38.6.2.6 Hardware Countermeasures against Differential Power Analysis Attacks

The AES module features four types of hardware countermeasures that are useful for protecting data against differential power analysis attacks:

• Type 1: Randomly add one cycle to data processing
• Type 2: Randomly add one cycle to data processing (other version)
• Type 3: Add a random number of clock cycles to data processing, subject to a maximum of 11/13/15 clock cycles for key sizes of 128/192/256 bits
• Type 4: Add random spurious power consumption during data processing

By default, all countermeasures are enabled, but require a write in the DRNGSEED register to be effective. One or more of the countermeasures can be disabled by programming the Countermeasure Type field in the Control A (CTRLA.CTYPE) register. The countermeasures use random numbers generated by a deterministic random number generator embedded in AES module. The seed for the random number generator is written to the RANDSEED register. Note also that a new seed must be written after a change in the keysize. Note that enabling countermeasures reduces AES module’s throughput. In short, the throughput is highest with all the countermeasures disabled. On the other hand, with all of the countermeasures enabled, the best protection is achieved but the throughput is worst.