Abstract: A cryptosystem prevents replay attacks within existing authentication protocols, susceptible to such attacks but containing a random component, without requiring modification to said protocols. The entity charged with authentication maintains a list of previously used bit patterns, extracted from a portion of the authentication message connected to the random component. If the bit pattern has been seen before, the message is rejected; if the bit pattern has not been seen before, the bit pattern is added to the stored list and the message is accepted.