=pod

=head1 NAME

RAND_DRBG - the deterministic random bit generator

=head1 SYNOPSIS

 #include <openssl/rand_drbg.h>

=head1 DESCRIPTION

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.

=head2 Disclaimer

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 L<RAND_bytes(3)> or
L<RAND_priv_bytes(3)>, see also L<RAND(7)>.

=head2 Typical Use Cases

Typical examples for such special use cases are the following:

=over 2

=item *

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).

=item *

You need to integrate a previously unsupported entropy source.

=item *

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

=back


=head1 CHAINING

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 I<parent> DRBG,
the other instance the I<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.

=head1 THE THREE SHARED DRBG INSTANCES

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

=head2 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 L<RAND_add(3)>.

=head2 The <public> DRBG instance

This instance is used per default by L<RAND_bytes(3)>.

=head2 The <private> DRBG instance

This instance is used per default by L<RAND_priv_bytes(3)>


=head1 LOCKING

The <master> DRBG is intended to be accessed concurrently for reseeding
by its child DRBG instances. The necessary locking is done internally.
It is I<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 threads.

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.

=head1 THE OVERALL PICTURE

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 L<RAND_DRBG_bytes(3)> is a convenience method
wrapping the L<RAND_DRBG_generate(3)> function, which serves the actual
request for random data.

=head1 RESEEDING

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.

=head2 Automatic Reseeding

Before satisfying a generate request (L<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 I<reseed_interval>.
This behaviour can be disabled by setting the I<reseed_interval> to 0.

- the time elapsed since the last reseeding exceeds a certain time
interval, the so called I<reseed_time_interval>.
This can be disabled by setting the I<reseed_time_interval> to 0.

- the DRBG is in an error state.

B<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.

=head2 Manual Reseeding

In addition to automatic reseeding, the caller can request an immediate
reseeding of the DRBG with fresh entropy by setting the
I<prediction resistance> parameter to 1 when calling L<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.
A request for prediction resistance can only be satisfied by pulling fresh
entropy from a live entropy source (section 5.5.2 of [NIST SP 800-90C]).
It is up to the user to ensure that a live entropy source is configured
and is being used.


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


=head2 Entropy Input vs. Additional Data

The DRBG distinguishes two different types of random input: I<entropy>,
which comes from a trusted source, and I<additional input>',
which can optionally be added by the user and is considered untrusted.
It is possible to add I<additional input> not only during reseeding,
but also for every generate request.
This is in fact done automatically by L<RAND_DRBG_bytes(3)>.


=head2 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.


=head2 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 B<randomness> argument:

=over 4

=item 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.


=item 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.

=back

=head2 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
I<security strength> of the DRBG, which is set to 256 by default.

=head1 SEE ALSO

L<RAND_DRBG_bytes(3)>,
L<RAND_DRBG_generate(3)>,
L<RAND_DRBG_reseed(3)>,
L<RAND_DRBG_get0_master(3)>,
L<RAND_DRBG_get0_public(3)>,
L<RAND_DRBG_get0_private(3)>,
L<RAND_DRBG_set_reseed_interval(3)>,
L<RAND_DRBG_set_reseed_time_interval(3)>,
L<RAND_DRBG_set_reseed_defaults(3)>,
L<RAND(7)>,

=head1 COPYRIGHT

Copyright 2017-2018 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (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
