impact of encryption on battery performance

[Amit Chahar]

I want to use AES ECB block present on nrf51 and nrf52 boards for resolvable private address (RPA) generation.

1. Are there any measurements about how much battery the AES-ECB block consumes for an operation?

2. For eg. If the battery life of my board is 2 years without any encryption present, then how much it will be after adding a single encryption operation per advertisement with an advertisment interval of 1 second?

3. If I use the softdevice functionality for generating RPA, then what will be the effect of that on battery life because RPA generation needs fetching 3 bytes from the random bytes pool and an AES-ECB operation to encrypt those 3 bytes.

Please provide this info for both nrf51 and nrf52. Thanks in advance.

[bjorn-spockeli]

Hi Amit,

Q1: the run current of the AES ECB peripheral and the time it takes to encrypt a 16byte block on the nRF51 is stated in the nRF51x22 Product Specifications.

Unfortunatley, we do not have any run current measurements for the AES ECB module on the nRF52832, so you will have to measure this.

Q2: You can calculate the average current consumption of a single AES ECB block encrypt(i.e. 16bytes) by multiplying 550uA*8.5us = 4.675nA

Q3: RPA? Do you mean the Random Number Generator(RNG)?

[Amit Chahar]

Q1. Can you tell if the current consumption for AES-ECB module on nRF52 would be higher or lower than the current consumption by nRF51 ?

Q3. RPA means Resolvable Private Address. To generate a RPA, 3 random bytes are needed. I am assuming that softdevice uses RNG for these 3 random bytes.

[bjorn-spockeli]

Q1: I cannot find any data on this in our internal Product Spec, so I would assume that the current consumption is either identical or lower.

Q3: Of course, sorry for that. We do not have any measurements showing the current consumption impact of using a RPA. Yes, the RNG is used for that and you can find the Electrical specifications for it here.

[Amit Chahar]

Does softdevice random number generator function use bias correction?

[bjorn-spockeli]

Yes, our SoftDevices enable the Bias Correction of the RNG.