Updated: 2022/Sep/29

Please read Privacy Policy. It's for your privacy.

RAND(7)                             OpenSSL                            RAND(7)

       RAND - the OpenSSL random generator

       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.

       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].

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

       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

3.0.12                            2023-05-07                           RAND(7)