5 RAND_DRBG - the deterministic random bit generator
9 #include <openssl/rand_drbg.h>
13 The default OpenSSL RAND method is based on the RAND_DRBG class,
14 which implements a deterministic random bit generator (DRBG).
15 A DRBG is a certain type of cryptographically-secure pseudo-random
16 number generator (CSPRNG), which is described in
17 [NIST SP 800-90A Rev. 1].
19 While the RAND API is the 'frontend' which is intended to be used by
20 application developers for obtaining random bytes, the RAND_DRBG API
21 serves as the 'backend', connecting the former with the operating
22 systems's entropy sources and providing access to the DRBG's
23 configuration parameters.
27 Unless you have very specific requirements for your random generator,
28 it is in general not necessary to utilize the RAND_DRBG API directly.
29 The usual way to obtain random bytes is to use L<RAND_bytes(3)> or
30 L<RAND_priv_bytes(3)>, see also L<RAND(7)>.
32 =head2 Typical Use Cases
34 Typical examples for such special use cases are the following:
40 You want to use your own private DRBG instances.
41 Multiple DRBG instances which are accessed only by a single thread provide
42 additional security (because their internal states are independent) and
43 better scalability in multithreaded applications (because they don't need
48 You need to integrate a previously unsupported entropy source.
52 You need to change the default settings of the standard OpenSSL RAND
53 implementation to meet specific requirements.
60 A DRBG instance can be used as the entropy source of another DRBG instance,
61 provided it has itself access to a valid entropy source.
62 The DRBG instance which acts as entropy source is called the I<parent> DRBG,
63 the other instance the I<child> DRBG.
65 This is called chaining. A chained DRBG instance is created by passing
66 a pointer to the parent DRBG as argument to the RAND_DRBG_new() call.
67 It is possible to create chains of more than two DRBG in a row.
69 =head1 THE THREE SHARED DRBG INSTANCES
71 Currently, there are three shared DRBG instances,
72 the <master>, <public>, and <private> DRBG.
73 While the <master> DRBG is a single global instance, the <public> and <private>
74 DRBG are created per thread and accessed through thread-local storage.
76 By default, the functions L<RAND_bytes(3)> and L<RAND_priv_bytes(3)> use
77 the thread-local <public> and <private> DRBG instance, respectively.
79 =head2 The <master> DRBG instance
81 The <master> DRBG is not used directly by the application, only for reseeding
82 the two other two DRBG instances. It reseeds itself by obtaining randomness
83 either from os entropy sources or by consuming randomness which was added
84 previously by L<RAND_add(3)>.
86 =head2 The <public> DRBG instance
88 This instance is used per default by L<RAND_bytes(3)>.
90 =head2 The <private> DRBG instance
92 This instance is used per default by L<RAND_priv_bytes(3)>
97 The <master> DRBG is intended to be accessed concurrently for reseeding
98 by its child DRBG instances. The necessary locking is done internally.
99 It is I<not> thread-safe to access the <master> DRBG directly via the
101 The <public> and <private> DRBG are thread-local, i.e. there is an
102 instance of each per thread. So they can safely be accessed without
103 locking via the RAND_DRBG interface.
105 Pointers to these DRBG instances can be obtained using
106 RAND_DRBG_get0_master(),
107 RAND_DRBG_get0_public(), and
108 RAND_DRBG_get0_private(), respectively.
109 Note that it is not allowed to store a pointer to one of the thread-local
110 DRBG instances in a variable or other memory location where it will be
111 accessed and used by multiple threads.
113 All other DRBG instances created by an application don't support locking,
114 because they are intended to be used by a single thread.
115 Instead of accessing a single DRBG instance concurrently from different
116 threads, it is recommended to instantiate a separate DRBG instance per
117 thread. Using the <master> DRBG as entropy source for multiple DRBG
118 instances on different threads is thread-safe, because the DRBG instance
119 will lock the <master> DRBG automatically for obtaining random input.
121 =head1 THE OVERALL PICTURE
123 The following picture gives an overview over how the DRBG instances work
124 together and are being used.
126 +--------------------+
127 | os entropy sources |
128 +--------------------+
130 v +-----------------------------+
131 RAND_add() ==> <master> <-| shared DRBG (with locking) |
132 / \ +-----------------------------+
133 / \ +---------------------------+
134 <public> <private> <- | per-thread DRBG instances |
135 | | +---------------------------+
137 RAND_bytes() RAND_priv_bytes()
140 +------------------+ +------------------------------------+
141 | general purpose | | used for secrets like session keys |
142 | random generator | | and private keys for certificates |
143 +------------------+ +------------------------------------+
146 The usual way to obtain random bytes is to call RAND_bytes(...) or
147 RAND_priv_bytes(...). These calls are roughly equivalent to calling
148 RAND_DRBG_bytes(<public>, ...) and RAND_DRBG_bytes(<private>, ...),
149 respectively. The method L<RAND_DRBG_bytes(3)> is a convenience method
150 wrapping the L<RAND_DRBG_generate(3)> function, which serves the actual
151 request for random data.
155 A DRBG instance seeds itself automatically, pulling random input from
156 its entropy source. The entropy source can be either a trusted operating
157 system entropy source, or another DRBG with access to such a source.
159 Automatic reseeding occurs after a predefined number of generate requests.
160 The selection of the trusted entropy sources is configured at build
161 time using the --with-rand-seed option. The following sections explain
162 the reseeding process in more detail.
