Re: [Rats] Review of draft-birkholz-rats-daa

[Christopher Newton <[e-mail]>]

Hi Thomas,

Thank you for your comments.

> Is it really necessary to introduce the new "DAA Issuer" role?
> 
> It seems to me that if the JOIN and SIGN phases are considered as two separate attestation protocols, the Issuer could be mapped to a couple of well-known RATS roles depending on the phase it is involved in:
> * Verifier for JOIN - plus an authorisation RP on top that grants the group credentials to the authenticated Attester;
> * Endorser for SIGN.

Yes, the DAA Issuer is a separate role. it is effectively a Certificate Authority for the DAA keys. The difference from a PKI CA is that the Verifier uses the public key of the DAA Issuer to directly verify the DAA signature. The signer's public key is not available to the verifier.

For a TPM, the Endorser is the chip manufacturer who provides a PKI certificate for the TPM's endorsement key. However, the endorsement key cannot be used as a DAA key as it is an encryption key. it is only used to enable the DAA Issuer to authenticate the TPM in a deniable way. After the Issuer authenticates the TPM it provides a DAA credential to the DAA signing key. The Endorser can take on the role of the DAA Issuer, but this is not a requirement. 

We hope that this makes things a little clearer, we could add more detail in the RATS DAA document if this would help.

Regards,

Chris and Liqun.


--
Dr Christopher Newton
Surrey Centre for Cyber Security
Department of Computer Science
University of Surrey
Guildford, Surrey, GU2 7XH, UK
--

-----Original Message-----
From: Thomas Fossati <[e-mail]> 
Sent: 26 May 2021 13:04
To: [e-mail]
Cc: [e-mail]; Thomas Fossati <[e-mail]>
Subject: Review of draft-birkholz-rats-daa

Hi RATS-DAA authors,

I have reviewed draft-birkholz-rats-daa-00 and I think this is a useful document, plus it is short and sweet.

I may have a few editorial suggestions, but I'd like to ask one meta question first - apologies if this was brought up in previous
conversations:

Is it really necessary to introduce the new "DAA Issuer" role?

It seems to me that if the JOIN and SIGN phases are considered as two separate attestation protocols, the Issuer could be mapped to a couple of well-known RATS roles depending on the phase it is involved in:
* Verifier for JOIN - plus an authorisation RP on top that grants
  the group credentials to the authenticated Attester;
* Endorser for SIGN.

Cheers, thank you.

t








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