Add PSA Extensions

Signed-off-by: Yogesh Deshpande <[email protected]>

musings/psa-extension.md

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+# INTRODUCTION
+
+This document describes how the existing ARM PSA profile can be extended
+to incorporate specific requirement of the user applications to support
+user data attestation.
+
+Throughout the document, RATS Passport Based Model is used.
+
+## Purpose 
+
+The document solves two main purpose.
+
+* Describe a procedure that allows Client Applications to bind application 
+
+provided "user data" to the ARM PSA Attestation Context.
+
+* It provides client applications enough details that enable them to build a binding
+
+layer beneath their applications which undertake full Passport based Attestation Protocol
+
+with an Attestation Verification Service, such as Veraison and return an Attestation Result
+
+as a Passport.
+
+### BACKGROUND
+
+The native ARM PSA Platform API as documented [here](https://arm-software.github.io/psa-api/attestation/1.0/IHI0085-PSA_Certified_Attestation_API-1.0.3.pdf)
+allows a client to request an Attestation Evidence (claims pertaining to the platform) 
+
+from the underlying platform by passing the "Freshness Parameter" known as "nonce"
+
+to get the Attesation token which is known as Evidence in the RATS protocol.
+
+### Extended PSA Evidence
+
+In order to bind the client supplied "user data" to the PSA Evidence the 
+
+PSA Evidence is augmented. The CDDL definition of the Augmented Evidence is as under:
+
+extendedPsaEvidence = {
+
+    "utoken" => UCCS-UAT,
+
+    "pat" => PSA-token
+
+}
+
+UCCS-UAT = <TBD601>({
+
+  &(eat_nonce: 10) => bstr .size (8..64) ; received freshness parameter from Verification Service
+
+  &(data: -7000) => bstr ; Supplied user data from the client
+
+  &(alg: -7001) => txt  ; The algorithm used to hash the utoken 
+
+})
+
+* Details of the extendedPSAEvidence
+
+1. `pat`: An ARM PSA Attestation token as define in [here](https://datatracker.ietf.org/doc/draft-tschofenig-rats-psa-token/). Note that this token represents the entire credential issued by the PSA Root of Trust
+
+2. `utoken` : A sidecar token used to link the application supplied user data to the PSA Platform token
+
+* Cryptographic linkage of "utoken" with "pat"
+
+In order to cryptographically link "utoken" with "pat", the sequence to be followed is as under:
+
+Upon receipt of "user data" from client application, 
+
+a. The binding layer initiates a Session with the "Attestation Verification Service" and receives 
+
+a "nonce" from the Service. The session intiation details can be found [here](https://github.com/veraison/docs/tree/main/api/challenge-response#challengeresponse)
+
+b. The binding layer encodes a "utoken" as a CBOR data.
+
+c. The encoded "utoken" is hashed using a suitable hash algorithm as detailed in the utoken. 
+
+d. The Hash("utoken") is used as a "challenge parameter" to obtain the `pat` from the underlying PSA platform.
+
+* CBOR data produced by encoding bytes .cbor extendedPsaEvidence is used as a attestation token that is sent to Verification Service (Veraison) to obtain the Attestation Results.
+
+* A new mediaType defined, as `application/eat-collection; profile=http://arm.com/psa-extension/1.0.0`
+will be used to exchange this Evidence. This needs to be set in ContentType when submitting Evidence.
+
+The exact steps are detailed [here](https://github.com/veraison/docs/tree/main/api/challenge-response)
+
+
+### Enhancements to Veraison 
+In order to support the extended PSA evidence Veraison Verification will be enhanced to:
+
+1. Support new Media type, i.e. "application/eat-collection; profile=http://arm.com/psa-extension/1.0.0"
+
+2. Veraison will support decoding of the extended PSA Evidence.
+
+3. A new Verification Plugin will be added to support Verification based on 
+   the new attestation scheme for psa extension
+
+4. A new Veraison extension will be added to the Eat Attestation Result[EAR](https://github.com/veraison/ear)to support setting the user data received from the Extended PSA Evidence, upon successful verification of the received Evidence