mirror of https://github.com/openssl/openssl.git
92 lines
3.7 KiB
Plaintext
92 lines
3.7 KiB
Plaintext
=pod
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=head1 NAME
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RAND
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- the OpenSSL random generator
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=head1 DESCRIPTION
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Random numbers are a vital part of cryptography, they are needed to provide
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unpredictability for tasks like key generation, creating salts, and many more.
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Software-based generators must be seeded with external randomness before they
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can be used as a cryptographically-secure pseudo-random number generator
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(CSPRNG).
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The availability of common hardware with special instructions and
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modern operating systems, which may use items such as interrupt jitter
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and network packet timings, can be reasonable sources of seeding material.
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OpenSSL comes with a default implementation of the RAND API which is based on
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the deterministic random bit generator (DRBG) model as described in
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[NIST SP 800-90A Rev. 1]. The default random generator will initialize
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automatically on first use and will be fully functional without having
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to be initialized ('seeded') explicitly.
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It seeds and reseeds itself automatically using trusted random sources
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provided by the operating system.
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As a normal application developer, you do not have to worry about any details,
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just use L<RAND_bytes(3)> to obtain random data.
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Having said that, there is one important rule to obey: Always check the error
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return value of L<RAND_bytes(3)> and do not take randomness for granted.
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Although (re-)seeding is automatic, it can fail because no trusted random source
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is available or the trusted source(s) temporarily fail to provide sufficient
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random seed material.
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In this case the CSPRNG enters an error state and ceases to provide output,
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until it is able to recover from the error by reseeding itself.
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For more details on reseeding and error recovery, see L<EVP_RAND(7)>.
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For values that should remain secret, you can use L<RAND_priv_bytes(3)>
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instead.
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This method does not provide 'better' randomness, it uses the same type of
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CSPRNG.
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The intention behind using a dedicated CSPRNG exclusively for private
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values is that none of its output should be visible to an attacker (e.g.,
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used as salt value), in order to reveal as little information as
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possible about its internal state, and that a compromise of the "public"
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CSPRNG instance will not affect the secrecy of these private values.
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In the rare case where the default implementation does not satisfy your special
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requirements, the default RAND internals can be replaced by your own
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L<EVP_RAND(3)> objects.
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Changing the default random generator should be necessary
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only in exceptional cases and is not recommended, unless you have a profound
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knowledge of cryptographic principles and understand the implications of your
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changes.
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Finally, it is possible for a provider to bypass the default RAND setup for
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L<RAND_bytes(3)> and associated functions. A provider can be specified as the
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single randomness source via the L<RAND_set1_random_provider(3)> function or via
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configuration using the B<random_provider> option in L<config(5)>. Once specified,
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the nominated provider will be used directly when calling the L<RAND_bytes(3)>
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family of functions.
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=head1 DEFAULT SETUP
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The default OpenSSL RAND method is based on the EVP_RAND deterministic random
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bit generator (DRBG) classes.
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A DRBG is a certain type of cryptographically-secure pseudo-random
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number generator (CSPRNG), which is described in [NIST SP 800-90A Rev. 1].
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=head1 SEE ALSO
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L<RAND_bytes(3)>,
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L<RAND_priv_bytes(3)>,
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L<EVP_RAND(3)>,
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L<RAND_get0_primary(3)>,
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L<config(5)>,
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L<EVP_RAND(7)>,
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L<RAND_set1_random_provider(3)>.
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=head1 COPYRIGHT
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Copyright 2018-2025 The OpenSSL Project Authors. All Rights Reserved.
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Licensed under the Apache License 2.0 (the "License"). You may not use
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this file except in compliance with the License. You can obtain a copy
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in the file LICENSE in the source distribution or at
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L<https://www.openssl.org/source/license.html>.
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=cut
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