NAME

       RAND - the OpenSSL random generator

DESCRIPTION

       Random numbers are a vital part of cryptography, they are needed to provide
       unpredictability for tasks like key generation, creating salts, and many more.  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.

       OpenSSL comes with a default implementation of the RAND API which is based on the
       deterministic random bit generator (DRBG) model as described in [NIST SP 800-90A Rev. 1].
       The default random generator will initialize automatically on first use and will be fully
       functional without having to be initialized ('seeded') explicitly.  It seeds and reseeds
       itself automatically using trusted random sources provided by the operating system.

       As a normal application developer, you do not have to worry about any details, just use
       RAND_bytes(3) to obtain random data.  Having said that, there is one important rule to
       obey: Always check the error return value of RAND_bytes(3) and do not take randomness for
       granted.

       For values that should remain secret, you can use RAND_priv_bytes(3) instead.  This method
       does not provide 'better' randomness, it uses the same type of CSPRNG.  The intention
       behind using a dedicated CSPRNG exclusively for private values is that none of its output
       should be visible to an attacker (e.g., used as salt value), in order to reveal as little
       information as possible about its internal state, and that a compromise of the "public"
       CSPRNG instance will not affect the secrecy of these private values.

       In the rare case where the default implementation does not satisfy your special
       requirements, there are two options:

       • Replace the default RAND method by your own RAND method using RAND_set_rand_method(3).

       • Modify the default settings of the OpenSSL RAND method by modifying the security
         parameters of the underlying DRBG, which is described in detail in RAND_DRBG(7).

       Changing the default random generator or its default parameters should be necessary only
       in exceptional cases and is not recommended, unless you have a profound knowledge of
       cryptographic principles and understand the implications of your changes.

SEE ALSO

       RAND_add(3), RAND_bytes(3), RAND_priv_bytes(3), RAND_get_rand_method(3),
       RAND_set_rand_method(3), RAND_OpenSSL(3), RAND_DRBG(7)

COPYRIGHT

       Copyright 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>.