Typo in documentation? ECB peripheral doesn't decrypt?

RE: Typo in documentation? ECB peripheral doesn't decrypt?

endnode — Sun, 09 Jul 2017 04:30:23 GMT

Yes Eric, "encryption only, not decryption".

RE: Typo in documentation? ECB peripheral doesn't decrypt?

BLE_lp — Sun, 09 Jul 2017 03:42:12 GMT

Thanks. I'm using mbed, and I was able to use a modified version of the main mbed TLS library for software AES decryption. Initial results seem like it works pretty well as far as speed is concerned.

It looks like both the nRF51822 and nRF82832 have the same ECB driver interface, so I'm assuming the nRF82832 doesn't do hardware AES decryption either, right?

RE: Typo in documentation? ECB peripheral doesn't decrypt?

endnode — Sat, 08 Jul 2017 16:09:28 GMT

Hi,

Yes that's type, HW "accelerated" AES encryption on nRF51 doesn't implement inverse decryption function. This is because Bluetooth doesn't need it (it uses CRT mode where AES encrypt function is combined with XOR stream cypher to perform both encrypt and decrypt) so there was no need to have it in HW. Indeed it makes it harder for application developers if they need it but the workarounds are easy: either use some modern methods such as AES-CRT which don't need it or implement it in SW (few hundred bytes in resulting binary if you know what you are doing...) There are numerous Q&As on this forum about this as well as nice blog post.

Cheers Jan

RE: Typo in documentation? ECB peripheral doesn't decrypt?

Turbo J — Sat, 08 Jul 2017 15:50:01 GMT

Yes this is a typo. Seems to be corrected only in the 13.x.y SDK, which is not available for NRF51x AFAIK.

The ECB hardware as documented in the reference manual can only encrypt AES-128 - only this mode is used for BT LE. Note that you can still use this hardware in some AES modes - like AES CFB mode.