ESP32-S2 HMAC

[renatopinheiro]

Hi,

If I set an HMAC efuse key is possible to rewrite? (https://docs.espressif.com/projects/esp ... on-outline)
If not, is possible to test the HMAC without writing the key to the efuse?

[chegewara]

Probably an espressif team member should confirm that, but HMAC is supported:
https://esp32.com/viewtopic.php?f=10&p=56361

[renatopinheiro]

Anyone can help?

[ESP_Sprite]

Not sure about the HMAC capabilities, but in general it's physically impossible to re-write an eFuse once it's blown.

[ESP_Angus]

Hi renatopinheiro,

Sprite is correct, eFuse blocks are write-once and cannot be updated, and the hardware HMAC peripheral can only source the key from the eFuse block. If you want to calculate an HMAC from a key in memory, you can do this with the mbedTLS APIs in ESP-IDF (the SHA operation is still accelerated so the operation is just as fast).

Note also that there are 5 eFuse key blocks available in the ESP32-S2, and you can select which key block to read the key from each time the HMAC peripheral is used. So if you write one key block to do some initial tests, you still have some options to write other blocks later.

Angus

[renatopinheiro]

Thanks, Angus and Sprite.