RAND(7) - man - phpMan

RAND(7SSL)                                     OpenSSL                                    RAND(7SSL)



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>.



3.0.2                                        2026-04-07                                   RAND(7SSL)