RAND(7ossl)

📦 version 3.5.0, in opensuse-leap 16.0

🌸 RAND(7ossl) — the OpenSSL random generator, bestie edition 🌸

okay so this is the RAND page from OpenSSL, and it's literally the page about where all your random numbers come from. pull up a chair 💅

✨ 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 tasks like key generation, creating salts, and so many more things. Software-based generators have to be seeded with external randomness before you can use them as a cryptographically-secure pseudo-random number generator (CSPRNG). And ngl, the seeding material situation is pretty good these days: common hardware with special instructions and modern operating systems — which may use things like interrupt jitter and network packet timings — can be reasonable sources of seeding material. slay 🦋

OpenSSL comes with a default implementation of the RAND API, and it's 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. honestly iconic behavior 👑

As a normal application developer, you do not have to worry about any details — just use RAND_bytes(3) to obtain random data. but bestie. listen. 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 because the trusted source(s) temporarily fail to provide sufficient random seed material. when that happens 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). 😭

For values that should remain secret, you can use RAND_priv_bytes(3) instead. and no, 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), so as to reveal as little information as possible about its internal state, and so that a compromise of the "public" CSPRNG instance will not affect the secrecy of these private values. it's giving separate group chats for a reason 💅

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. 👀

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. like, this is not a "let me just tweak it" situation, bestie 🚨

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. ✨

🧚 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]. that's the whole section, she's short and she knows it 🎀

🌈 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 page!! 💜 this was RAND(7ossl) from OpenSSL version 3.5.0, dated 2026-09-25 — go get you some cryptographically secure randomness and check your return values, mwah 🫶🍓

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