Hello Andrej

Let me add an additional explanation.

I use the term of “secure” RTC to mean:

- From host CPU side, the RTC could be accessed only by secure access.

- The RTC does not integrated in SoC.

- The RTC is connected by external bus like I2C to CPU and BMC.

 - The RTC is configured by BMC before starting a secure boot.

Let me ask you a question from another point of view.

If tf-a does not check the validity period, are there any security concern?

I am particularly concerned about the security of debug certificates.

I know tf-a does not support debug certificate, but if tf-a does, I think some protection is needed because debug certificate does not have tolerance against replay attack at least in my recognition.

Best Regards,

Masato Fukumori

From: Andrej Butok <andrey.butok@nxp.com>
Sent: Thursday, December 10, 2020 11:31 PM
To: Fukumori, Masato/福森 将人 <fukumori.masato@fujitsu.com>
Cc: tf-a <tf-a@lists.trustedfirmware.org>
Subject: RE: Question about validity period of X509 certificate

Hello Masato Fukumori,

So, we need a “secure” RTC, a “secure” entity to initialize/set it before the RTX can be used.

And it should be ready from very beginning, during a secure boot.

Yes, this check may be enabled.

Thanks,

Andrej

From: fukumori.masato@fujitsu.com <fukumori.masato@fujitsu.com>
Sent: Thursday, December 10, 2020 2:59 PM
To: Andrej Butok <andrey.butok@nxp.com>
Cc: tf-a <tf-a@lists.trustedfirmware.org>
Subject: RE: Question about validity period of X509 certificate

Hello Andrej

I think one way is implement RTC as a secure RTC, and configure it by secure entity like BMC.

Is there a reliability issue with this?

Best Regards,

Masato Fukumori

From: Andrej Butok <andrey.butok@nxp.com>
Sent: Thursday, December 10, 2020 10:33 PM
To: Fukumori, Masato/福森 将人 <fukumori.masato@fujitsu.com>
Cc: tf-a <tf-a@lists.trustedfirmware.org>
Subject: RE: Question about validity period of X509 certificate

Hello Masato Fukumori,

To check a "validity period" of a X 509 certificate,

you must be sure that your system date & time is set, correct and not changed.

Do you have a reliable way to achieve this?

Best regards,

Andrej Butok

From: TF-A <tf-a-bounces@lists.trustedfirmware.org> On Behalf Of fukumori.masato--- via TF-A
Sent: Thursday, December 10, 2020 2:23 PM
To: 'tf-a@lists.trustedfirmware.org' <tf-a@lists.trustedfirmware.org>
Subject: [TF-A] Question about validity period of X509 certificate

Hello.

I have a question about checking the X 509 certificate with tf-a.

My understanding is that tf-a does not check the "validity period" of the X 509 certificate.

I 'm not sure why tf-a doesn't check. Does anyone know this background?

Best Regards,

Masato Fukumori