56.4.4.2 Key Writing and Automatic Hash Subkey Calculation

Whenever a new key is written to the hardware, two automatic actions are processed:

• GCM Hash Subkey H generation—The GCM hash subkey (H) is automatically generated. The GCM hash subkey generation must be complete before doing any other action. AES_ISR.DATRDY indicates when the subkey generation is complete (with interrupt if configured). The GCM hash subkey calculation is processed with the formula H = CIPHER(Key, <128 bits to zero>). The generated GCM H value is then available in AES_GCMHRx. If the application software requires a specific hash subkey, the automatically generated H value can be overwritten in AES_GCMHRx.
AES_GCMHRx can be written after the end of the hash subkey generation (see AES_ISR.DATRDY) and prior to starting the input data feed.
• AES_GHASHRx Clear—AES_GHASHRx are automatically cleared. If a hash initial value is needed for the GHASH, it must be written to AES_GHASHRx
  • after a write to the AES Key Register, if any
  • before starting the input data feed