EVP_RAND(7SSL) OpenSSL EVP_RAND(7SSL)
NAME
EVP_RAND - the random bit generator
SYNOPSIS
#include <openssl/evp.h>
#include <rand.h>
DESCRIPTION
The default OpenSSL RAND method is based on the EVP_RAND classes to
provide non-deterministic inputs to other cryptographic algorithms.
While the RAND API is the 'frontend' which is intended to be used by
application developers for obtaining random bytes, the EVP_RAND API
serves as the 'backend', connecting the former with the operating
systems's entropy sources and providing access to deterministic random
bit generators (DRBG) and their configuration parameters. A DRBG is a
certain type of cryptographically-secure pseudo-random number generator
(CSPRNG), which is described in [NIST SP 800-90A Rev. 1].
Disclaimer
Unless you have very specific requirements for your random generator,
it is in general not necessary to utilize the EVP_RAND API directly.
The usual way to obtain random bytes is to use RAND_bytes(3) or
RAND_priv_bytes(3), see also RAND(7).
Typical Use Cases
Typical examples for such special use cases are the following:
• You want to use your own private DRBG instances. Multiple DRBG
instances which are accessed only by a single thread provide
additional security (because their internal states are independent)
and better scalability in multithreaded applications (because they
don't need to be locked).
• You need to integrate a previously unsupported entropy source. Refer
to provider-rand(7) for the implementation details to support adding
randomness sources to EVP_RAND.
• You need to change the default settings of the standard OpenSSL RAND
implementation to meet specific requirements.
EVP_RAND CHAINING
An EVP_RAND instance can be used as the entropy source of another
EVP_RAND instance, provided it has itself access to a valid entropy
source. The EVP_RAND instance which acts as entropy source is called
the parent, the other instance the child. Typically, the child will be
a DRBG because it does not make sense for the child to be an entropy
source.
This is called chaining. A chained EVP_RAND instance is created by
passing a pointer to the parent EVP_RAND_CTX as argument to the
EVP_RAND_CTX_new() call. It is possible to create chains of more than
two DRBG in a row. It is also possible to use any EVP_RAND_CTX class
as the parent, however, only a live entropy source may ignore and not
use its parent.
THE THREE SHARED DRBG INSTANCES
Currently, there are three shared DRBG instances, the <primary>,
<public>, and <private> DRBG. While the <primary> DRBG is a single
global instance, the <public> and <private> DRBG are created per thread
and accessed through thread-local storage.
By default, the functions RAND_bytes(3) and RAND_priv_bytes(3) use the
thread-local <public> and <private> DRBG instance, respectively.
The <primary> DRBG instance
The <primary> DRBG is not used directly by the application, only for
reseeding the two other two DRBG instances. It reseeds itself by
obtaining randomness either from os entropy sources or by consuming
randomness which was added previously by RAND_add(3).
The <public> DRBG instance
This instance is used per default by RAND_bytes(3).
The <private> DRBG instance
This instance is used per default by RAND_priv_bytes(3)
LOCKING
The <primary> DRBG is intended to be accessed concurrently for
reseeding by its child DRBG instances. The necessary locking is done
internally. It is not thread-safe to access the <primary> DRBG
directly via the EVP_RAND interface. The <public> and <private> DRBG
are thread-local, i.e. there is an instance of each per thread. So they
can safely be accessed without locking via the EVP_RAND interface.
Pointers to these DRBG instances can be obtained using
RAND_get0_primary(), RAND_get0_public() and RAND_get0_private(),
respectively. Note that it is not allowed to store a pointer to one of
the thread-local DRBG instances in a variable or other memory location
where it will be accessed and used by multiple threads.
All other DRBG instances created by an application don't support
locking, because they are intended to be used by a single thread.
Instead of accessing a single DRBG instance concurrently from different
threads, it is recommended to instantiate a separate DRBG instance per
thread. Using the <primary> DRBG as entropy source for multiple DRBG
instances on different threads is thread-safe, because the DRBG
instance will lock the <primary> DRBG automatically for obtaining
random input.
THE OVERALL PICTURE
The following picture gives an overview over how the DRBG instances
work together and are being used.
+--------------------+
| os entropy sources |
+--------------------+
|
v +-----------------------------+
RAND_add() ==> <primary> <-| shared DRBG (with locking) |
/ \ +-----------------------------+
/ \ +---------------------------+
<public> <private> <- | per-thread DRBG instances |
| | +---------------------------+
v v
RAND_bytes() RAND_priv_bytes()
| ^
| |
+------------------+ +------------------------------------+
| general purpose | | used for secrets like session keys |
| random generator | | and private keys for certificates |
+------------------+ +------------------------------------+
The usual way to obtain random bytes is to call RAND_bytes(...) or
RAND_priv_bytes(...). These calls are roughly equivalent to calling
EVP_RAND_generate(<public>, ...) and EVP_RAND_generate(<private>, ...),
respectively.
