add required_keys util to signatures

Signed-off-by: Jason Volk <jason@zemos.net>

crates/ruma-signatures/src/functions.rs

@@ -10,7 +10,8 @@ use base64::{alphabet, Engine};
 use ruma_common::{
     canonical_json::{redact, JsonType},
     serde::{base64::Standard, Base64},
-    CanonicalJsonObject, CanonicalJsonValue, OwnedEventId, OwnedServerName, RoomVersionId, UserId,
+    CanonicalJsonObject, CanonicalJsonValue, OwnedEventId, OwnedServerName,
+    OwnedServerSigningKeyId, RoomVersionId, UserId,
 };
 use serde_json::to_string as to_json_string;
 use sha2::{digest::Digest, Sha256};
@@ -632,6 +633,34 @@ fn canonical_json_with_fields_to_remove(
     to_json_string(&owned_object).map_err(|e| Error::Json(e.into()))
 }
 
+/// Extracts the server names and key ids to check signatures for given event.
+pub fn required_keys(
+    object: &CanonicalJsonObject,
+    version: &RoomVersionId,
+) -> Result<BTreeMap<OwnedServerName, Vec<OwnedServerSigningKeyId>>, Error> {
+    use CanonicalJsonValue::Object;
+
+    let mut map = BTreeMap::<OwnedServerName, Vec<OwnedServerSigningKeyId>>::new();
+    let Some(Object(signatures)) = object.get("signatures") else {
+        return Ok(map);
+    };
+
+    for server in servers_to_check_signatures(object, version)? {
+        let Some(Object(set)) = signatures.get(server.as_str()) else {
+            continue;
+        };
+
+        let entry = map.entry(server.clone()).or_default();
+        set.into_iter()
+            .map(|(k, _)| k.clone())
+            .map(TryInto::try_into)
+            .filter_map(Result::ok)
+            .for_each(|key_id| entry.push(key_id));
+    }
+
+    Ok(map)
+}
+
 /// Extracts the server names to check signatures for given event.
 ///
 /// It will return the sender's server (unless it's a third party invite) and the event id server
@@ -639,7 +668,7 @@ fn canonical_json_with_fields_to_remove(
 ///
 /// Starting with room version 8, if join_authorised_via_users_server is present, a signature from
 /// that user is required.
-fn servers_to_check_signatures(
+pub fn servers_to_check_signatures(
     object: &CanonicalJsonObject,
     version: &RoomVersionId,
 ) -> Result<BTreeSet<OwnedServerName>, Error> {

crates/ruma-signatures/src/lib.rs

@@ -49,8 +49,8 @@ use ruma_common::serde::{AsRefStr, DisplayAsRefStr};
 pub use self::{
     error::{Error, JsonError, ParseError, VerificationError},
     functions::{
-        canonical_json, content_hash, hash_and_sign_event, reference_hash, sign_json, verify_event,
-        verify_json,
+        canonical_json, content_hash, hash_and_sign_event, reference_hash, required_keys,
+        servers_to_check_signatures, sign_json, verify_event, verify_json,
     },
     keys::{Ed25519KeyPair, KeyPair, PublicKeyMap, PublicKeySet},
     signatures::Signature,