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<RAND_DRBG(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 CSPRNG. 41The intention behind using a dedicated CSPRNG exclusively for private 42values is that none of its output should be visible to an attacker (e.g., 43used as salt value), in order to reveal as little information as 44possible about its internal state, and that a compromise of the "public" 45CSPRNG instance will not affect the secrecy of these private values. 46 47In the rare case where the default implementation does not satisfy your special 48requirements, there are two options: 49 50=over 2 51 52=item * 53 54Replace the default RAND method by your own RAND method using 55L<RAND_set_rand_method(3)>. 56 57=item * 58 59Modify the default settings of the OpenSSL RAND method by modifying the security 60parameters of the underlying DRBG, which is described in detail in L<RAND_DRBG(7)>. 61 62=back 63 64Changing the default random generator or its default parameters should be necessary 65only in exceptional cases and is not recommended, unless you have a profound knowledge 66of cryptographic principles and understand the implications of your changes. 67 68=head1 SEE ALSO 69 70L<RAND_add(3)>, 71L<RAND_bytes(3)>, 72L<RAND_priv_bytes(3)>, 73L<RAND_get_rand_method(3)>, 74L<RAND_set_rand_method(3)>, 75L<RAND_OpenSSL(3)>, 76L<RAND_DRBG(7)> 77 78=head1 COPYRIGHT 79 80Copyright 2018-2019 The OpenSSL Project Authors. All Rights Reserved. 81 82Licensed under the OpenSSL license (the "License"). You may not use 83this file except in compliance with the License. You can obtain a copy 84in the file LICENSE in the source distribution or at 85L<https://www.openssl.org/source/license.html>. 86 87=cut 88