A virtual private network (VPN) is a private data network that makes use of the public or shared infrastructure, maintaining privacy through the use of a tunneling protocol and security procedures. A virtual private network can be contrasted with a system of owned or leased lines that can only be used by one company. The main purpose of a VPN is to give the company the same capabilities of a private dedicated network at much lower cost by using a shared infrastructure. Phone companies have provided private shared resources for voice messages for over a decade. A virtual private network makes it possible to have the same protected sharing of public resources for data.
The main reason that companies use secure VPNs is so that they can transmit sensitive information over a shared network without needing to worry about who might see it. Everything that goes over a secure VPN is encrypted to such a level that even if someone captured a copy of the traffic, they could not read the traffic. Further, using a secure VPN allows the company to know that an attacker cannot alter the contents of their transmissions, such as by changing the value of financial transactions. Secure VPNs are particularly valuable for remote access where a user is connected to the Internet at a location not controlled by the network administrator, such as from a hotel room, airport kiosk, or home.
All traffic on the secure VPN must be encrypted and authenticated. Many of the protocols that are used to create secure VPNs allow the creation of VPNs that have authentication but no encryption. Although such a network is more secure than a network with no authentication, it is not a VPN because there is no privacy.
The security properties of the VPN must be agreed to by all parties in the VPN. Secure VPNs have one or more tunnels, and each tunnel has two endpoints. The administrators of the two endpoints of each tunnel must be able to agree on the security properties of the tunnel.
No one outside the VPN can affect the security properties of the VPN. It must be impossible for an attacker to change the security properties of any part of a VPN, such as to weaken the encryption or to affect which encryption keys are used.