TARDIS: Time and Remanence Decay in SRAM to Implement Secure Protocols on Embedded Devices without Clocks

Lack of a locally trustworthy clock makes security protocols challenging to implement on batteryless embedded devices such as contact smartcards, contactless smartcards, and RFID tags. A device that knows how much time has elapsed between queries from an untrusted reader could better protect against attacks that depend on the existence of a rate-unlimited encryption oracle. The TARDIS (Time and Remanence Decay in SRAM) helps locally maintain a sense of time elapsed without power and without special-purpose hardware. The TARDIS software computes the expiration state of a timer by analyzing the decay of existing on-chip SRAM. The TARDIS enables coarse-grained, hourglass-like timers such that cryptographic software can more deliberately decide how to throttle its response rate. Our experiments demonstrate that the TARDIS can measure time ranging from seconds to several hours depending on hardware parameters. Key challenges to implementing a practical TARDIS include compensating for temperature and handling variation across hardware. Our contributions are (1) the algorithmic building blocks for computing elapsed time from SRAM decay; (2) characterizing TARDIS behavior under different temperatures, capacitors, SRAM sizes, and chips; and (3) three proof-of-concept implementations that use the TARDIS to enable privacy-preserving RFID tags, to deter double swiping of contactless credit cards, and to increase the difficulty of brute-force attacks against e-passports.