• noun a set of security protocols that allows information to be transferred securely over the Internet and is used to set up and support secure virtual private networks. The system works with packets of data at the IP layer and supports two types of public-key data encryption. The first, called Transport mode, encrypts the data within a packet, but does not touch the header information, which contains the destination address, subject and source of a packet. The second mode, Tunnel mode, provides a greater level of security by encrypting all of the packet, including the header information.