=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 prediction_resistance);

 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 primary_reseed_interval,
                                   unsigned int secondary_reseed_interval,
                                   time_t primary_reseed_time_interval,
                                   time_t secondary_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.
An immediate reseeding can be requested by setting the
B<prediction_resistance> flag to 1.
Requesting prediction resistance is a relative expensive operation.
See NOTES section for more details.

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<primary_reseed_interval> and B<secondary_reseed_interval>)
and the reseed time interval
(B<primary_reseed_time_interval> and B<secondary_reseed_tme_interval>)
of DRBG instances.
The default values are set independently for primary DRBG instances (which don't
have a parent) and secondary 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().

The reseeding default values are applied only during creation of a DRBG instance.
To ensure that they are applied to the global and thread-local DRBG instances
(<primary>, resp. <public> and <private>), it is necessary to call
RAND_DRBG_set_reseed_defaults() before creating any thread and before calling
any cryptographic routines that obtain random data directly or indirectly.

=head1 SEE ALSO

L<RAND_DRBG_generate(3)>,
L<RAND_DRBG_bytes(3)>,
L<RAND_DRBG_set_callbacks(3)>.
L<RAND_DRBG(7)>

=head1 HISTORY

The RAND_DRBG functions were added in OpenSSL 1.1.1.

Prediction resistance is supported from OpenSSL 3.0.

=head1 COPYRIGHT

Copyright 2017-2019 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>.

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