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.
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
· 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.
RAND_add(3), RAND_bytes(3), RAND_priv_bytes(3),
RAND_get_rand_method(3), RAND_set_rand_method(3), RAND_OpenSSL(3),
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
1.1.1c 2019-05-28 RAND(7)