NAME

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

INTRODUCTION

       The OpenSSL "libssl" library provides implementations of several secure network communications protocols.
       Specifically it provides SSL/TLS (SSLv3, TLSv1, TLSv1.1, TLSv1.2 and TLSv1.3), DTLS (DTLSv1 and DTLSv1.2)
       and QUIC (client side only). The library depends on "libcrypto" for its underlying cryptographic
       operations (see ossl-guide-libcrypto-introduction(7)).

       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.

       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 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. 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). 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.

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 is to
           represent the client or the server, and whether it is to use SSL/TLS, DTLS or QUIC (client only).  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. 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.

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

       Copyright 2000-2023 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>.

3.4.1                                              2025-04-03               OSSL-GUIDE-LIBSSL-INTRODUCTION(7SSL)