While the hostile algorithms that have captured the general public's imagination are viruses and worms, the more common direct problem on Unix systems are Trojan horses and time bombs. A Trojan horse is a program that appears to be something it is not. An example of a Trojan horse is a program that appears to be a calculator or other useful utility which has a hidden payload of inserting a back door onto its host system. A simple Trojan horse can be created by modifying any source code with the addition of a payload.