Abstract: A method, system, and devices are provided in which modified digital signatures are used to provide a dynamically generated number suitable for use in transactions requiring validation. The method uses symmetric key encryption to encode a message comprising authorization information and may use compression algorithms to provide a truncated message digest such that the dynamic number may be processed by existing credit card or other authorization systems. In part, this method is an improvement over other validation methods as decryption, which requires greater computing power, is not required. The method may be performed through the use of various devices. For example, credit cards may utilize the method to dispose the dynamic number in a magnetic strip or to transmit the dynamic number via radio transmitter. Smart cards, smart phones, or USB devices, optionally may be utilized to perform the inventive method.