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RAND_DRBG(7)                        OpenSSL                       RAND_DRBG(7)

       RAND_DRBG - the deterministic random bit generator

       libcrypto, -lcrypto

        #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

       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

       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

       - 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

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

   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_defaults(3), RAND(7),

       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

1.1.1i                            2018-12-08                      RAND_DRBG(7)