OpenVPN is a free and open-source software application that implements virtual private network (VPN) techniques to create secure point-to-point or site-to-site connections in routed or bridged configurations and remote access facilities. It uses a custom security protocol that utilizes SSL/TLS for key exchange. It is capable of traversing network address translators (NATs) and firewalls. It was written by James Yonan and is published under the GNU General Public License (GPL).

OpenVPN allows peers to authenticate each other using pre-shared secret keys, certificates or username/password. When used in a multiclient-server configuration, it allows the server to release an authentication certificate for every client, using signatures and certificate authority. It uses the OpenSSL encryption library extensively, as well as the TLS protocol, and contains many security and control features.

OVPN has been ported and embedded to several systems. For example, DD-WRT has the server function. SoftEther VPN, a multi-protocol VPN server, has an implementation of Open VPN protocol.

Encryption
OVPN uses the OpenSSL library to provide encryption of both the data and control channels. It lets OpenSSL do all the encryption and authentication work, allowing OpenVPN to use all the ciphers available in the OpenSSL package. It can also use the HMAC packet authentication feature to add an additional layer of security to the connection (referred to as an “HMAC Firewall” by the creator). It can also use hardware acceleration to get better encryption performance.[11][12] Support for mbed TLS is available starting from version 2.3

Security
OpenVPN offers various internal security features. It has up to 256-bit encryption through OpenSSL library, although some service providers may offer lower rates, effectively providing some of the fastest VPN available to consumers.[19][20] It runs in userspace instead of requiring IP stack (therefore kernel) operation. OpenVPN has the ability to drop root privileges, use mlockall to prevent swapping sensitive data to disk, enter a chroot jail after initialization and apply a SELinux context after initialization.

OpenVPN runs a custom security protocol based on SSL and TLS rather than support IKE, IPsec, L2TP or PPTP. OpenVPN offers support of smart cards via PKCS#11-based cryptographic tokens.

OpenVPN setup
You can setup OpenVPN for all devices and OS by the below links easily:

OpenVPN for Windows (Video tutorials): https://potentvpn.com/openvpn-for-windows-10/

OpenVPN for Android: https://potentvpn.com/android-openvpn-setup/