although, for this implementation, you don't need to read out the chip anyway. you can just sniff the i2c bus to learn the keys.
I doubt there is 100% secure device, you know, it's all about tradeoffs. Given enough time and resources, nearly anything can be hacked.
By the way, if an attacker has gained physical access to the primary token (and also to the another factor, a password, since token on its own is not too helpful), they don't need to hack it in any way: they can just use it to log into the account, add some other token and revoke the existing one.
you might want to read up on EAL certification. Yes, there is no 100% secure device, but "secure" is about resistance to attack and an actual secure element is very resistant, as well as tamper evident.
u2f-zero could be hacked in 10s flat if you prep ahead of time.
> if an attacker has gained physical access to the primary token ... they don't need to hack it in any way
but they need long term access. with this device, short term access is enough to learn the key and then i can get access at a time of my choosing. i can also learn the counter value and you won't notice that i have gained access.
I'm not sure of any methods to bypass the read protection on normal MCUs in a 10s "drive by" attack. AFAIK, the special companies that provide flash readout (http://www.break-ic.com/), do so by decapping the chip and using involved imaging techniques. I suspect they get good at identifying various flash technologies, many of which are common to many chips. But don't think it's feasible for a drive by.
The I2C eavesdropping shouldn't be an issue because the ATECC508A does apply a mask.
If I lose my primary token, how can I not notice that? I look at it a few times per day, and I use it not rarely either, so I can't see how I can not notice if I lose it (even if it was replaced by the similarly-looking device).
RMASK and WMASK, which smelled not useful to you, are there exactly to prevent this from happening.
the key managed here
https://github.com/conorpp/u2f-zero/blob/master/firmware/src...
and here
https://github.com/conorpp/u2f-zero/blob/master/firmware/src...
just means that the "actual key" (unmasked) only lives in MCU memory for a short time -- the time from when the mask is applied and then until return to caller and memory is cleared, in the enrollment case I linked. In the authenticate case, it lives quite a bit longer because the stack space used for key storage isn't zero'd.
The atecc508 doesn't use or know how to use the mask. The actual key used for the encryption is passed in the clear over i2c.
(note that the key derivation you suggest is wrong because of the extra xor masking.)
http://ww1.microchip.com/downloads/en/DeviceDoc/20005927A.pd...