=pod

=head1 NAME

RAND_bytes, RAND_priv_bytes, RAND_bytes_ex, RAND_priv_bytes_ex,
RAND_pseudo_bytes, RAND_set1_random_provider - generate random data

=head1 SYNOPSIS

 #include <openssl/rand.h>

 int RAND_bytes(unsigned char *buf, int num);
 int RAND_priv_bytes(unsigned char *buf, int num);

 int RAND_bytes_ex(OSSL_LIB_CTX *ctx, unsigned char *buf, size_t num,
                   unsigned int strength);
 int RAND_priv_bytes_ex(OSSL_LIB_CTX *ctx, unsigned char *buf, size_t num,
                        unsigned int strength);

 int RAND_set1_random_provider(OSSL_LIB_CTX *ctx, OSSL_PROVIDER *p);

The following function has been deprecated since OpenSSL 1.1.0, and can be
hidden entirely by defining B<OPENSSL_API_COMPAT> with a suitable version value,
see L<openssl_user_macros(7)>:

 int RAND_pseudo_bytes(unsigned char *buf, int num);

=head1 DESCRIPTION

RAND_bytes() generates B<num> random bytes using a cryptographically
secure pseudo random generator (CSPRNG) and stores them in B<buf>. B<buf> B<MUST NOT> be NULL.

RAND_priv_bytes() has the same semantics as RAND_bytes().  It is intended to
be used for generating values that should remain private. If using the
default RAND_METHOD, this function uses a separate "private" PRNG
instance so that a compromise of the "public" PRNG instance will not
affect the secrecy of these private values, as described in L<RAND(7)>
and L<EVP_RAND(7)>.

RAND_bytes_ex() and RAND_priv_bytes_ex() are the same as RAND_bytes() and
RAND_priv_bytes() except that they both take additional I<strength> and
I<ctx> parameters. The bytes generated will have a security strength of at
least I<strength> bits.
The DRBG used for the operation is the public or private DRBG associated with
the specified I<ctx>. The parameter can be NULL, in which case
the default library context is used (see L<OSSL_LIB_CTX(3)>.
If the default RAND_METHOD has been changed then for compatibility reasons the
RAND_METHOD will be used in preference and the DRBG of the library context
ignored.

RAND_set1_random_provider() specifies a provider, I<prov>, which will be used
by the library context I<ctx> for all of the generate calls above instead
of the built-in in DRBGs and entropy source.  Pass NULL for the provider
to disable the random provider functionality.  In this case, the built-in DRBGs
and entropy source will be used.  This call should not be considered thread safe.

=head1 NOTES

By default, the OpenSSL CSPRNG supports a security level of 256 bits, provided it
was able to seed itself from a trusted entropy source.
On all major platforms supported by OpenSSL (including the Unix-like platforms
and Windows), OpenSSL is configured to automatically seed the CSPRNG on first use
using the operating systems's random generator.

If the entropy source fails or is not available, the CSPRNG will enter an
error state and refuse to generate random bytes. For that reason, it is important
to always check the error return value of RAND_bytes() and RAND_priv_bytes() and
not take randomness for granted.

On other platforms, there might not be a trusted entropy source available
or OpenSSL might have been explicitly configured to use different entropy sources.
If you are in doubt about the quality of the entropy source, don't hesitate to ask
your operating system vendor or post a question on GitHub or the openssl-users
mailing list.

=head1 RETURN VALUES

RAND_bytes() and RAND_priv_bytes()
return 1 on success, -1 if not supported by the current
RAND method, or 0 on other failure. The error code can be
obtained by L<ERR_get_error(3)>.

RAND_set1_random_provider() returns 1 on success and 0 on failure.

=head1 SEE ALSO

L<RAND_add(3)>,
L<RAND_bytes(3)>,
L<RAND_priv_bytes(3)>,
L<ERR_get_error(3)>,
L<RAND(7)>,
L<EVP_RAND(7)>

=head1 HISTORY

=over 2

=item *

RAND_pseudo_bytes() was deprecated in OpenSSL 1.1.0; use RAND_bytes() instead.

=item *

The RAND_priv_bytes() function was added in OpenSSL 1.1.1.

=item *

The RAND_bytes_ex() and RAND_priv_bytes_ex() functions were added in OpenSSL 3.0

=item *

The RAND_set1_random_provider() function was added in OpenSSL 3.5

=back

=head1 COPYRIGHT

Copyright 2000-2025 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
