1RAND_DRBG(7) OpenSSL RAND_DRBG(7)
2
6 RAND_DRBG - the deterministic random bit generator
7
9 #include <openssl/rand_drbg.h>
10
12 The default OpenSSL RAND method is based on the RAND_DRBG class, which
13 implements a deterministic random bit generator (DRBG). A DRBG is a
14 certain type of cryptographically-secure pseudo-random number generator
15 (CSPRNG), which is described in [NIST SP 800-90A Rev. 1].
16 While the RAND API is the 'frontend' which is intended to be used by
18 application developers for obtaining random bytes, the RAND_DRBG API
19 serves as the 'backend', connecting the former with the operating
20 systems's entropy sources and providing access to the DRBG's
21 configuration parameters.
22 Disclaimer
24 Unless you have very specific requirements for your random generator,
25 it is in general not necessary to utilize the RAND_DRBG API directly.
26 The usual way to obtain random bytes is to use RAND_bytes(3) or
27 RAND_priv_bytes(3), see also RAND(7).
28 Typical Use Cases
30 Typical examples for such special use cases are the following:
31 · You want to use your own private DRBG instances. Multiple DRBG
33 instances which are accessed only by a single thread provide
34 additional security (because their internal states are independent)
35 and better scalability in multithreaded applications (because they
36 don't need to be locked).
37 · You need to integrate a previously unsupported entropy source.
39 · You need to change the default settings of the standard OpenSSL RAND
41 implementation to meet specific requirements.
42
44 A DRBG instance can be used as the entropy source of another DRBG
45 instance, provided it has itself access to a valid entropy source. The
46 DRBG instance which acts as entropy source is called the parent DRBG,
47 the other instance the child DRBG.
48 This is called chaining. A chained DRBG instance is created by passing
50 a pointer to the parent DRBG as argument to the RAND_DRBG_new() call.
51 It is possible to create chains of more than two DRBG in a row.
52
54 Currently, there are three shared DRBG instances, the <master>,
55 <public>, and <private> DRBG. While the <master> DRBG is a single
56 global instance, the <public> and <private> DRBG are created per thread
57 and accessed through thread-local storage.
58 By default, the functions RAND_bytes(3) and RAND_priv_bytes(3) use the
60 thread-local <public> and <private> DRBG instance, respectively.
61 The <master> DRBG instance
63 The <master> DRBG is not used directly by the application, only for
64 reseeding the two other two DRBG instances. It reseeds itself by
65 obtaining randomness either from os entropy sources or by consuming
66 randomness which was added previously by RAND_add(3).
67 The <public> DRBG instance
69 This instance is used per default by RAND_bytes(3).
70 The <private> DRBG instance
72 This instance is used per default by RAND_priv_bytes(3)
73
75 The <master> DRBG is intended to be accessed concurrently for reseeding
76 by its child DRBG instances. The necessary locking is done internally.
77 It is not thread-safe to access the <master> DRBG directly via the
78 RAND_DRBG interface. The <public> and <private> DRBG are thread-local,
79 i.e. there is an instance of each per thread. So they can safely be
80 accessed without locking via the RAND_DRBG interface.
81 Pointers to these DRBG instances can be obtained using
83 RAND_DRBG_get0_master(), RAND_DRBG_get0_public(), and
84 RAND_DRBG_get0_private(), respectively. Note that it is not allowed to
85 store a pointer to one of the thread-local DRBG instances in a variable
86 or other memory location where it will be accessed and used by multiple
87 threads.
88 All other DRBG instances created by an application don't support
90 locking, because they are intended to be used by a single thread.
91 Instead of accessing a single DRBG instance concurrently from different
92 threads, it is recommended to instantiate a separate DRBG instance per
93 thread. Using the <master> DRBG as entropy source for multiple DRBG
94 instances on different threads is thread-safe, because the DRBG
95 instance will lock the <master> DRBG automatically for obtaining random
96 input.
97
99 The following picture gives an overview over how the DRBG instances
100 work together and are being used.
101 +--------------------+
103 | os entropy sources |
104 +--------------------+
105 |
106 v +-----------------------------+
107 RAND_add() ==> <master> <-| shared DRBG (with locking) |
108 / \ +-----------------------------+
109 / \ +---------------------------+
110 <public> <private> <- | per-thread DRBG instances |
111 | | +---------------------------+
112 v v
113 RAND_bytes() RAND_priv_bytes()
114 | ^
115 | |
116 +------------------+ +------------------------------------+
117 | general purpose | | used for secrets like session keys |
118 | random generator | | and private keys for certificates |
119 +------------------+ +------------------------------------+
120 The usual way to obtain random bytes is to call RAND_bytes(...) or
122 RAND_priv_bytes(...). These calls are roughly equivalent to calling
123 RAND_DRBG_bytes(<public>, ...) and RAND_DRBG_bytes(<private>, ...),
124 respectively. The method RAND_DRBG_bytes(3) is a convenience method
125 wrapping the RAND_DRBG_generate(3) function, which serves the actual
126 request for random data.
127
129 A DRBG instance seeds itself automatically, pulling random input from
130 its entropy source. The entropy source can be either a trusted
131 operating system entropy source, or another DRBG with access to such a
132 source.
