=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>.

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