. The following sections
       explain the reseeding process in more detail.

   Automatic Reseeding
       Before satisfying a generate request (RAND_DRBG_generate(3)), the DRBG
       reseeds itself automatically, if one of the following conditions holds:

       - the DRBG was not instantiated (=seeded) yet or has been uninstantiated.

       - 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 can be disabled by
       setting the reseed_time_interval to 0.

       - the DRBG is in an error state.

       Note: An error state is entered if the entropy source fails while the
       DRBG is seeding or reseeding.  The last case ensures that the DRBG
       automatically recovers from the error as soon as the entropy source is
       available again.

   Manual Reseeding
       In addition to automatic reseeding, the caller can request an immediate
       reseeding of the DRBG with fresh entropy by setting the prediction
       resistance parameter to 1 when calling RAND_DRBG_generate(3).

       The document [NIST SP 800-90C] describes prediction resistance requests
       in detail and imposes strict conditions on the entropy sources that are
       approved for providing prediction resistance.  Since the default DRBG
       implementation does not have access to such an approved entropy source, a
       request for prediction resistance will currently always fail.  In other
       words, prediction resistance is currently not supported yet by the DRBG.

       For the three shared DRBGs (and only for these) there is another way to
       reseed them manually: If RAND_add(3) is called with a positive randomness
       argument (or RAND_seed(3)), then 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>.

       The last feature has been added to support the common practice used with
       previous OpenSSL versions to call RAND_add() before calling RAND_bytes().

   Entropy Input vs. Additional Data
       The DRBG distinguishes two different types of random input: entropy,
       which comes from a trusted source, and additional input', which can
       optionally be added by the user and is considered untrusted.  It is
       possible to add additional input not only during reseeding, but also for
       every generate request.  This is in fact done automatically by

   Configuring the Random Seed Source
       In most cases OpenSSL will automatically choose a suitable seed source
       for automatically seeding and reseeding its <master> DRBG. In some cases
       however, it will be necessary to explicitly specify a seed source during
       configuration, using the --with-rand-seed option. For more information,
       see the INSTALL instructions. There are also operating systems where no
       seed source is available and automatic reseeding is disabled by default.

       The following two sections describe the reseeding process of the master
       DRBG, depending on whether automatic reseeding is available or not.

   Reseeding the master DRBG with automatic seeding enabled
       Calling RAND_poll() or RAND_add() is not necessary, because the DRBG
       pulls the necessary entropy from its source automatically.  However, both
       calls are permitted, and do reseed the RNG.

       RAND_add() can be used to add both kinds of random input, depending on
       the value of the randomness argument:

       randomness == 0:
           The random bytes are mixed as additional input into the current state
           of the DRBG.  Mixing in additional input is not considered a full
           reseeding, hence the reseed counter is not reset.

       randomness > 0:
           The random bytes are used as entropy input for a full reseeding
           (resp. reinstantiation) if the DRBG is instantiated (resp.
           uninstantiated or in an error state).  The number of random bits
           required for reseeding is determined by the security strength of the
           DRBG. Currently it defaults to 256 bits (32 bytes).  It is possible
           to provide less randomness than required.  In this case the missing
           randomness will be obtained by pulling random input from the trusted
           entropy sources.

   Reseeding the master DRBG with automatic seeding disabled
       Calling RAND_poll() will always fail.

       RAND_add() needs to be called for initial seeding and periodic reseeding.
       At least 48 bytes (384 bits) of randomness have to be provided, otherwise
       the (re-)seeding of the DRBG will fail. This corresponds to one and a
       half times the security strength of the DRBG. The extra half is used for
       the nonce during instantiation.

       More precisely, the number of bytes needed for seeding depend on the
       security strength of the DRBG, which is set to 256 by default.

       RAND_DRBG_bytes(3), RAND_DRBG_generate(3), RAND_DRBG_reseed(3),
       RAND_DRBG_get0_master(3), RAND_DRBG_get0_public(3),
       RAND_DRBG_get0_private(3), RAND_DRBG_set_reseed_interval(3),
       RAND_DRBG_set_reseed_time_interval(3), RAND_DRBG_set_reseed_defaults(3),

       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

1.1.1j                             2021-02-16                       RAND_DRBG(7)