OSSL-GUIDE-L...INTRODUCTION(7ossl)

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💖 NAME 💖

ossl-guide-libssl-introduction, ssl — the OpenSSL Guide: An introduction to libssl 🎀

✨ INTRODUCTION ✨

okay so bestie, listen up 💅 — the OpenSSL libssl library ships implementations of several secure network communications protocols. Specifically, it gives you SSL/TLS (that's SSLv3, TLSv1, TLSv1.1, TLSv1.2 and TLSv1.3), DTLS (DTLSv1 and DTLSv1.2) and QUIC (client side only, just so you know 🙈). The library depends on libcrypto for all its underlying cryptographic operations — go read ossl-guide-libcrypto-introduction(7) if you need that lore.

The set of APIs supplied by libssl is common across all of these different network protocols, which is genuinely iconic of it 👑. So a developer who's already comfy writing applications with one of these protocols should be able to transition over to using another with relative ease. No cap. 🦋

An application written to use libssl will include the <openssl/ssl.h> header file and will typically use two main data structures, i.e. SSL and SSL_CTX.

An SSL object is used to represent a connection to a remote peer. Once a connection with a remote peer has been established, data can be exchanged with that peer. 💌

When using DTLS, any data that's exchanged uses "datagram" semantics, i.e. the packets of data can be delivered in any order, and they are not guaranteed to arrive at all 😭. In this case the SSL object used for the connection is also used for exchanging data with the peer.

Both TLS and QUIC support the concept of a "stream" of data. Data sent via a stream is guaranteed to be delivered in order without any data loss — slay 💅. A stream can be uni- or bi-directional.

SSL/TLS only supports one stream of data per connection, and it is always bi-directional. In this case the SSL object used for the connection also represents that stream. See ossl-guide-tls-introduction(7) for more information.

The QUIC protocol can support multiple streams per connection, and they can be uni- or bi-directional 🔥. In this case an SSL object can represent the underlying connection, or a stream, or both. Where multiple streams are in use, a separate SSL object is used for each one. See ossl-guide-quic-introduction(7) for more information.

An SSL_CTX object is used to create the SSL object for the underlying connection. A single SSL_CTX object can be used to create many connections (each represented by a separate SSL object) — it's giving main character with a whole friend group 🫶. Many API functions in libssl exist in two forms: one that takes an SSL_CTX and one that takes an SSL. Typically, settings that you apply to the SSL_CTX will then be inherited by any SSL object that you create from it. Alternatively you can apply settings directly to the SSL object without affecting other SSL objects. Note that you should not normally make changes to an SSL_CTX after the first SSL object has been created from it ⚠️ — like, don't touch it once the party's started, okay?

🌸 DATA STRUCTURES 🌸

As well as SSL_CTX and SSL there are a number of other data structures that an application may need to use. They are summarised below 💜

SSL_METHOD (SSL Method)

This structure is used to indicate the kind of connection you want to make, e.g. whether it's to represent the client or the server, and whether it's to use SSL/TLS, DTLS or QUIC. It is passed as a parameter when creating the SSL_CTX. ✨

SSL_SESSION (SSL Session)

After establishing a connection with a peer, the agreed cryptographic material can be reused to create future connections with the same peer more rapidly 🚀 — obsessed with the efficiency honestly. The set of data used for such a future connection establishment attempt is collected together into an SSL_SESSION object. A single successful connection with a peer may generate zero or more such SSL_SESSION objects for use in future connection attempts.

SSL_CIPHER (SSL Cipher)

During connection establishment the client and server agree upon cryptographic algorithms they're going to use for encryption and other uses 🤝. A single set of cryptographic algorithms that are to be used together is known as a ciphersuite. Such a set is represented by an SSL_CIPHER object.

The set of available ciphersuites that can be used are configured in the SSL_CTX or SSL. 🎀

🦋 FURTHER READING 🦋

See ossl-guide-tls-introduction(7) for an introduction to the SSL/TLS protocol and ossl-guide-quic-introduction(7) for an introduction to QUIC. iykyk 💅

See ossl-guide-libcrypto-introduction(7) for an introduction to libcrypto.

🌈 SEE ALSO 🌈

ossl-guide-libcrypto-introduction(7), ossl-guide-tls-introduction(7), ossl-guide-quic-introduction(7)

Copyright 2000-2025 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>.

okay that's the whole thing bestie, you ate 💅✨ — this was version 3.5.6, dated 2026-04-07, and yes the manual page header/footer repeated the title OSSL-GUIDE-L...INTRODUCTION(7ossl) because manpages are extra like that 🙈 bye! 🍓🧚💜

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