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

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version 3.5.9 · 8 versions
, in alpine 3.23, alpine 3.24

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

okay so this is OSSL-GUIDE-L...INTRODUCTION(7ossl) — the 7ossl manual page, bestie, and it's literally your lil' orientation for libssl. let's get into it 🫶

💖 INTRODUCTION 💖

The OpenSSL libssl library provides implementations of several secure network communications protocols — like, several. Specifically it provides:

  • SSL/TLS — SSLv3, TLSv1, TLSv1.1, TLSv1.2 and TLSv1.3
  • DTLS — DTLSv1 and DTLSv1.2
  • QUIC — client side only, okurrr 👑

The library depends on libcrypto for its underlying cryptographic operations (see ossl-guide-libcrypto-introduction(7)). no cap, that's its whole support system.

The set of APIs supplied by libssl is common across all of these different network protocols, so a developer familiar with writing applications using one of these protocols should be able to transition to using another with relative ease. lowkey iconic design ngl 💅

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. two queens. that's the whole cast.

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. periodt.

When using DTLS any data that is 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 — no lost packets, no chaos, we love that for us ✨. 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. one stream, one SSL object, one vibe.

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. QUIC said "more streams, more problems, more SSL objects" 💀

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). 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 — heirloom settings, passing down the family traits 🧬. 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 baby's born 🙈

🌸 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. the extended universe 🌈

💜 SSL_METHOD (SSL Method)

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

💜 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 — speedrun strats ✨. 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 are 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. pick your fighters, bestie 🎀

🦋 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.

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>.

sign-off szn 🎀 — this lil' masterpiece is version 3.5.9, dated 2026-09-29, and it's filed under OSSL-GUIDE-L...INTRODUCTION(7ossl). mwah, go write some secure network code bestie 💖✨
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