doc/man3/RAND_bytes.pod

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