[Questions]DAC private key length in factory data confusing

[susepc]

Dear sir:

        The DAC private key is generated using P256, and the key length is 256 bits, which is 32 bytes.

        As below link, why is 68 bytes planned in the factory data?

developer.nordicsemi.com/.../nrfconnect_factory_data_configuration.html

dac_key

DAC private key

68 B

byte string

mandatory

The private key associated with the Device Attestation Certificate (DAC). This key should be encrypted and maximum security should be guaranteed while generating and providing it to factory data.

[AHaug]

Hi,

I've had a look in the Matter 1.2 specification and I could not find a specific reason myself for why this was the case, so I've asked the developers for some input. I'll get back to you with a verification when I hear from them.

Kind regards,
Andreas

[AHaug]

Ah, it makes sense that it is 68 Bytes.

The field is written in byte string format, i.e if the raw DAC key is 32 Bytes length, then the dac_key field value should have 32*2+len(hex:) => 64+4 => 68 Bytes.

Kind regards,
Andreas

[susepc]

 AHaug 

        Thanks for your quickly response!

        Does the last 4 bytes of data represent the dac_key field value length or the ASCII of "hex:"?

        If it is the former, what is the role of this length?

[AHaug]

Hi,

The byte string item is explained as: 

Kind regards,
Andreas

[susepc]

 AHaug 

        Thanks for your great support!

        Sorry to bother you again, I have one last question：

        Can I use raw byte to store key not using byte string?

        If not, please let me know the reason, thx!

BR

Chuan

[AHaug]

Hi,

No worries! :) 

susepc said:

    Can I use raw byte to store key not using byte string?

This parameter is a byte string based on the Matter spec due to how octets, keys (which is an octet string AFAIK) and more are defined, so according to my understanding of the specification you will have to comply with the byte string format to avoid breaking the spec.

Ref: Matter Core Spec v 1.2, chapter 4.13.2.4, 6.5.10, 10.3.6

Kind regards,
Andreas

[susepc]

AHaug 

        Thanks for your strong support!

         I'll close this ticket soon.

BR

Chuan