OSSL-GUIDE-LI...-INTRODUCTION(7SSL) OpenSSL OSSL-GUIDE-LI...-INTRODUCTION(7SSL)
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. 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 2000-2025 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>.
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