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. 31 32For values that should remain secret, you can use L<RAND_priv_bytes(3)> 33instead. 34This method does not provide 'better' randomness, it uses the same type of CSPRNG. 35The intention behind using a dedicated CSPRNG exclusively for private 36values is that none of its output should be visible to an attacker (e.g., 37used as salt value), in order to reveal as little information as 38possible about its internal state, and that a compromise of the "public" 39CSPRNG instance will not affect the secrecy of these private values. 40 41In the rare case where the default implementation does not satisfy your special 42requirements, there are two options: 43 44=over 2 45 46=item * 47 48Replace the default RAND method by your own RAND method using 49L<RAND_set_rand_method(3)>. 50 51=item * 52 53Modify the default settings of the OpenSSL RAND method by modifying the security 54parameters of the underlying DRBG, which is described in detail in L<RAND_DRBG(7)>. 55 56=back 57 58Changing the default random generator or its default parameters should be necessary 59only in exceptional cases and is not recommended, unless you have a profound knowledge 60of cryptographic principles and understand the implications of your changes. 61 62=head1 SEE ALSO 63 64L<RAND_add(3)>, 65L<RAND_bytes(3)>, 66L<RAND_priv_bytes(3)>, 67L<RAND_get_rand_method(3)>, 68L<RAND_set_rand_method(3)>, 69L<RAND_OpenSSL(3)>, 70L<RAND_DRBG(7)> 71 72=head1 COPYRIGHT 73 74Copyright 2018 The OpenSSL Project Authors. All Rights Reserved. 75 76Licensed under the OpenSSL license (the "License"). You may not use 77this file except in compliance with the License. You can obtain a copy 78in the file LICENSE in the source distribution or at 79L<https://www.openssl.org/source/license.html>. 80 81=cut 82