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.  Although (re-)seeding is automatic, it can fail because no trusted random source
       is available or the trusted source(s) temporarily fail to provide sufficient random seed
       material.  In this case the CSPRNG enters an error state and ceases to provide output,
       until it is able to recover from the error by reseeding itself.  For more details on
       reseeding and error recovery, see EVP_RAND(7).

       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, the default RAND internals can be replaced by your own EVP_RAND(3) objects.

       Changing the default random generator 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.

DEFAULT SETUP

       The default OpenSSL RAND method is based on the EVP_RAND deterministic random bit
       generator (DRBG) classes.  A DRBG is a certain type of cryptographically-secure pseudo-
       random number generator (CSPRNG), which is described in [NIST SP 800-90A Rev. 1].

SEE ALSO

       RAND_bytes(3), RAND_priv_bytes(3), EVP_RAND(3), RAND_get0_primary(3), EVP_RAND(7)

COPYRIGHT

       Copyright 2018-2021 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 <https://www.openssl.org/source/license.html>.