LPC55S69 Signed Image Boot - Questions on using Certificate Chains?

[jlongo]

I've read through Application Note 12283 (LPC55Sxx Secure Boot) and am trying to work through exactly how a certificate chain, if it is used, is walked on the processor.  My confusion is that the App Note only ever talks about one certificate - the root.  It mentions in section 3.4 that you can and should create a certificate chain, but then, how does the ROM validate the chain?  Figure 5 (Signed image format) only shows one x509 certificate that would be included in the firmware file.  What if my chain has a root -> intermediary -> end entity?  Is it expecting that one x509 certificate to include the entire chain in it?  If so, are there any length restrictions?  This just isn't clear :-(

Also, when generating certificate chains, there is the sequence id that has the revocation id, but each cert in the chain could have their own sequence numbers.  What is best practice here?  Should all of the sequence numbers in the chain be the same for the purpose of anti-rollback?  Or, is only the end entity sequence number checked?

I'm sorry if these questions seem silly, but considering the probability of me bricking my dev kit is high if I make a mistake in this stuff, I'd rather ask silly questions than be left with a paperweight.

[Sabina_Bruce · Accepted solution]

Hello Jeffrey,

Hope you are doing well.

I recommend to take a look at Chapter 7 of the reference manual. It explains with the details of what you are looking for, such as the structure, length, considerations, etc.

Specifically, section 7.3.5 where you will find the certificate block structure and its requirements and recommendations.

For the revocation ID, The x509 serial number field in the image signing certificate is used the following way: byte 0
shall be 0x3c, byte 1 shall be 0xc3, byte 2 and byte 3 form an unsigned 16-bit integer whose value is compared with the IMAGE_KEY_REVOKE value in the PFR. On mismatch, the image authentication process will fail.

The image signing certificate is the final certificate of the certificate table. It's not a problem if you use one certificate, for more details on the structure you can refer to the last part of section 7.3.5.2.

Hope this helps!

If you need any further information or have any further questions, please do not hesitate to get back to me. 

Best Regards,

Sabina

-----------------------------------------------------------------------------------------------------------------------

Note: If this post answers your question, please click the Correct Answer button. Thank you!

----------------------------------------------------------------------------------------------------------------------- 

[jlongo]

sabinabruce,

Thanks for the pointer to the additional information in the UM.  The explanation in the user manual that the "Certificate" is really a "Certificate Table" where it can contain multiple certificates was immensely helpful and cleared up a lot of my confusion around that.

And, from what I understand from your explanation, it is the revocation ID in the end entity (image signing certificate) that is used to enforce rollback protection.  If I put any revocation IDs anywhere else up in the chain, they would be ignored.

If you get a chance to confirm my understanding, that would be great. Thanks so much for your reply!!!

[Sabina_Bruce]

Hello,

 If I put any revocation IDs anywhere else up in the chain, they would be ignored.

Yes this is correct, it has to be the final certificate.

Not a problem, glad to help!

If you have any questions in the future don't hesitate to contact us again.

Best Regards,

Sabina

-----------------------------------------------------------------------------------------------------------------------

Note: If this post answers your question, please click the Correct Answer button. Thank you!

-----------------------------------------------------------------------------------------------------------------------