topic Re: Signing firmware , how to restrict ? in LPC Microcontrollers

Signing firmware , how to restrict ?

trescurieux — Mon, 03 Aug 2020 14:49:14 GMT

Sorry in advance if this is a dumb question about signed firmware.

I've read the Secure Boot AN and the LPC55 user manual, but there is something i'm missing

The secure boot firmware contains the signing public key (in the certificate(s)), and is signed with it if it got it correctly.

So far so good.

The part i dont understand is how a LPC55 is pinned to a certificate or a set of certificates ?

i.e. how can i prevent a completely valid secure firmware e. signed by somebody else to be used ?

There is something in the PFR to deal with that i guess, but i could not figure it out.

I expected the root public key somewhere there, so that i could be used to validate the whole chain, and reject every signature not coming from MY certificate chain but i didnt find it.

If someone could kindly redirect me to the relevant part of the doc /and or shed some light that would be appreciated

Thank you in advance

Re: Signing firmware , how to restrict ?

trescurieux — Mon, 03 Aug 2020 15:41:51 GMT

Hello again

Maybe just the hash of the root certificate is stored in the PFR and is checked against the one in the firmware image ?

So only firmware(s) with the right root certificate hash are accepted ?

Thanks

Re: Signing firmware , how to restrict ?

Alice_Yang — Tue, 04 Aug 2020 08:14:47 GMT

Hello Tres,

Yes, pay attention in "5.5CMPA page preparation" of secure boot AN, program RKTH

to chip, this hash is generated from certificates during signing process. So it corresponding to your private key and certificate.

Regards,

Alice

Re: Signing firmware , how to restrict ?

trescurieux — Tue, 04 Aug 2020 11:26:53 GMT

Thanks a lot !

Best Regards

Tres