RAND(7SSL)

📦
version 4.0.3 · 13 versions
, in ubuntu stonking

💖 RAND — the OpenSSL random generator 💖

okay bestie, welcome to RAND(7SSL), aka the OpenSSL randomness manual, and honestly? this one is kind of the main character of the whole crypto ecosystem. let's get into it 🎀

✨ NAME ✨

RAND — the OpenSSL random generator. that's it, that's the whole vibe. 🦋

🌸 DESCRIPTION 🌸

Random numbers are a vital part of cryptography, fr fr. they're what gives you unpredictability for stuff like key generation, creating salts, and so many more things. literally everywhere.

Software-based generators must be seeded with external randomness before they can be used as a cryptographically-secure pseudo-random number generator (CSPRNG). and the good news? common hardware with special instructions and modern operating systems (which may use items such as interrupt jitter and network packet timings) can be reasonable sources of seeding material. slay 🌈

OpenSSL comes with a default implementation of the RAND API which is based on the deterministic random bit generator (DRBG) model as described in [NIST SP 800-90A Rev. 1]. The default random generator will initialize automatically on first use and will be fully functional without having to be initialized ('seeded') explicitly. it seeds and reseeds itself automatically using trusted random sources provided by the operating system. no manual labour, it just works 👑

As a normal application developer, you do not have to worry about any details, just use RAND_bytes(3) to obtain random data. you're welcome 💅

Having said that, there is one important rule to obey: always check the error return value of RAND_bytes(3) and do not take randomness for granted. even though (re-)seeding is automatic, it can fail because no trusted random source is available or the trusted source(s) temporarily fail to provide sufficient random seed material. In this case the CSPRNG enters an error state and ceases to provide output, until it is able to recover from the error by reseeding itself. for more details on reseeding and error recovery, see EVP_RAND(7). 🫶

no cap: if you skip the error check, you are not beating the "took randomness for granted" allegations. always check. 😭

For values that should remain secret, you can use RAND_priv_bytes(3) instead. this method does not provide 'better' randomness — it uses the same type of CSPRNG. The intention behind using a dedicated CSPRNG exclusively for private values is that none of its output should be visible to an attacker (e.g., used as salt value), in order to reveal as little information as possible about its internal state, and that a compromise of the "public" CSPRNG instance will not affect the secrecy of these private values. two separate vibes, one shared energy 💜

In the rare case where the default implementation does not satisfy your special requirements, the default RAND internals can be replaced by your own EVP_RAND(3) objects. DIY era ✨

Changing the default random generator should be necessary only in exceptional cases and is not recommended, unless you have a profound knowledge of cryptographic principles and understand the implications of your changes. read that again. not a casual weekend project. 💀

Finally, it is possible for a provider to bypass the default RAND setup for RAND_bytes(3) and associated functions. A provider can be specified as the single randomness source via the RAND_set1_random_provider(3) function or via configuration using the random_provider option in config(5). Once specified, the nominated provider will be used directly when calling the RAND_bytes(3) family of functions. okay so that's the whole plot twist 🍓

👑 DEFAULT SETUP 👑

The default OpenSSL RAND method is based on the EVP_RAND deterministic random bit generator (DRBG) classes. A DRBG is a certain type of cryptographically-secure pseudo-random number generator (CSPRNG), which is described in [NIST SP 800-90A Rev. 1]. iconic, cited, standardized ⭐

🌈 SEE ALSO 🌈

Copyright 2018-2025 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!! 🌸 this was RAND(7SSL) from OpenSSL, version 4.0.3, dated 2026-09-29. go forth, call RAND_bytes(3), and check that return value like the icon you are 💖✨🦋

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