RT1020 - How is the OTPMK set and used?

[rshipman]

Hi,

I have some questions regarding the OTPMK key:

1. How do I set the OTPMK for the RT1020?
2. When setting BEE_KEY0_SEL=b10 to use the OTPMK, does it use the OTPMK unchanged, or does it combine it with other things, like the unique ID? In other words, when you select OTPMK as the key source, does it use a derived key tied to the chip, or does it use the OTPMK as it is burnt into fuses?
3. What is the difference between using the OTPMK and SW_GP2 other than they are different fuses?

Thanks and regards,

Ronnie

[FelipeGarcia]

Hi Ronnie,

Please see my answers below.

1. The OTPMK fuse value is programmed by NXP during chip manufacture. You cannot set this value.

2. The OTPMK key value is derived, it can be used on the device but cannot be read even by NXP or directly from fuses.

3. The main difference is that SW-GP2 is user-defined key, not provisioned by NXP. If the SWGP2 fuse is used as a key you can write and read locking of the fuse using SW_GP2_LOCK and SW_GP2_RLOCK.

I hope this helps!

Best regards,

Felipe

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[rshipman]

Thank you Felipe.

More questions:

1. Is the 'derived OTPMK' actually the 'crypto key' which is also based on the 'unique id' and possibly other stuff?

2. In otherwords the 'derived OPTMK' is tied inextricably to the chip. So I cannot download and update the KIB in the field if I needed to because it needs to be generated by the chip. The only way to do this without the same chip is to use the SW_GP2 key? I'm not saying I need to do this, I just want to confirm I understand this. And this is also the reason for the DEK blob stuff if I wanted to use that.

Kind regards,

Ronnie

[FelipeGarcia]

Hi Ronnie,

1. OTPMK is unique secret key programmed and locked by NXP. The structure cannot be shared.

2. That is correct.

Best regards,

Felipe