RAND(7ossl) OpenSSL RAND(7ossl)
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(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 RAND(7) 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(7), RAND_get0_primary(3),
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>.
3.3.2 2024-09-04 RAND(7ossl)
openssl 3.3.2 - Generated Tue Oct 1 15:26:38 CDT 2024