Session transcript definition

Somewhat related to #8

We need an agreement on and common description of what data should be signed over by the device and what protection guarantees we need.

Historically in OpenID4VP, there was a bit of divergence, where the data being signed over by the Wallet was slightly different between the formats. We should align on a generic definition what exactly we sign over and how that is constructed. SessionTranscript is reconstructed, whereas KB-JWT contained the values and both contain different values (e.g., JwkTumbprint)

SessionTranscript for mDoc (vanilla OpenID4VP):

OpenID4VPHandover = [
"OpenID4VPHandover", ; A fixed identifier for this handover type
OpenID4VPHandoverInfoHash ; A cryptographic hash of OpenID4VPHandoverInfo
]

; Contains the sha-256 hash of OpenID4VPHandoverInfoBytes
OpenID4VPHandoverInfoHash = bstr

; Contains the bytes of OpenID4VPHandoverInfo encoded as CBOR
OpenID4VPHandoverInfoBytes = bstr .cbor OpenID4VPHandoverInfo

OpenID4VPHandoverInfo = [
clientId,
nonce,
jwkThumbprint,
responseUri
] ; Array containing handover parameters

clientId = tstr

nonce = tstr

jwkThumbprint = bstr

responseUri = tstr

This is kb-jwt for SD-JWT VC (vanilla OpenID4VP)

The following is a non-normative example of the unsecured payload of the Key Binding JWT of a Verifiable Presentation.

{
  "nonce": "n-0S6_WzA2Mj",
  "aud": "x509_san_dns:client.example.org",
  "iat": 1709838604,
  "sd_hash": "Dy-RYwZfaaoC3inJbLslgPvMp09bH-clYP_3qbRqtW4"
}

Start by reviewing issue #8 and the two examples in this issue: the mDoc SessionTranscript and the SD-JWT VC Key Binding JWT payload. Compare which values are signed or included and document a generic definition of the signed data and its protection guarantees. Done means the working group has agreed on the construction and common description.