RAND

RAND(7)                              OpenSSL                             RAND(7)



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 RAND_DRBG(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, 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 2018-2019 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>.



1.1.1j                             2021-02-16                            RAND(7)