kbs: document attestation policy selection

Describe the "id" field of the RCAR Request in the protocol document,
including that its accepted values are deployment specific and that an
unmapped id is rejected, both of which a KBC implementation has to know.
State that selecting an id is not an authorization decision so that
readers do not mistake it for one.

Document policy_id_map for each attestation service type, and note that
for Intel TA a selected id replaces the configured policy_ids.

Signed-off-by: Xynnn007 <xynnn@linux.alibaba.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

Author: Xynnn007

kbs/docs/config.md

@@ -154,6 +154,15 @@ Concrete attestation service can be set via `type` field. Supported attestation
 
 Due to different `type` field, properties are different.
 
+`timeout` and `policy_id_map` apply to every type. The latter is a table mapping
+each ID a client may select to one or more policy IDs:
+
+```toml
+[attestation_service.policy_id_map]
+alice = ["alice-strict"]
+bob = ["bob-cpu", "bob-gpu"]
+```
+
 #### Built-In CoCo AS
 
 When `type` is set to `coco_as_builtin`, the following properties can be set.
@@ -164,6 +173,7 @@ When `type` is set to `coco_as_builtin`, the following properties can be set.
 | Property                   | Type                        | Description                                              | Default                                                                                                       |
 |----------------------------|-----------------------------|----------------------------------------------------------|---------------------------------------------------------------------------------------------------------------|
 | `timeout`                  | Integer                     | The maximum time (in minutes) of the attestation session | 5                                                                                                             |
+| `policy_id_map`            | Map of String array         | AS policies selectable by a client, keyed by ID. See [RCAR `Request`][ps]| `{}`                                                                        |
 | `rvps_config`              | [RVPSConfiguration][2]      | RVPS configuration                                       | See [RVPSConfiguration][2]                                                                                    |
 | `attestation_token_broker` | [AttestationTokenBroker][1] | Attestation result token configuration.                  | See [AttestationTokenBroker][1]                                                                               |
 | `verifier_config`          | Object                      | Optional verifier specific configuration (for example TPM)| See [Verifier Configuration][vcfg]                                                                            |
@@ -173,6 +183,7 @@ When `type` is set to `coco_as_builtin`, the following properties can be set.
 [3]: #keyvaluestorage
 [4]: #tokensignerconfig
 [vcfg]: ../../attestation-service/docs/config.md#verifier-configuration
+[ps]: ./kbs_attestation_protocol.md#request
 
 ##### AttestationTokenBroker
 
@@ -234,6 +245,7 @@ The following properties can be set.
 | Property    | Type    | Description                                                                                                                   | Default                  |
 |-------------|---------|-------------------------------------------------------------------------------------------------------------------------------|--------------------------|
 | `timeout`   | Integer | The maximum time (in minutes) between RCAR handshake's `auth` and `attest` requests                                           | 5                        |
+| `policy_id_map` | Map of String array | AS policies selectable by a client, keyed by ID. See [RCAR `Request`][ps]                                    | `{}`                     |
 | `as_addr`   | String  | The URL of the remote CoCoAS                                                                                                  | `http://127.0.0.1:50004` |
 | `pool_size` | Integer | The connections between KBS and CoCoAS are maintained in a conenction pool. This property determines the max size of the pool | `100`                    |
 
@@ -251,6 +263,7 @@ The following properties can be set.
 | `api_key`                | String       | Intel Trust Authority API key.                                                                         | Yes      | -       |
 | `certs_file`             | String       | URL to an Intel Trust Authority portal or path to JWKS file used for token verification.               | Yes      | -       |
 | `policy_ids`             | String array | Quoted and comma-separated list of policy IDs defined in ITA portal.                                   | No       | `[]`    |
+| `policy_id_map`          | Map of String array | Policies selectable by a client, keyed by ID. A selected ID replaces `policy_ids`. See [RCAR `Request`][ps]     | No       | `{}`    |
 | `allow_unmatched_policy` | Boolean      | Whether policy matching is required. If no `policy_ids` are specified, policy matching is not checked. | No       | false   |
 
 Detailed [documentation](https://docs.trustauthority.intel.com).

kbs/docs/kbs_attestation_protocol.md

@@ -117,6 +117,21 @@ transfer some specific information. For example, some attestations follow the
 Diffie–Hellman key exchange protocol to first build a secure channel and
 transfer secret messages (Such as AMD SEV(-ES) pre-attestation).
 
+`extra-params` may also carry an `id`, which selects the Attestation Service
+policies that evaluate this session's evidence:
+
+```json
+"extra-params": { "id": "alice" }
+```
+
+KBS maps each `id` to one or more attestation policies, so the accepted values
+are specific to a deployment and must be known to the KBC in advance. An
+unmapped `id` is rejected, and omitting the field selects a default policy.
+
+The KBC picks the `id` itself, so doing so is not an authorization decision. The
+policies that were applied are reported in the
+[Attestation Results Token](#attestation-results-token).
+
 ## `Challenge`
 
 If the KBC does not own any KBS generated HTTP Cookie, or if the Cookie validity