=pod

=head1 NAME

ossl-guide-introduction
- OpenSSL Guide: An introduction to OpenSSL

=head1 WHAT IS OPENSSL?

OpenSSL is a robust, commercial-grade, full-featured toolkit for general-purpose
cryptography and secure communication. Its features are made available via a
command line application that enables users to perform various cryptography
related functions such as generating keys and certificates. Additionally it
supplies two libraries that application developers can use to implement
cryptography based capabilities and to securely communicate across a network.
Finally, it also has a set of providers that supply implementations of a broad
set of cryptographic algorithms.

OpenSSL is fully open source. Version 3.0 and above are distributed under the
Apache v2 license.

=head1 GETTING AND INSTALLING OPENSSL

The OpenSSL Project develops and distributes the source code for OpenSSL. You
can obtain that source code via the OpenSSL website
(L<https://www.openssl.org/source>).

Many Operating Systems (notably Linux distributions) supply pre-built OpenSSL
binaries either pre-installed or available via the package management system in
use for that OS. It is worth checking whether this applies to you before
attempting to build OpenSSL from the source code.

Some third parties also supply OpenSSL binaries (e.g. for Windows and some other
platforms). The OpenSSL project maintains a list of these third parties at
L<https://github.com/openssl/openssl/wiki/Binaries>.

If you build and install OpenSSL from the source code then you should download
the appropriate files for the version that you want to use from the link given
above. Extract the contents of the B<tar.gz> archive file that you downloaded
into an appropriate directory. Inside that archive you will find a file named
B<INSTALL.md> which will supply detailed instructions on how to build and
install OpenSSL from source. Make sure you read the contents of that file
carefully in order to achieve a successful build. In the directory you will also
find a set of B<NOTES> files that provide further platform specific information.
Make sure you carefully read the file appropriate to your platform. As well as
the platform specific B<NOTES> files there is also a B<NOTES-PERL.md> file that
provides information about setting up Perl for use by the OpenSSL build system
across multiple platforms.

Sometimes you may want to build and install OpenSSL from source on a system
which already has a pre-built version of OpenSSL installed on it via the
Operating System package management system (for example if you want to use a
newer version of OpenSSL than the one supplied by your Operating System). In
this case it is strongly recommended to install OpenSSL to a different location
than where the pre-built version is installed. You should B<never> replace the
pre-built version with a different version as this may break your system.

=head1 CONTENTS OF THE OPENSSL GUIDE

The OpenSSL Guide is a series of documentation pages (starting with this one)
that introduce some of the main concepts in OpenSSL. The guide can either be
read end-to-end in order, or alternatively you can simply skip to the parts most
applicable to your use case. Note however that later pages may depend on and
assume knowledge from earlier pages.

The pages in the guide are as follows:

=over 4

=item L<ossl-guide-libraries-introduction(7)>: An introduction to the OpenSSL libraries

=item L<ossl-guide-libcrypto-introduction(7)>: An introduction to libcrypto

=item L<ossl-guide-libssl-introduction(7)>: An introduction to libssl

=item L<ossl-guide-tls-introduction(7)>: An introduction to SSL/TLS in OpenSSL

=item L<ossl-guide-tls-client-block(7)>: Writing a simple blocking TLS client

=item L<ossl-guide-tls-client-non-block(7)>: Writing a simple nonblocking TLS client

=item L<ossl-guide-tls-server-block(7)>: Writing a simple blocking TLS server

=item L<ossl-guide-quic-introduction(7)>: An introduction to QUIC in OpenSSL

=item L<ossl-guide-quic-client-block(7)>: Writing a simple blocking QUIC client

=item L<ossl-guide-quic-server-block(7)>: Writing a simple blocking QUIC server

=item L<ossl-guide-quic-multi-stream(7)>: Writing a simple multi-stream QUIC client

=item L<ossl-guide-quic-server-non-block(7)>: Writing a simple nonblocking QUIC server

=item L<ossl-guide-quic-client-non-block(7)>: Writing a simple nonblocking QUIC client

=item L<ossl-guide-migration(7)>: Migrating from older OpenSSL versions

=back

=head1 COPYRIGHT

Copyright 2023-2025 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
