=pod

=head1 NAME

RAND_add, RAND_poll, RAND_poll_ex, RAND_poll_cb,
RAND_seed, RAND_status, RAND_event, RAND_screen
- add randomness to the PRNG or get its status

=head1 SYNOPSIS

 #include <openssl/rand.h>

 int RAND_status(void);

 typedef void (*RAND_poll_cb)(void *arg,
                              const void *buf, int num, double randomness);
 int RAND_poll_ex(RAND_poll_cb cb, void *arg);
 int RAND_poll();

 void RAND_add(const void *buf, int num, double randomness);
 void RAND_seed(const void *buf, int num);

Deprecated:

 #if OPENSSL_API_COMPAT < 0x10100000L
 int RAND_event(UINT iMsg, WPARAM wParam, LPARAM lParam);
 void RAND_screen(void);
 #endif

=head1 DESCRIPTION

Random numbers are a vital part of cryptography, including key generation,
creating salts, etc., and software-based
generators must be "seeded" with external randomness before they can be
used as a cryptographically-secure pseudo-random number generator (CSPRNG).
The availability of common hardware with special instructions and
modern operating systems, which may use items such as interrupt jitter
and network packet timings, can be reasonable sources of seeding material.

RAND_status() indicates whether or not the CSPRNG has been sufficiently
seeded. If not, functions such as RAND_bytes(3) will fail.

RAND_poll_ex() uses the system's capabilities to obtain a buffer
containing random bits which can then be used to seed a CSPRNG. The
exact features used depends on how OpenSSL was configured, and a summary
can be displayed with the OpenSSL L<version(1)> command.  This function
is normally called as needed by the CSPRNG.  The B<arg> parameter is an
arbitrary pointer which will be passed as an argument to the callback.
The B<cb> function is called each time there is data to add.

RAND_poll() invokes RAND_poll_ex() with B<cb> and B<arg> set so that it
will call RAND_add(), to add the randomness to the global CSPRNG.

RAND_add() mixes the B<num> bytes at B<buf> into the PRNG state.
The B<randomness> argument is an estimate of how much randomness is
contained in
B<buf>, in bytes, and should be a number between zero and B<num>.
Details about sources of randomness and how to estimate their randomness
can be found in the literature; for example NIST SP 800-90B.
The content of B<buf> cannot be recovered from subsequent CSPRNG output.
This function will not normally be needed, as RAND_poll() should have been
configured to do the appropriate seeding for the local platform.
Applications that need to keep random state in an external file should
use L<RAND_load_file(3)>.

RAND_seed() is equivalent to RAND_add() with B<randomness> set to B<num>.

RAND_event() and RAND_screen() are equivalent to RAND_poll().

=head1 RETURN VALUES

RAND_status() returns 1 if the CSPRNG has been seeded
with enough data, 0 otherwise.

RAND_poll() returns 1 if it generated seed data, 0 otherwise.

RAND_event() returns RAND_status().

The other functions do not return values.

=head1 HISTORY

RAND_event() and RAND_screen() were deprecated in OpenSSL 1.1.0 and should
not be used.

=head1 SEE ALSO

L<RAND_bytes(3)>, L<RAND_egd(3)>,
L<RAND_load_file(3)>

=head1 COPYRIGHT

Copyright 2000-2017 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>.

=cut