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
(3) and do not take randomness for granted.
For values that should remain secret, you can use RAND_priv_bytes
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
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
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.
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