xref: /openssl/doc/man7/RAND.pod (revision 0c75a738)
1=pod
2
3=head1 NAME
4
5RAND
6- the OpenSSL random generator
7
8=head1 DESCRIPTION
9
10Random numbers are a vital part of cryptography, they are needed to provide
11unpredictability for tasks like key generation, creating salts, and many more.
12Software-based generators must be seeded with external randomness before they
13can be used as a cryptographically-secure pseudo-random number generator
14(CSPRNG).
15The availability of common hardware with special instructions and
16modern operating systems, which may use items such as interrupt jitter
17and network packet timings, can be reasonable sources of seeding material.
18
19OpenSSL comes with a default implementation of the RAND API which is based on
20the deterministic random bit generator (DRBG) model as described in
21[NIST SP 800-90A Rev. 1]. The default random generator will initialize
22automatically on first use and will be fully functional without having
23to be initialized ('seeded') explicitly.
24It seeds and reseeds itself automatically using trusted random sources
25provided by the operating system.
26
27As a normal application developer, you do not have to worry about any details,
28just use L<RAND_bytes(3)> to obtain random data.
29Having said that, there is one important rule to obey: Always check the error
30return value of L<RAND_bytes(3)> and do not take randomness for granted.
31Although (re-)seeding is automatic, it can fail because no trusted random source
32is available or the trusted source(s) temporarily fail to provide sufficient
33random seed material.
34In this case the CSPRNG enters an error state and ceases to provide output,
35until it is able to recover from the error by reseeding itself.
36For more details on reseeding and error recovery, see L<EVP_RAND(7)>.
37
38For values that should remain secret, you can use L<RAND_priv_bytes(3)>
39instead.
40This method does not provide 'better' randomness, it uses the same type of
41CSPRNG.
42The intention behind using a dedicated CSPRNG exclusively for private
43values is that none of its output should be visible to an attacker (e.g.,
44used as salt value), in order to reveal as little information as
45possible about its internal state, and that a compromise of the "public"
46CSPRNG instance will not affect the secrecy of these private values.
47
48In the rare case where the default implementation does not satisfy your special
49requirements, the default RAND internals can be replaced by your own
50L<EVP_RAND(3)> objects.
51
52Changing the default random generator should be necessary
53only in exceptional cases and is not recommended, unless you have a profound
54knowledge of cryptographic principles and understand the implications of your
55changes.
56
57=head1 DEFAULT SETUP
58
59The default OpenSSL RAND method is based on the EVP_RAND deterministic random
60bit generator (DRBG) classes.
61A DRBG is a certain type of cryptographically-secure pseudo-random
62number generator (CSPRNG), which is described in [NIST SP 800-90A Rev. 1].
63
64
65=head1 SEE ALSO
66
67L<RAND_bytes(3)>,
68L<RAND_priv_bytes(3)>,
69L<EVP_RAND(3)>,
70L<RAND_get0_primary(3)>,
71L<EVP_RAND(7)>
72
73=head1 COPYRIGHT
74
75Copyright 2018-2021 The OpenSSL Project Authors. All Rights Reserved.
76
77Licensed under the Apache License 2.0 (the "License").  You may not use
78this file except in compliance with the License.  You can obtain a copy
79in the file LICENSE in the source distribution or at
80L<https://www.openssl.org/source/license.html>.
81
82=cut
83