1EVP_RAND(7ossl) OpenSSL EVP_RAND(7ossl)
2
6 EVP_RAND - the random bit generator
7
9 #include <openssl/evp.h>
10 #include <rand.h>
11
13 The default OpenSSL RAND method is based on the EVP_RAND classes to
14 provide non-deterministic inputs to other cryptographic algorithms.
15 While the RAND API is the 'frontend' which is intended to be used by
17 application developers for obtaining random bytes, the EVP_RAND API
18 serves as the 'backend', connecting the former with the operating
19 systems's entropy sources and providing access to deterministic random
20 bit generators (DRBG) and their configuration parameters. A DRBG is a
21 certain type of cryptographically-secure pseudo-random number generator
22 (CSPRNG), which is described in [NIST SP 800-90A Rev. 1].
23 Disclaimer
25 Unless you have very specific requirements for your random generator,
26 it is in general not necessary to utilize the EVP_RAND API directly.
27 The usual way to obtain random bytes is to use RAND_bytes(3) or
28 RAND_priv_bytes(3), see also RAND(7).
29 Typical Use Cases
31 Typical examples for such special use cases are the following:
32 • You want to use your own private DRBG instances. Multiple DRBG
34 instances which are accessed only by a single thread provide
35 additional security (because their internal states are independent)
36 and better scalability in multithreaded applications (because they
37 don't need to be locked).
38 • You need to integrate a previously unsupported entropy source. Refer
40 to provider-rand(7) for the implementation details to support adding
41 randomness sources to EVP_RAND.
42 • You need to change the default settings of the standard OpenSSL RAND
44 implementation to meet specific requirements.
45
47 An EVP_RAND instance can be used as the entropy source of another
48 EVP_RAND instance, provided it has itself access to a valid entropy
49 source. The EVP_RAND instance which acts as entropy source is called
50 the parent, the other instance the child. Typically, the child will be
51 a DRBG because it does not make sense for the child to be an entropy
52 source.
53 This is called chaining. A chained EVP_RAND instance is created by
55 passing a pointer to the parent EVP_RAND_CTX as argument to the
56 EVP_RAND_CTX_new() call. It is possible to create chains of more than
57 two DRBG in a row. It is also possible to use any EVP_RAND_CTX class
58 as the parent, however, only a live entropy source may ignore and not
59 use its parent.
60
62 Currently, there are three shared DRBG instances, the <primary>,
63 <public>, and <private> DRBG. While the <primary> DRBG is a single
64 global instance, the <public> and <private> DRBG are created per thread
65 and accessed through thread-local storage.
66 By default, the functions RAND_bytes(3) and RAND_priv_bytes(3) use the
68 thread-local <public> and <private> DRBG instance, respectively.
69 The <primary> DRBG instance
71 The <primary> DRBG is not used directly by the application, only for
72 reseeding the two other two DRBG instances. It reseeds itself by
73 obtaining randomness either from os entropy sources or by consuming
74 randomness which was added previously by RAND_add(3).
75 The <public> DRBG instance
77 This instance is used per default by RAND_bytes(3).
78 The <private> DRBG instance
80 This instance is used per default by RAND_priv_bytes(3)
81
83 The <primary> DRBG is intended to be accessed concurrently for
84 reseeding by its child DRBG instances. The necessary locking is done
85 internally. It is not thread-safe to access the <primary> DRBG
86 directly via the EVP_RAND interface. The <public> and <private> DRBG
87 are thread-local, i.e. there is an instance of each per thread. So they
88 can safely be accessed without locking via the EVP_RAND interface.
89 Pointers to these DRBG instances can be obtained using
91 RAND_get0_primary(), RAND_get0_public() and RAND_get0_private(),
92 respectively. Note that it is not allowed to store a pointer to one of
93 the thread-local DRBG instances in a variable or other memory location
94 where it will be accessed and used by multiple threads.
95 All other DRBG instances created by an application don't support
97 locking, because they are intended to be used by a single thread.
98 Instead of accessing a single DRBG instance concurrently from different
99 threads, it is recommended to instantiate a separate DRBG instance per
100 thread. Using the <primary> DRBG as entropy source for multiple DRBG
101 instances on different threads is thread-safe, because the DRBG
102 instance will lock the <primary> DRBG automatically for obtaining
103 random input.
104
106 The following picture gives an overview over how the DRBG instances
107 work together and are being used.
108 +--------------------+
110 | os entropy sources |
111 +--------------------+
112 |
113 v +-----------------------------+
114 RAND_add() ==> <primary> <-| shared DRBG (with locking) |
115 / \ +-----------------------------+
116 / \ +---------------------------+
117 <public> <private> <- | per-thread DRBG instances |
118 | | +---------------------------+
119 v v
120 RAND_bytes() RAND_priv_bytes()
121 | ^
122 | |
123 +------------------+ +------------------------------------+
124 | general purpose | | used for secrets like session keys |
125 | random generator | | and private keys for certificates |
126 +------------------+ +------------------------------------+
127 The usual way to obtain random bytes is to call RAND_bytes(...) or
129 RAND_priv_bytes(...). These calls are roughly equivalent to calling
130 EVP_RAND_generate(<public>, ...) and EVP_RAND_generate(<private>, ...),
131 respectively.
132
134 A DRBG instance seeds itself automatically, pulling random input from
135 its entropy source. The entropy source can be either a trusted
136 operating system entropy source, or another DRBG with access to such a
137 source.