133 Automatic reseeding occurs after a predefined number of generate
135 requests. The selection of the trusted entropy sources is configured
136 at build time using the --with-rand-seed option. The following sections
137 explain the reseeding process in more detail.
138 Automatic Reseeding
140 Before satisfying a generate request (RAND_DRBG_generate(3)), the DRBG
141 reseeds itself automatically, if one of the following conditions holds:
142 - the DRBG was not instantiated (=seeded) yet or has been
144 uninstantiated.
145 - the number of generate requests since the last reseeding exceeds a
147 certain threshold, the so called reseed_interval. This behaviour can
148 be disabled by setting the reseed_interval to 0.
149 - the time elapsed since the last reseeding exceeds a certain time
151 interval, the so called reseed_time_interval. This can be disabled by
152 setting the reseed_time_interval to 0.
153 - the DRBG is in an error state.
155 Note: An error state is entered if the entropy source fails while the
157 DRBG is seeding or reseeding. The last case ensures that the DRBG
158 automatically recovers from the error as soon as the entropy source is
159 available again.
160 Manual Reseeding
162 In addition to automatic reseeding, the caller can request an immediate
163 reseeding of the DRBG with fresh entropy by setting the prediction
164 resistance parameter to 1 when calling RAND_DRBG_generate(3).
165 The document [NIST SP 800-90C] describes prediction resistance requests
167 in detail and imposes strict conditions on the entropy sources that are
168 approved for providing prediction resistance. Since the default DRBG
169 implementation does not have access to such an approved entropy source,
170 a request for prediction resistance will currently always fail. In
171 other words, prediction resistance is currently not supported yet by
172 the DRBG.
173 For the three shared DRBGs (and only for these) there is another way to
175 reseed them manually: If RAND_add(3) is called with a positive
176 randomness argument (or RAND_seed(3)), then this will immediately
177 reseed the <master> DRBG. The <public> and <private> DRBG will detect
178 this on their next generate call and reseed, pulling randomness from
179 <master>.
180 The last feature has been added to support the common practice used
182 with previous OpenSSL versions to call RAND_add() before calling
183 RAND_bytes().
184 Entropy Input vs. Additional Data
186 The DRBG distinguishes two different types of random input: entropy,
187 which comes from a trusted source, and additional input', which can
188 optionally be added by the user and is considered untrusted. It is
189 possible to add additional input not only during reseeding, but also
190 for every generate request. This is in fact done automatically by
191 RAND_DRBG_bytes(3).
192 Configuring the Random Seed Source
194 In most cases OpenSSL will automatically choose a suitable seed source
195 for automatically seeding and reseeding its <master> DRBG. In some
196 cases however, it will be necessary to explicitly specify a seed source
197 during configuration, using the --with-rand-seed option. For more
198 information, see the INSTALL instructions. There are also operating
199 systems where no seed source is available and automatic reseeding is
200 disabled by default.
201 The following two sections describe the reseeding process of the master
203 DRBG, depending on whether automatic reseeding is available or not.
204 Reseeding the master DRBG with automatic seeding enabled
206 Calling RAND_poll() or RAND_add() is not necessary, because the DRBG
207 pulls the necessary entropy from its source automatically. However,
208 both calls are permitted, and do reseed the RNG.
209 RAND_add() can be used to add both kinds of random input, depending on
211 the value of the randomness argument:
212 randomness == 0:
214 The random bytes are mixed as additional input into the current
215 state of the DRBG. Mixing in additional input is not considered a
216 full reseeding, hence the reseed counter is not reset.
217 randomness > 0:
219 The random bytes are used as entropy input for a full reseeding
220 (resp. reinstantiation) if the DRBG is instantiated (resp.
221 uninstantiated or in an error state). The number of random bits
222 required for reseeding is determined by the security strength of
223 the DRBG. Currently it defaults to 256 bits (32 bytes). It is
224 possible to provide less randomness than required. In this case
225 the missing randomness will be obtained by pulling random input
226 from the trusted entropy sources.
227 Reseeding the master DRBG with automatic seeding disabled
229 Calling RAND_poll() will always fail.
230 RAND_add() needs to be called for initial seeding and periodic
232 reseeding. At least 48 bytes (384 bits) of randomness have to be
233 provided, otherwise the (re-)seeding of the DRBG will fail. This
234 corresponds to one and a half times the security strength of the DRBG.
235 The extra half is used for the nonce during instantiation.
236 More precisely, the number of bytes needed for seeding depend on the
238 security strength of the DRBG, which is set to 256 by default.
239
241 RAND_DRBG_bytes(3), RAND_DRBG_generate(3), RAND_DRBG_reseed(3),
242 RAND_DRBG_get0_master(3), RAND_DRBG_get0_public(3),
243 RAND_DRBG_get0_private(3), RAND_DRBG_set_reseed_interval(3),
244 RAND_DRBG_set_reseed_time_interval(3),
245 RAND_DRBG_set_reseed_defaults(3), RAND(7),
246
248 Copyright 2017-2018 The OpenSSL Project Authors. All Rights Reserved.
249 Licensed under the OpenSSL license (the "License"). You may not use
251 this file except in compliance with the License. You can obtain a copy
252 in the file LICENSE in the source distribution or at
253 <https://www.openssl.org/source/license.html>.
2541.1.1c 2019-05-28 RAND_DRBG(7)