NAME

       RAND_DRBG_generate, RAND_DRBG_bytes - generate random bytes using the given drbg instance

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);

DESCRIPTION

       RAND_DRBG_generate() generates outlen random bytes using the given DRBG instance drbg and
       stores them in the buffer at out.

       Before generating the output, the DRBG instance checks whether the maximum number of
       generate requests (reseed interval) or the maximum timespan (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 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 adin to a buffer of length 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 adin to NULL and adinlen to 0;

       RAND_DRBG_bytes() generates outlen random bytes using the given DRBG instance drbg and
       stores them in the buffer at 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).

RETURN VALUES

       RAND_DRBG_generate() and RAND_DRBG_bytes() return 1 on success, and 0 on failure.

NOTES

       The reseed interval and reseed time interval of the drbg are set to reasonable default
       values, which in general do not have to be adjusted.  If necessary, they can be changed
       using RAND_DRBG_set_reseed_interval(3) and 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.

HISTORY

       The RAND_DRBG functions were added in OpenSSL 1.1.1.

SEE ALSO

       RAND_bytes(3), RAND_DRBG_set_reseed_interval(3), RAND_DRBG_set_reseed_time_interval(3),
       RAND_DRBG(7)

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 <https://www.openssl.org/source/license.html>.