Port-knocking has recently become a popular concept, and a common source of discussion. Many groups and communities have argued about the importance and viability of port-knocking as a security concept, or as an additional security measure. One of the main complaints about port-knocking is that one can implement trivial replay attacks against any static port-knocking system. This problem can be remedied by implementing a cryptographic system in tandem with a traditional port-knocking ideas. One of the simplest means of implementing a cryptographic port knock is by using a one-time-password system (otp or s/key). Such a system has been implemented in COK, which is trivially extensible, and flexible enough to be useful in a production environment, which will allow for port-knocking from public locations with a minimum amount of pain.