Cryptography and SSL/TLS Toolkit



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


The OpenSSL Project develops and distributes the source code for OpenSSL. You can obtain that source code via the OpenSSL website (

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

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 tar.gz archive file that you downloaded into an appropriate directory. Inside that archive you will find a file named 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 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 NOTES files there is also a 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 never replace the pre-built version with a different version as this may break your system.


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:

ossl-guide-libraries-introduction(7): An introduction to the OpenSSL libraries
ossl-guide-libcrypto-introduction(7): An introduction to libcrypto
ossl-guide-libssl-introduction(7): An introduction to libssl
ossl-guide-tls-introduction(7): An introduction to SSL/TLS in OpenSSL
ossl-guide-tls-client-block(7): Writing a simple blocking TLS client
ossl-guide-tls-client-non-block(7): Writing a simple nonblocking TLS client
ossl-guide-quic-introduction(7): An introduction to QUIC in OpenSSL
ossl-guide-quic-client-block(7): Writing a simple blocking QUIC client
ossl-guide-quic-multi-stream(7): Writing a simple multi-stream QUIC client
ossl-guide-quic-client-non-block(7): Writing a simple nonblocking QUIC client
ossl-guide-migration(7): Migrating from older OpenSSL versions

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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