EVP_RAND(7ossl)

📦
version 15.1 · 3 versions
, in freebsd 15.1

💜 NAME 💜

EVP_RAND — the random bit generator, and yes bestie this is the OpenSSL manual page EVP_RAND(7ossl) and we're about to get into it 🎀

✨ SYNOPSIS ✨

#include <openssl/evp.h>
#include <rand.h>

🌸 DESCRIPTION 🌸

Okay so here's the tea ☕ — the default OpenSSL RAND method is built on top of the EVP_RAND classes, and its whole job is feeding non-deterministic inputs into all your other cryptographic algorithms. That's the gig. That's what it does. 💅

So picture it: the RAND API is the 'frontend' — the cute one that application developers actually talk to when they want random bytes. Meanwhile the EVP_RAND API is the 'backend', the one behind the scenes connecting the frontend to the operating system's entropy sources, and also giving you access to deterministic random bit generators (DRBG) and all their configuration parameters. A DRBG is a specific type of cryptographically-secure pseudo-random number generator (CSPRNG), and it's described in [NIST SP 800-90A Rev. 1]. Iconic, honestly. 👑

🙈 Disclaimer 🙈

Real talk though — unless you have very specific requirements for your random generator (like, very specific, main-character requirements), 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). That's it. That's the normal path. Don't make it weird. 🫶

🦋 Typical Use Cases 🦋

Typical examples for such special use cases are the following, bestie:

  • 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). Lowkey a glow-up for your threads. ✨
  • 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. Go read it, I'll wait. 💅
  • You need to change the default settings of the standard OpenSSL RAND implementation to meet specific requirements. Customising. We love to see it. 🎀

🔗 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 instance acting as entropy source is called the parent, and the other instance is the child. Typically the child will be a DRBG, because it does not make sense for the child to be an entropy source. Like, why would it. 😭

This is called chaining. A chained EVP_RAND instance is created by passing a pointer to the parent EVP_RAND_CTX as an argument to the EVP_RAND_CTX_new() call. And yes, you can go nested — 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 way that's a whole family tree 🌈

👑 THE THREE SHARED DRBG INSTANCES 👑

Currently, there are three shared DRBG instances: the <primary>, <public>, and <private> DRBG. And here's the distinction bestie, write it down 📝 — 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 — it only exists for reseeding the 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). Behind the scenes icon, fr fr. 💜

🧚 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) (no period, that's how the original rolls, and we respect it)

🔒 LOCKING 🔒

The <primary> DRBG is intended to be accessed concurrently for reseeding by its child DRBG instances, and the necessary locking is done internally. But — and this is important, don't @ me 👀 — 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. Slay. ✨

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. Just don't. That's not the vibe. 💀

All other DRBG instances created by an application don't support locking, because they are intended to be used by a single thread. So 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. See? It handles itself. 🫶

🌈 THE OVERALL PICTURE 🌈

Here's the diagram, bestie — it gives an overview over how the DRBG instances work together and are being used. Study it, it's literally the whole map 🗺️

                  +--------------------+
                  | 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. And that's where we'll leave it for now 🌸 — more to come, don't go anywhere.

🌱 RESEEDING 🌱

okay so bestie, a DRBG instance seeds itself automatically — it just pulls random input from its entropy source on its own, no hand-holding. and that entropy source can be either a trusted operating system entropy source, or another DRBG that has access to such a source. either way, it's giving self-sufficient queen energy 💅

automatic reseeding happens 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 next couple sections break down the whole reseeding process in more detail, so stay with me 🫶

✨ Automatic Reseeding ✨

before satisfying a generate request (EVP_RAND_generate(3)), the DRBG reseeds itself automatically if any one of these 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. you can disable this behaviour 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. that last case is literally the glow-up — it ensures the DRBG automatically recovers from the error as soon as the entropy source is available again 😭✨

🌸 Manual Reseeding 🌸

besides the automatic stuff, 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]). and it is up to the user to ensure that a live entropy source is configured and is being used — no cap, that part's on you 🙈

for the three shared DRBGs (and only for these) there's 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>. iconique behavior tbh 👑

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. and bestie, it's possible to add additional input not only during reseeding, but also for every single generate request. versatile much? 💖

🎀 Configuring the Random Seed Source 🎀

in most cases OpenSSL will automatically choose a suitable seed source for automatically seeding and reseeding its <primary> DRBG. the default seed source can be configured when OpenSSL is compiled by setting -DOPENSSL_DEFAULT_SEED_SRC=SEED-SRC. if not set then "SEED-SRC" is used. you can specify a third-party provider seed-source, or -DOPENSSL_DEFAULT_SEED_SRC=JITTER if available.

in some cases however, it will be necessary to explicitly specify a seed source used by "SEED-SRC" during configuration, using the --with-rand-seed option. for more info, 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. so like, optional but valid 💅

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 — the DRBG just tops itself up, slay 🫶
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. so like, don't try it, fr fr 💀

⭐ Reseeding the primary DRBG with automatic seeding disabled ⭐

calling RAND_poll() will always fail. periodt.

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 2017-2024 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>.

and that's the whole page bestie, we ate and left no crumbs 🍓✨ this lil gem is from OpenSSL 3.5.6, dated 2026-04-07, and it's filed under EVP_RAND(7ossl) — go forth and reseed responsibly, mwah 💋🎀

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