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 .