RESEEDING
A DRBG instance seeds itself automatically, pulling random input from
its entropy source. The entropy source can be either a trusted
operating system entropy source, or another DRBG with access to such a
source.
Automatic reseeding occurs after a predefined number of generate
requests. The selection of the trusted entropy sources is configured
at build time using the --with-rand-seed option. The following sections
explain the reseeding process in more detail.
Automatic Reseeding
Before satisfying a generate request (EVP_RAND_generate(3)), the DRBG
reseeds itself automatically, if one of the following conditions holds:
- the DRBG was not instantiated (=seeded) yet or has been
uninstantiated.
- the number of generate requests since the last reseeding exceeds a
certain threshold, the so called reseed_interval. This behaviour can
be disabled by setting the reseed_interval to 0.
- the time elapsed since the last reseeding exceeds a certain time
interval, the so called reseed_time_interval. This can be disabled by
setting the reseed_time_interval to 0.
- the DRBG is in an error state.
Note: An error state is entered if the entropy source fails while the
DRBG is seeding or reseeding. The last case ensures that the DRBG
automatically recovers from the error as soon as the entropy source is
available again.
Manual Reseeding
In addition to automatic reseeding, the caller can request an immediate
reseeding of the DRBG with fresh entropy by setting the prediction
resistance parameter to 1 when calling EVP_RAND_generate(3).
The document [NIST SP 800-90C] describes prediction resistance requests
in detail and imposes strict conditions on the entropy sources that are
approved for providing prediction resistance. A request for prediction
resistance can only be satisfied by pulling fresh entropy from a live
entropy source (section 5.5.2 of [NIST SP 800-90C]). It is up to the
user to ensure that a live entropy source is configured and is being
used.
For the three shared DRBGs (and only for these) there is another way to
reseed them manually: If RAND_add(3) is called with a positive
randomness argument (or RAND_seed(3)), then this will immediately
reseed the <primary> DRBG. The <public> and <private> DRBG will detect
this on their next generate call and reseed, pulling randomness from
<primary>.
The last feature has been added to support the common practice used
with previous OpenSSL versions to call RAND_add() before calling
RAND_bytes().
Entropy Input and Additional Data
The DRBG distinguishes two different types of random input: entropy,
which comes from a trusted source, and additional input', which can
optionally be added by the user and is considered untrusted. It is
possible to add additional input not only during reseeding, but also
for every generate request.
Configuring the Random Seed Source
In most cases OpenSSL will automatically choose a suitable seed source
for automatically seeding and reseeding its <primary> DRBG. In some
cases however, it will be necessary to explicitly specify a seed source
during configuration, using the --with-rand-seed option. For more
information, see the INSTALL instructions. There are also operating
systems where no seed source is available and automatic reseeding is
disabled by default.
The following two sections describe the reseeding process of the
primary DRBG, depending on whether automatic reseeding is available or
not.
Reseeding the primary DRBG with automatic seeding enabled
Calling RAND_poll() or RAND_add() is not necessary, because the DRBG
pulls the necessary entropy from its source automatically. However,
both calls are permitted, and do reseed the RNG.
RAND_add() can be used to add both kinds of random input, depending on
the value of the randomness argument:
randomness == 0:
The random bytes are mixed as additional input into the current
state of the DRBG. Mixing in additional input is not considered a
full reseeding, hence the reseed counter is not reset.
randomness > 0:
The random bytes are used as entropy input for a full reseeding
(resp. reinstantiation) if the DRBG is instantiated (resp.
uninstantiated or in an error state). The number of random bits
required for reseeding is determined by the security strength of
the DRBG. Currently it defaults to 256 bits (32 bytes). It is
possible to provide less randomness than required. In this case
the missing randomness will be obtained by pulling random input
from the trusted entropy sources.
NOTE: Manual reseeding is *not allowed* in FIPS mode, because [NIST
SP-800-90Ar1] mandates that entropy *shall not* be provided by the
consuming application for instantiation (Section 9.1) or reseeding
(Section 9.2). For that reason, the randomness argument is ignored and
the random bytes provided by the RAND_add(3) and RAND_seed(3) calls are
treated as additional data.
Reseeding the primary DRBG with automatic seeding disabled
Calling RAND_poll() will always fail.
RAND_add() needs to be called for initial seeding and periodic
reseeding. At least 48 bytes (384 bits) of randomness have to be
provided, otherwise the (re-)seeding of the DRBG will fail. This
corresponds to one and a half times the security strength of the DRBG.
The extra half is used for the nonce during instantiation.
More precisely, the number of bytes needed for seeding depend on the
security strength of the DRBG, which is set to 256 by default.
SEE ALSO
RAND(7), EVP_RAND(3)
HISTORY
This functionality was added in OpenSSL 3.0.
COPYRIGHT
Copyright 2017-2020 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the Apache License 2.0 (the "License"). You may not use
this file except in compliance with the License. You can obtain a copy
in the file LICENSE in the source distribution or at
<https://www.openssl.org/source/license.html>.
3.0.13 2025-09-18 EVP_RAND(7SSL)
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