5 RAND_add, RAND_poll, RAND_seed, RAND_status, RAND_event, RAND_screen
6 - add randomness to the PRNG or get its status
10 #include <openssl/rand.h>
12 int RAND_status(void);
15 void RAND_add(const void *buf, int num, double randomness);
16 void RAND_seed(const void *buf, int num);
20 #if OPENSSL_API_COMPAT < 0x10100000L
21 int RAND_event(UINT iMsg, WPARAM wParam, LPARAM lParam);
22 void RAND_screen(void);
27 Random numbers are a vital part of cryptography, including key generation,
28 creating salts, etc., and software-based
29 generators must be "seeded" with external randomness before they can be
30 used as a cryptographically-secure pseudo-random number generator (CSPRNG).
31 The availability of common hardware with special instructions and
32 modern operating systems, which may use items such as interrupt jitter
33 and network packet timings, can be reasonable sources of seeding material.
35 RAND_status() indicates whether or not the CSPRNG has been sufficiently
36 seeded. If not, functions such as RAND_bytes(3) will fail.
38 RAND_poll() uses the system's capabilities to seed the CSPRNG using
39 random input obtained from polling various trusted entropy sources.
40 The default choice of the entropy source can be modified at build time
41 using the --with-rand-seed configure option, see also the B<NOTES> section.
42 A summary of the configure options can be displayed with the OpenSSL
43 L<version(1)> command.
45 RAND_add() mixes the B<num> bytes at B<buf> into the PRNG state.
46 The B<randomness> argument is an estimate of how much randomness is
48 B<buf>, in bytes, and should be a number between zero and B<num>.
49 Details about sources of randomness and how to estimate their randomness
50 can be found in the literature; for example NIST SP 800-90B.
51 The content of B<buf> cannot be recovered from subsequent CSPRNG output.
52 This function will not normally be needed, as RAND_poll() should have been
53 configured to do the appropriate seeding for the local platform.
54 Applications that need to keep random state in an external file should
55 use L<RAND_load_file(3)>.
57 RAND_seed() is equivalent to RAND_add() with B<randomness> set to B<num>.
59 RAND_event() and RAND_screen() are equivalent to RAND_poll().
63 RAND_status() returns 1 if the CSPRNG has been seeded
64 with enough data, 0 otherwise.
66 RAND_poll() returns 1 if it generated seed data, 0 otherwise.
68 RAND_event() returns RAND_status().
70 The other functions do not return values.
74 The new OpenSSL DRBG has some peculiarities which need to be taken
75 into account when it is selected as the default OpenSSL CSPRNG, i.e.,
76 when RAND_get_rand_method() == RAND_OpenSSL().
77 This applies in particular to the way reseeding is done by the DRBG:
83 The DRBG seeds itself automatically, pulling random input from trusted
85 Automatic reseeding occurs after a predefined number of generate requests.
86 The selection of the trusted entropy sources is configured at build
87 time using the --with-rand-seed option.
91 The DRBG distinguishes two different types of random input:
92 'entropy', which comes from a trusted source, and 'additional input',
93 which can optionally be added by the user and is considered untrusted.
97 Automatic seeding can be disabled using the --with-rand-seed=none option.
99 =head2 DRBG with automatic seeding enabled
101 Calling RAND_poll() or RAND_add() is not necessary, because the DRBG
102 polls the entropy source automatically.
103 However, both calls are permitted, and do reseed the RNG.
105 RAND_add() can be used to add both kinds of random input, depending on the
106 value of the B<randomness> argument:
110 =item randomness == 0:
112 The random bytes are mixed as additional input into the current state of
114 Mixing in additional input is not considered a full reseeding, hence the
115 reseed counter is not reset.
118 =item randomness > 0:
120 The random bytes are used as entropy input for a full reseeding
121 (resp. reinstantiation) if the DRBG is instantiated
122 (resp. uninstantiated or in an error state).
123 A reseeding requires 16 bytes (128 bits) of randomness.
124 It is possible to provide less randomness than required.
125 In this case the missing randomness will be obtained by pulling random input
126 from the trusted entropy sources.
130 =head2 DRBG with automatic seeding disabled (--with-rand-seed=none)
132 Calling RAND_poll() will always fail.
134 RAND_add() needs to be called for initial seeding and periodic reseeding.
135 At least 16 bytes (128 bits) of randomness have to be provided, otherwise
136 the (re-)seeding of the DRBG will fail.
140 RAND_event() and RAND_screen() were deprecated in OpenSSL 1.1.0 and should
145 L<RAND_bytes(3)>, L<RAND_egd(3)>,
150 Copyright 2000-2017 The OpenSSL Project Authors. All Rights Reserved.
152 Licensed under the OpenSSL license (the "License"). You may not use
153 this file except in compliance with the License. You can obtain a copy
154 in the file LICENSE in the source distribution or at
155 L<https://www.openssl.org/source/license.html>.