1=pod 2 3=head1 NAME 4 5RAND_bytes, RAND_priv_bytes, RAND_bytes_ex, RAND_priv_bytes_ex, 6RAND_pseudo_bytes, RAND_set1_random_provider - generate random data 7 8=head1 SYNOPSIS 9 10 #include <openssl/rand.h> 11 12 int RAND_bytes(unsigned char *buf, int num); 13 int RAND_priv_bytes(unsigned char *buf, int num); 14 15 int RAND_bytes_ex(OSSL_LIB_CTX *ctx, unsigned char *buf, size_t num, 16 unsigned int strength); 17 int RAND_priv_bytes_ex(OSSL_LIB_CTX *ctx, unsigned char *buf, size_t num, 18 unsigned int strength); 19 20 int RAND_set1_random_provider(OSSL_LIB_CTX *ctx, OSSL_PROVIDER *p); 21 22The following function has been deprecated since OpenSSL 1.1.0, and can be 23hidden entirely by defining B<OPENSSL_API_COMPAT> with a suitable version value, 24see L<openssl_user_macros(7)>: 25 26 int RAND_pseudo_bytes(unsigned char *buf, int num); 27 28=head1 DESCRIPTION 29 30RAND_bytes() generates B<num> random bytes using a cryptographically 31secure pseudo random generator (CSPRNG) and stores them in B<buf>. B<buf> B<MUST NOT> be NULL. 32 33RAND_priv_bytes() has the same semantics as RAND_bytes(). It is intended to 34be used for generating values that should remain private. If using the 35default RAND_METHOD, this function uses a separate "private" PRNG 36instance so that a compromise of the "public" PRNG instance will not 37affect the secrecy of these private values, as described in L<RAND(7)> 38and L<EVP_RAND(7)>. 39 40RAND_bytes_ex() and RAND_priv_bytes_ex() are the same as RAND_bytes() and 41RAND_priv_bytes() except that they both take additional I<strength> and 42I<ctx> parameters. The bytes generated will have a security strength of at 43least I<strength> bits. 44The DRBG used for the operation is the public or private DRBG associated with 45the specified I<ctx>. The parameter can be NULL, in which case 46the default library context is used (see L<OSSL_LIB_CTX(3)>. 47If the default RAND_METHOD has been changed then for compatibility reasons the 48RAND_METHOD will be used in preference and the DRBG of the library context 49ignored. 50 51RAND_set1_random_provider() specifies a provider, I<prov>, which will be used 52by the library context I<ctx> for all of the generate calls above instead 53of the built-in in DRBGs and entropy source. Pass NULL for the provider 54to disable the random provider functionality. In this case, the built-in DRBGs 55and entropy source will be used. This call should not be considered thread safe. 56 57=head1 NOTES 58 59By default, the OpenSSL CSPRNG supports a security level of 256 bits, provided it 60was able to seed itself from a trusted entropy source. 61On all major platforms supported by OpenSSL (including the Unix-like platforms 62and Windows), OpenSSL is configured to automatically seed the CSPRNG on first use 63using the operating systems's random generator. 64 65If the entropy source fails or is not available, the CSPRNG will enter an 66error state and refuse to generate random bytes. For that reason, it is important 67to always check the error return value of RAND_bytes() and RAND_priv_bytes() and 68not take randomness for granted. 69 70On other platforms, there might not be a trusted entropy source available 71or OpenSSL might have been explicitly configured to use different entropy sources. 72If you are in doubt about the quality of the entropy source, don't hesitate to ask 73your operating system vendor or post a question on GitHub or the openssl-users 74mailing list. 75 76=head1 RETURN VALUES 77 78RAND_bytes() and RAND_priv_bytes() 79return 1 on success, -1 if not supported by the current 80RAND method, or 0 on other failure. The error code can be 81obtained by L<ERR_get_error(3)>. 82 83RAND_set1_random_provider() returns 1 on success and 0 on failure. 84 85=head1 SEE ALSO 86 87L<RAND_add(3)>, 88L<RAND_bytes(3)>, 89L<RAND_priv_bytes(3)>, 90L<ERR_get_error(3)>, 91L<RAND(7)>, 92L<EVP_RAND(7)> 93 94=head1 HISTORY 95 96=over 2 97 98=item * 99 100RAND_pseudo_bytes() was deprecated in OpenSSL 1.1.0; use RAND_bytes() instead. 101 102=item * 103 104The RAND_priv_bytes() function was added in OpenSSL 1.1.1. 105 106=item * 107 108The RAND_bytes_ex() and RAND_priv_bytes_ex() functions were added in OpenSSL 3.0 109 110=item * 111 112The RAND_set1_random_provider() function was added in OpenSSL 3.5 113 114=back 115 116=head1 COPYRIGHT 117 118Copyright 2000-2025 The OpenSSL Project Authors. All Rights Reserved. 119 120Licensed under the Apache License 2.0 (the "License"). You may not use 121this file except in compliance with the License. You can obtain a copy 122in the file LICENSE in the source distribution or at 123L<https://www.openssl.org/source/license.html>. 124 125=cut 126