man 7 ssl

OSSL-GUIDE-LIBSSL-INTRODUCTION(7osslOpenSOSSL-GUIDE-LIBSSL-INTRODUCTION(7ossl)

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>.

3.2.2                             2025-01OSSL-GUIDE-LIBSSL-INTRODUCTION(7ossl)