Abstract: This report describes a class of techniques which allow either the attack of a computer or to catch the key of a cryptosystem by using a pair of (or combined) viruses, one of them being hidden in ciphertext. These technique- s are valid for any operating system and can be efficiently implemented in any programming language. In order to avoid detection, the viral infection is very limited and uses polymorphic techniques. Moreover the main virus erases itself after the payload action. The general structure of the two viruses is presented and the methods of protection against such attacks is finally envisaged.