138 Automatic reseeding occurs after a predefined number of generate
140 requests. The selection of the trusted entropy sources is configured
141 at build time using the --with-rand-seed option. The following sections
142 explain the reseeding process in more detail.
143 Automatic Reseeding
145 Before satisfying a generate request (EVP_RAND_generate(3)), the DRBG
146 reseeds itself automatically, if one of the following conditions holds:
147 - the DRBG was not instantiated (=seeded) yet or has been
149 uninstantiated.
150 - the number of generate requests since the last reseeding exceeds a
152 certain threshold, the so called reseed_interval. This behaviour can
153 be disabled by setting the reseed_interval to 0.
154 - the time elapsed since the last reseeding exceeds a certain time
156 interval, the so called reseed_time_interval. This can be disabled by
157 setting the reseed_time_interval to 0.
158 - the DRBG is in an error state.
160 Note: An error state is entered if the entropy source fails while the
162 DRBG is seeding or reseeding. The last case ensures that the DRBG
163 automatically recovers from the error as soon as the entropy source is
164 available again.
165 Manual Reseeding
167 In addition to automatic reseeding, the caller can request an immediate
168 reseeding of the DRBG with fresh entropy by setting the prediction
169 resistance parameter to 1 when calling EVP_RAND_generate(3).
170 The document [NIST SP 800-90C] describes prediction resistance requests
172 in detail and imposes strict conditions on the entropy sources that are
173 approved for providing prediction resistance. A request for prediction
174 resistance can only be satisfied by pulling fresh entropy from a live
175 entropy source (section 5.5.2 of [NIST SP 800-90C]). It is up to the
176 user to ensure that a live entropy source is configured and is being
177 used.
178 For the three shared DRBGs (and only for these) there is another way to
180 reseed them manually: If RAND_add(3) is called with a positive
181 randomness argument (or RAND_seed(3)), then this will immediately
182 reseed the <primary> DRBG. The <public> and <private> DRBG will detect
183 this on their next generate call and reseed, pulling randomness from
184 <primary>.
185 The last feature has been added to support the common practice used
187 with previous OpenSSL versions to call RAND_add() before calling
188 RAND_bytes().
189 Entropy Input and Additional Data
191 The DRBG distinguishes two different types of random input: entropy,
192 which comes from a trusted source, and additional input', which can
193 optionally be added by the user and is considered untrusted. It is
194 possible to add additional input not only during reseeding, but also
195 for every generate request.
196 Configuring the Random Seed Source
198 In most cases OpenSSL will automatically choose a suitable seed source
199 for automatically seeding and reseeding its <primary> DRBG. In some
200 cases however, it will be necessary to explicitly specify a seed source
201 during configuration, using the --with-rand-seed option. For more
202 information, see the INSTALL instructions. There are also operating
203 systems where no seed source is available and automatic reseeding is
204 disabled by default.
205 The following two sections describe the reseeding process of the
207 primary DRBG, depending on whether automatic reseeding is available or
208 not.
209 Reseeding the primary DRBG with automatic seeding enabled
211 Calling RAND_poll() or RAND_add() is not necessary, because the DRBG
212 pulls the necessary entropy from its source automatically. However,
213 both calls are permitted, and do reseed the RNG.
214 RAND_add() can be used to add both kinds of random input, depending on
216 the value of the randomness argument:
217 randomness == 0:
219 The random bytes are mixed as additional input into the current
220 state of the DRBG. Mixing in additional input is not considered a
221 full reseeding, hence the reseed counter is not reset.
222 randomness > 0:
224 The random bytes are used as entropy input for a full reseeding
225 (resp. reinstantiation) if the DRBG is instantiated (resp.
226 uninstantiated or in an error state). The number of random bits
227 required for reseeding is determined by the security strength of
228 the DRBG. Currently it defaults to 256 bits (32 bytes). It is
229 possible to provide less randomness than required. In this case
230 the missing randomness will be obtained by pulling random input
231 from the trusted entropy sources.
232 NOTE: Manual reseeding is *not allowed* in FIPS mode, because [NIST
234 SP-800-90Ar1] mandates that entropy *shall not* be provided by the
235 consuming application for instantiation (Section 9.1) or reseeding
236 (Section 9.2). For that reason, the randomness argument is ignored and
237 the random bytes provided by the RAND_add(3) and RAND_seed(3) calls are
238 treated as additional data.
239 Reseeding the primary DRBG with automatic seeding disabled
241 Calling RAND_poll() will always fail.
242 RAND_add() needs to be called for initial seeding and periodic
244 reseeding. At least 48 bytes (384 bits) of randomness have to be
245 provided, otherwise the (re-)seeding of the DRBG will fail. This
246 corresponds to one and a half times the security strength of the DRBG.
247 The extra half is used for the nonce during instantiation.
248 More precisely, the number of bytes needed for seeding depend on the
250 security strength of the DRBG, which is set to 256 by default.
251
253 RAND(7), EVP_RAND(3)
254
256 This functionality was added in OpenSSL 3.0.
257
259 Copyright 2017-2020 The OpenSSL Project Authors. All Rights Reserved.
260 Licensed under the Apache License 2.0 (the "License"). You may not use
262 this file except in compliance with the License. You can obtain a copy
263 in the file LICENSE in the source distribution or at
264 <https://www.openssl.org/source/license.html>.
2653.0.9 2023-07-27 EVP_RAND(7ossl)