The SPYRUS Metering System allows intellectual property suppliers to control access to electronic information with content-based meters. Information is distributed to users in encrypted form, and each user has a hardware token that contains the cryptographic keys necessary to decrypt the information as well as the meters that control the use of each key. The hardware token will not grant access to the information by decrypting it unless the supplier provided meter constraints are met. Since the hardware token includes a built-in real time clock, time-based meters can be enforced without relying on the easily modified computer system clock.
"Metering: a prepay technique", Proc. SPIE 3022, Storage and Retrieval for Image and Video Databases V, (15 January 1997); doi: 10.1117/12.263443; https://doi.org/10.1117/12.263443