5.3.3 GenDig Command

The GenDig command uses a SHA-256 hash to combine a stored or input value with the contents of TempKey, which must be validated prior to the execution of this command. The stored value can come from one of the data slots, the Configuration zone, either of the OTP pages or the monotonic counters. The specific mode of the device determines which data are to be included in the GenDig calculation.

In some cases, it is required to run the GenDig prior to the execution of some commands. The command can be run multiple times to include more data in the digest prior to executing a given command. The resulting digest is retained in TempKey and can be used in one of four ways:

  1. It can be included as part of the message used by the MAC, Sign or CheckMac commands. Because the MAC response output incorporates both the data used in the GenDig calculation and the secret key from the MAC command, it serves to authenticate the data stored in the Data and/or OTP zones.
  2. A subsequent Read or Write command can use the digest to provide authentication and/or confidentiality for the data, in which case, it is known as a data protection digest.
  3. The command can be used for secure personalization by using a value from the transport key array. The resulting data protection digest would, then, be used by write.
  4. The input value, typically a nonce from a remote device, is combined with the current TempKey value to create a shared nonce in which both devices can attest to the inclusion of the RNG.