164 =head2 Automatic Reseeding
166 Before satisfying a generate request (L<RAND_DRBG_generate(3)>), the DRBG
167 reseeds itself automatically, if one of the following conditions holds:
169 - the DRBG was not instantiated (=seeded) yet or has been uninstantiated.
171 - the number of generate requests since the last reseeding exceeds a
172 certain threshold, the so called I<reseed_interval>.
173 This behaviour can be disabled by setting the I<reseed_interval> to 0.
175 - the time elapsed since the last reseeding exceeds a certain time
176 interval, the so called I<reseed_time_interval>.
177 This can be disabled by setting the I<reseed_time_interval> to 0.
179 - the DRBG is in an error state.
181 B<Note>: An error state is entered if the entropy source fails while
182 the DRBG is seeding or reseeding.
183 The last case ensures that the DRBG automatically recovers
184 from the error as soon as the entropy source is available again.
186 =head2 Manual Reseeding
188 In addition to automatic reseeding, the caller can request an immediate
189 reseeding of the DRBG with fresh entropy by setting the
190 I<prediction resistance> parameter to 1 when calling L<RAND_DRBG_generate(3)>.
192 The dcoument [NIST SP 800-90C] describes prediction resistance requests
193 in detail and imposes strict conditions on the entropy sources that are
194 approved for providing prediction resistance.
195 Since the default DRBG implementation does not have access to such an approved
196 entropy source, a request for prediction resistance will currently always fail.
197 In other words, prediction resistance is currently not supported yet by the DRBG.
200 For the three shared DRBGs (and only for these) there is another way to
201 reseed them manually:
202 If L<RAND_add(3)> is called with a positive I<randomness> argument
203 (or L<RAND_seed(3)>), then this will immediately reseed the <master> DRBG.
204 The <public> and <private> DRBG will detect this on their next generate
205 call and reseed, pulling randomness from <master>.
207 The last feature has been added to support the common practice used with
208 previous OpenSSL versions to call RAND_add() before calling RAND_bytes().
211 =head2 Entropy Input vs. Additional Data
213 The DRBG distinguishes two different types of random input: I<entropy>,
214 which comes from a trusted source, and I<additional input>',
215 which can optionally be added by the user and is considered untrusted.
216 It is possible to add I<additional input> not only during reseeding,
217 but also for every generate request.
218 This is in fact done automatically by L<RAND_DRBG_bytes(3)>.
221 =head2 Configuring the Random Seed Source
223 In most cases OpenSSL will automatically choose a suitable seed source
224 for automatically seeding and reseeding its <master> DRBG. In some cases
225 however, it will be necessary to explicitly specify a seed source during
226 configuration, using the --with-rand-seed option. For more information,
227 see the INSTALL instructions. There are also operating systems where no
228 seed source is available and automatic reseeding is disabled by default.
230 The following two sections describe the reseeding process of the master
231 DRBG, depending on whether automatic reseeding is available or not.
234 =head2 Reseeding the master DRBG with automatic seeding enabled
236 Calling RAND_poll() or RAND_add() is not necessary, because the DRBG
237 pulls the necessary entropy from its source automatically.
238 However, both calls are permitted, and do reseed the RNG.
240 RAND_add() can be used to add both kinds of random input, depending on the
241 value of the B<randomness> argument:
245 =item randomness == 0:
247 The random bytes are mixed as additional input into the current state of
249 Mixing in additional input is not considered a full reseeding, hence the
250 reseed counter is not reset.
253 =item randomness > 0:
255 The random bytes are used as entropy input for a full reseeding
256 (resp. reinstantiation) if the DRBG is instantiated
257 (resp. uninstantiated or in an error state).
258 The number of random bits required for reseeding is determined by the
259 security strength of the DRBG. Currently it defaults to 256 bits (32 bytes).
260 It is possible to provide less randomness than required.
261 In this case the missing randomness will be obtained by pulling random input
262 from the trusted entropy sources.
266 =head2 Reseeding the master DRBG with automatic seeding disabled
268 Calling RAND_poll() will always fail.
270 RAND_add() needs to be called for initial seeding and periodic reseeding.
271 At least 48 bytes (384 bits) of randomness have to be provided, otherwise
272 the (re-)seeding of the DRBG will fail. This corresponds to one and a half
273 times the security strength of the DRBG. The extra half is used for the
274 nonce during instantiation.
276 More precisely, the number of bytes needed for seeding depend on the
277 I<security strength> of the DRBG, which is set to 256 by default.
281 L<RAND_DRBG_bytes(3)>,
282 L<RAND_DRBG_generate(3)>,
283 L<RAND_DRBG_reseed(3)>,
284 L<RAND_DRBG_get0_master(3)>,
285 L<RAND_DRBG_get0_public(3)>,
286 L<RAND_DRBG_get0_private(3)>,
287 L<RAND_DRBG_set_reseed_interval(3)>,
288 L<RAND_DRBG_set_reseed_time_interval(3)>,
289 L<RAND_DRBG_set_reseed_defaults(3)>,
294 Copyright 2017-2018 The OpenSSL Project Authors. All Rights Reserved.
296 Licensed under the OpenSSL license (the "License"). You may not use
297 this file except in compliance with the License. You can obtain a copy
298 in the file LICENSE in the source distribution or at
299 L<https://www.openssl.org/source/license.html>.