Tag Archives: AES-CMAC

Building a custom door chime with an ATtiny85 and AES-CMAC


AES-CMAC on an ATtiny85? You bet! 


Our friends at Hackaday recently brought to our attention a nifty little custom door chime, powered by an ATtiny85 and equipped with AES-CMAC for message signing. While Daniel Neville could’ve used a commercial product, it’s evident that the Maker wanted to pack a little extra security into the pint-sized device.

Prototype-chime-components

Controlled by the tinyAVR MCU, the gate buzzer features an LM380N audio amplifier as well as a low-cost 315 MHz receiver. Using AVR assembly, the Maker managed to cram everything into the 8 Kbytes of Flash on the ATtiny85, including an AES cypher-based message authentication code. The transmitting gadget signs the request with a key shared between both devices, and the receiver verifies that the message is indeed from a trusted sender.

chime-layout-annotated

“The chime learns up to eight transmitters with the same shared key but with different serial numbers and different secret AES-CMAC keys. Each transmitter can have either one or two sensors to monitor. Each sensor on each transmitter is associated with one of sixteen possible sets of sounds. Some sound sets include activation, deactivation and prolonged activation reminder sounds while others only include the activation sound,” Neville writes.

Intrigued? You can read all about the build and access its source codes here.