docs: add Decision Record about STS Accounts API (eclipse-edc#4488)

* docs: add decision record about STS Accounts API

* Update README.md

docs/developer/decision-records/2024-09-24-sts-accounts-api/README.md

@@ -0,0 +1,80 @@
+# Implementation of a Accounts API for the SecureTokenService
+
+## Decision
+
+An Accounts API for the SecureTokenService (STS) will be implemented in a separate module to manage STS accounts.
+In addition, the current class `StsClient` will be renamed to `StsAccount`, and `StsClientStore` to `StsAccountStore` to
+avoid confusion.
+
+## Rationale
+
+The STS acts as authorization backend for a CredentialService. In EDC, the STS implementation is designed to be deployed
+alongside the IdentityHub and acts as Identity Provider (IdP) and for its Presentation API. Although there is
+an [automatic way](https://github.com/eclipse-edc/IdentityHub/pull/458) to
+synchronize a IdentityHub `ParticipantContext` and an STS account, there still should be a way to administer (create,
+update, read and delete) STS accounts at runtime.
+
+In addition, in cases where STS is deployed as standalone component, the IdentityHub may synchronize the
+`ParticipantContext` and the STS account using this Accounts API.
+
+The reason for renaming the `StsClient` is that the term "client" is quite overloaded and could mislead developers into
+thinking that it is a "client to communicate with the STS token API".
+
+## Approach
+
+Implement a REST API that has the following endpoints:
+
+### `POST /v1/accounts`
+
+Create a new `StsAccount`. Key pairs must be managed _out-of-band_. If no `client_secret` is provided, one will be
+generated. This endpoint returns the _entire_ `StsAccount`, including the `client_secret`. Note that there is **no way**
+to obtain the `client_secret` again at a later point in time!
+
+### `PUT /v1/accounts/`
+
+Update an existing `StsAccount`. Note that there is a dedicated endpoint for rotating the `client_secret`.
+
+### `GET /v1/accounts/{id}`
+
+Fetch a particular `StsAccount` by ID. The ID is the entity ID (database ID) of the `StsAccount`. The `client_secret` is
+**not** returned, only its alias.
+
+### `POST /v1/accounts/query`
+
+Query STS accounts, taking a `QuerySpec` in the request body. The `client_secret` is **not** returned, only its alias.
+
+### `PUT /v1/accounts/{id}/secret`
+
+Updates the `client_secret`. The new secret alias - and optionally the new secret - are provided in the request body. If
+no secret is provided, one is generated at random. The old client secret is deleted from the Vault and replaced with the
+new client secret. If the process fails, manual intervention may be necessary, because Vaults are not transactional
+resources. This endpoint returns the current `secret_alias` to make that manual intervention easier.
+
+### `DELETE /v1/accounts/{id}`
+
+Deletes a particular account, removes the `client_secret` from the Vault. Note that this does **not** delete the key
+pair.
+
+> The `StsClientServiceImpl` will be extended to accommodate this functionality.
+
+### Handling key-pairs
+
+The STS is designed to be used alongside IdentityHub, either as embedded component, or as standalone runtime. In
+either case it should be the IdentityHub that exposes the control surface for handling key-pairs through its Identity
+API and its internal automatic synchronization mechanism (`StsAccountProvisioner`). For that reason, the STS Accounts
+API does not provide any endpoints to handle key pairs.
+
+_Note that if STS is deployed standalone, IdentityHub could use this Accounts API to handle synchronization via REST._
+
+### Web context
+
+A new web context `"accounts"` will be introduced for the STS Accounts API to provide it its own security realm and
+ingress configuration. This is particularly important when STS runs as embedded component in IdentityHub or the
+connector controlplane to avoid clashes and provide better separation.
+
+### Security
+
+Out-of-the-box, the STS Accounts API utilizes token-based authentication. This can be extended and
+overridden in downstream applications, for example using the delegated authentication feature.
+
+> The STS Accounts API should **never** be exposed to the internet without additional infrastructure!

docs/developer/decision-records/README.md

@@ -60,3 +60,4 @@
 - [2024-07-03 Additional CatalogRequest scope parameter](./2024-07-03-additional-catalogrequest-param/)
 - [2024-08-05 Custom JWSSigner Implementations](./2024-08-05-custom-jwssigners/)
 - [2024-08-16 Policy_Validation](./2024-08-16-policy-validation)
+- [2024-09-24 STS Accounts API](./2024-09-24-sts-accounts-api)