doc/man7/ossl-guide-libssl-introduction.pod - third_party/openssl - Git at Google

 =pod

 =head1 NAME

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

 =head1 INTRODUCTION

 The OpenSSL C<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 C<libcrypto> for its underlying cryptographic
 operations (see L<ossl-guide-libcrypto-introduction(7)>).

 The set of APIs supplied by C<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 C<libssl> will include the F<< <openssl/ssl.h> >>
 header file and will typically use two main data structures, i.e. B<SSL> and
 B<SSL_CTX>.

 An B<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 B<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 B<SSL> object used for the connection also
 represents that stream. See L<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 B<SSL> object can represent the
 underlying connection, or a stream, or both. Where multiple streams are in use
 a separate B<SSL> object is used for each one. See
 L<ossl-guide-quic-introduction(7)> for more information.

 An B<SSL_CTX> object is used to create the B<SSL> object for the underlying
 connection. A single B<SSL_CTX> object can be used to create many connections
 (each represented by a separate B<SSL> object). Many API functions in libssl
 exist in two forms: one that takes an B<SSL_CTX> and one that takes an B<SSL>.
 Typically settings that you apply to the B<SSL_CTX> will then be inherited by
 any B<SSL> object that you create from it. Alternatively you can apply settings
 directly to the B<SSL> object without affecting other B<SSL> objects. Note that
 you should not normally make changes to an B<SSL_CTX> after the first B<SSL>
 object has been created from it.

 =head1 DATA STRUCTURES

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

 =over 4

 =item B<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 B<SSL_CTX>.

 =item B<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 B<SSL_SESSION> object. A single successful connection with a
 peer may generate zero or more such B<SSL_SESSION> objects for use in future
 connection attempts.

 =item B<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 B<SSL_CIPHER> object.

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

 =back

 =head1 FURTHER READING

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

 See L<ossl-guide-libcrypto-introduction(7)> for an introduction to C<libcrypto>.

 =head1 SEE ALSO

 L<ossl-guide-libcrypto-introduction(7)>, L<ossl-guide-tls-introduction(7)>,
 L<ossl-guide-quic-introduction(7)>

 =head1 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
 L<https://www.openssl.org/source/license.html>.

 =cut
	=pod

	=head1 NAME

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

	=head1 INTRODUCTION

	The OpenSSL C<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 C<libcrypto> for its underlying cryptographic
	operations (see L<ossl-guide-libcrypto-introduction(7)>).

	The set of APIs supplied by C<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 C<libssl> will include the F<< <openssl/ssl.h> >>
	header file and will typically use two main data structures, i.e. B<SSL> and
	B<SSL_CTX>.

	An B<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 B<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 B<SSL> object used for the connection also
	represents that stream. See L<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 B<SSL> object can represent the
	underlying connection, or a stream, or both. Where multiple streams are in use
	a separate B<SSL> object is used for each one. See
	L<ossl-guide-quic-introduction(7)> for more information.

	An B<SSL_CTX> object is used to create the B<SSL> object for the underlying
	connection. A single B<SSL_CTX> object can be used to create many connections
	(each represented by a separate B<SSL> object). Many API functions in libssl
	exist in two forms: one that takes an B<SSL_CTX> and one that takes an B<SSL>.
	Typically settings that you apply to the B<SSL_CTX> will then be inherited by
	any B<SSL> object that you create from it. Alternatively you can apply settings
	directly to the B<SSL> object without affecting other B<SSL> objects. Note that
	you should not normally make changes to an B<SSL_CTX> after the first B<SSL>
	object has been created from it.

	=head1 DATA STRUCTURES

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

	=over 4

	=item B<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 B<SSL_CTX>.

	=item B<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 B<SSL_SESSION> object. A single successful connection with a
	peer may generate zero or more such B<SSL_SESSION> objects for use in future
	connection attempts.

	=item B<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 B<SSL_CIPHER> object.

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

	=back

	=head1 FURTHER READING

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

	See L<ossl-guide-libcrypto-introduction(7)> for an introduction to C<libcrypto>.

	=head1 SEE ALSO

	L<ossl-guide-libcrypto-introduction(7)>, L<ossl-guide-tls-introduction(7)>,
	L<ossl-guide-quic-introduction(7)>

	=head1 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
	L<https://www.openssl.org/source/license.html>.

	=cut