MSC-4004: unified view of identity service

Signed-off-by: Xavier Guimard <yadd@debian.org>

proposals/4004-unified-identity-service-view.md

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+# MSC4004: Unified view of identity service
+
+Centralized chat platforms are able to automatically detect which entries of
+the phone address book are known by the platform, then automatically propose
+those names when user clicks on "new discussion".
+
+The Matrix-Identity-Service has already a secured mechanism able to be used
+for this _(lookup)_. However, the identity service is only able to answer
+with data it knows _(ie submitted lookups)_.
+
+## Proposal
+
+The goal of this proposal is to add a mechanism to provide an unified
+view of identity service without centralizating user's data.
+
+### Changes
+
+All of this changes affects [identity-service-api.md](https://github.com/matrix-org/matrix-spec/blob/main/content/identity-service-api.md)
+
+#### Association lookup
+
+##### GET `/_matrix/identity/v2/hash_details`
+
+To ensure continuity of asociations when the pepper changes, it is required
+to have more than one pepper available. To avoid breaking changes, the current
+format is kept and an optional additional filed is provided:
+
+```json
+{
+  "algorithms": [
+    "none",
+    "sha256"
+  ],
+  "lookup_pepper": "matrixrocks",
+  "alt_lookup_peppers": ["oldmatrixrocks"]
+}
+
+```
+
+##### POST `/_matrix/identity/v2/lookup`
+
+A new key is added in response, `third_party_mappings`. It permits to the
+identity service to answer that it doesn't know this 3PID but knows where
+to find it:
+
+```json
+{
+  "mappings": {
+    "4kenr7N9drpCJ4AfalmlGQVsOn3o2RHjkADUpXJWZUc": "@alice:example.org"
+  },
+  "third_party_mappings": {
+    "matrix.domain.com:8448": [
+      "nlo35_T5fzSGZzJApqu8lgIudJvmOQtDaHtr-I4rU7I"
+    ]
+  }
+}
+```
+
+The client application has to do a new lookup query to `matrix.domain.com:8448`.
+The hash value given here isn't valid on this new server: the client application
+has to calculate a new hash using pepper/alg from `matrix.domain.com:8448`.
+
+#### Establishing associations
+
+##### POST `/_matrix/identity/v2/lookups`
+
+A new endpoint, reserved to trusted servers, will allow to declare a list
+of hashes owned by the (trusted) server. Hashes are calculated using the
+pepper of recipient server.
+
+Request body:
+
+```json
+{
+  "algorithm": "sha256",
+  "pepper": "matrix_rocks",
+  "mappings": {
+    "matrix.domain.com:8448": [
+      "nlo35_T5fzSGZzJApqu8lgIudJvmOQtDaHtr-I4rU7I"
+    ]
+  }
+}
+```
+
+## Potential issues
+
+To avoid conversation hijacking, only trusted server should be allowed to push
+a list of owned hashes.
+
+## Security considerations
+
+This proposal is based on current Matrix-Identity-Service security mechanisms.
+Only the new endpoint should accept request only from trusted server.