In version 4 of the Internet protocol, an IP "address" was used to identify both a computer and the point at which that computer was connected to the network. This is acceptable when the computer's point of connection never changes, but might become a problem when computers are mobile. The IETF is proposing a change to the Internet Protocol which allows a host's address to change over time. The last draft of the proposal was rejected as unacceptable because it introduced too many security problems. We present a cryptographic protocol which is intended to reduce these security problems to a manageable level.