Define usage of HPKE and info structure to be used.

Author: GarethCOliver

1.1/openid-4-verifiable-presentations-1_1.md

@@ -1419,7 +1419,78 @@ While this shows the payload of the above encrypted Authorization Response examp
 Note that for the ECDH JWE algorithms (from Section 4.6 of [@!RFC7518]), the `apu` and `apv` values are inputs
 into the key derivation process that is used to derive the content encryption key. Regardless of the algorithm used, the values are always part of the AEAD tag computation so will still be bound to the encrypted response.
 
-Note: For encryption, implementers have a variety of options available through JOSE, including the use of Hybrid Public Key Encryption (HPKE) as detailed in [@I-D.ietf-jose-hpke-encrypt]. 
+### Encryption using HPKE
+Hybrid Public Key Encryption MAY be used by specifying the `alg` value according to [@I-D.ietf-jose-hpke-encrypt]. When it is, a `session_info` structure is calculated by the Wallet and the Verifier independently and included as mutually known private information. This is to ensure that encryption fails closed when the session information needed for verification of the credential is invalid.
+
+When the response mode is `direct_post.jwt` the encoding is as follows: 
+
+```example
+  session_info = ASCII("OpenID4VP si") ||
+                 BYTE(255) ||
+                 ASCII(clientId) ||
+                 BYTE(255) ||
+                 ASCII(nonce) ||
+                 BYTE(255) ||
+                 BYTE(jwkThumbprint) ||
+                 BYTE(255) ||
+                 ASCII(responseUri)
+```
+
+Where:
+
+- ASCII("OpenID4VP si"): A fixed ASCII string identifying this session_info structure.
+- BYTE(255): In each occurence is a separator byte (0xFF) used to delimit fields.
+- ASCII(clientId): The `client_id` request parameter. If applicable, this includes the Client Identifier Prefix.
+- ASCII(nonce): The `nonce` request parameter.
+- Byte(jwkThumbprint): The `JWK SHA-256 Thumbprint as defined in [RFC7638], encoded as a Byte, of the Verifier's public key used to encrypt the response.
+- ASCII(responseUri): The `redirect_uri` or `response_uri` request parameter, depending on which is present, as determined by the Response Mode.
+
+The following is a non-normative example of the `session_info` structure for `direct_post.jwt` response mode:
+
+```example
+  "OpenID4VP si\xffx509_san_dns:example.com\xffexc7gBkxjx1rdc9udRrveKvSsJIq80avlXeLHhGwqtA\xffB\x83ì\x92zàò\x08Úª-\x02j\x81O+"Ü¥,ø_ú\x8f?\x86&ƽf\x90G\xffhttps://example.com/response"
+```
+
+The corresponding hexadecimal representation is:
+
+```
+4f70656e4944345650207369ff783530395f73616e5f646e733a6578616d706c652e636f6dff6578633767426b786a7831726463397564527276654b7653734a4971383061766c58654c48684777717441ff4283ec927ae0f208daaa2d026a814f2b22dca52cf85ffa8f3f8626c6bd669047ff68747470733a2f2f6578616d706c652e636f6d2f726573706f6e7365
+```
+
+When the response mode is `dc_api.jwt` the encoding is as follows:
+
+```example
+  session_info = ASCII("OpenID4VPDCAPI si") ||
+                 BYTE(255) ||
+                 ASCII(origin) ||
+                 BYTE(255) ||
+                 ASCII(nonce) ||
+                 BYTE(255) ||
+                 BYTE(jwkThumbprint) ||
+
+```
+
+Where:
+
+- ASCII("OpenID4VPDCAPI si"): A fixed ASCII string identifying this session_info structure.
+- BYTE(255): In each occurence is a separator byte (0xFF) used to delimit fields.
+- ASCII(origin): The ASCII string representing the Origin of the request as described in (#dc_api_request). It MUST NOT be prefixed with origin:.
+- ASCII(nonce): The `nonce` request parameter.
+- Byte(jwkThumbprint): The `JWK SHA-256 Thumbprint as defined in [RFC7638], encoded as a Byte, of the Verifier's public key used to encrypt the response.
+
+The following is a non-normative example of the `session_info` structure for `dc_api.jwt` response mode:
+
+```example
+  "OpenID4VPDCAPI si\xffhttps://example.com\xffexc7gBkxjx1rdc9udRrveKvSsJIq80avlXeLHhGwqtA\xffB\x83ì\x92zàò\x08Úª-\x02j\x81O+"Ü¥,ø_ú\x8f?\x86&ƽf\x90G"
+```
+
+The corresponding hexadecimal representation is:
+
+```
+4f70656e49443456504443415049207369ff68747470733a2f2f6578616d706c652e636f6dff6578633767426b786a7831726463397564527276654b7653734a4971383061766c58654c48684777717441ff4283ec927ae0f208daaa2d026a814f2b22dca52cf85ffa8f3f8626c6bd669047
+```
+
+The `session_info` structure's bytes are used as the value of the `info` parameter when using Integrated Encryption as the Key Management Mode. If the `recipient_structure` is being used then it is used as the value of the `recipient_extra_info` parameter instead.
 
 ### Response Mode "direct_post.jwt" {#direct_post_jwt}
 
@@ -3562,4 +3633,5 @@ The technology described in this specification was made available from contribut
 
    -01
 
-   * Clarify that `encrypted_response_enc_values_supported` applies only if JWE content encryption algorithm is used; e.g., it does not apply to JOSE HPKE
+   * Add usage of HPKE an specfication for the `info` parameter. 
+   * Clarify that `encrypted_response_enc_values_supported` applies only if JWE content encryption algorithm is used; e.g., it does not apply to JOSE HPKE