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The Texas Instruments digital signature transponder is a cryptographically enabled radio-frequency identification device used in a variety of wireless authentication applications. The largest deployments of the DST include the Exxon-Mobil Speedpass payment system , as well as a variety of vehicle immobilizer systems used in many late model Ford, Lincoln, Mercury, Toyota, and Nissan vehicles.
The DST is an unpowered "passive" transponder which uses a proprietary block cipher to implement a challenge–response authentication protocol. Each DST tag contains a quantity of non-volatile RAM, which stores a 40-bit encryption key. This key is used to encipher a 40-bit challenge issued by the reader, producing a 40-bit ciphertext, which is then truncated to produce a 24-bit response transmitted back to the reader. Verifiers verify this challenge by computing the expected result and comparing it to the tag response. Transponder encryption keys are user programmable, using a simple over-the-air protocol. Once correctly programmed, transponders may be "locked" through a separate command, which prevents further changes to the internal key value. Each transponder is factory provisioned with a 24-bit serial number and 8-bit manufacturer code. These values are fixed, and cannot be altered.