diff --git a/services/identity/src/client_service.rs b/services/identity/src/client_service.rs
--- a/services/identity/src/client_service.rs
+++ b/services/identity/src/client_service.rs
@@ -4,8 +4,9 @@
 
 use crate::{
   client_service::client_proto::{
-    DeleteUserRequest, Empty, GenerateNonceResponse, KeyserverKeysRequest,
-    KeyserverKeysResponse, OpaqueLoginFinishRequest, OpaqueLoginFinishResponse,
+    CompareUsersRequest, CompareUsersResponse, DeleteUserRequest, Empty,
+    GenerateNonceResponse, KeyserverKeysRequest, KeyserverKeysResponse,
+    OpaqueLoginFinishRequest, OpaqueLoginFinishResponse,
     OpaqueLoginStartRequest, OpaqueLoginStartResponse,
     ReceiverKeysForUserRequest, ReceiverKeysForUserResponse,
     RefreshUserPreKeysRequest, RegistrationFinishRequest,
@@ -150,4 +151,11 @@
   ) -> Result<tonic::Response<VerifyUserAccessTokenResponse>, tonic::Status> {
     unimplemented!();
   }
+
+  async fn compare_users(
+    &self,
+    _request: tonic::Request<CompareUsersRequest>,
+  ) -> Result<tonic::Response<CompareUsersResponse>, tonic::Status> {
+    unimplemented!();
+  }
 }
diff --git a/shared/protos/identity_client.proto b/shared/protos/identity_client.proto
--- a/shared/protos/identity_client.proto
+++ b/shared/protos/identity_client.proto
@@ -63,6 +63,11 @@
   // Called by other services to verify a user's access token
   rpc VerifyUserAccessToken(VerifyUserAccessTokenRequest) returns
     (VerifyUserAccessTokenResponse) {}
+
+  // Called by Ashoat's keyserver with a list of usernames in MySQL and returns:
+  // 1. a list of usernames that are in DynamoDB but not in the supplied list
+  // 2. a list of usernames that are in the supplied list but not in DynamoDB
+  rpc CompareUsers(CompareUsersRequest) returns (CompareUsersResponse) {}
 }
 
 // Helper types
@@ -318,3 +323,14 @@
 message VerifyUserAccessTokenResponse {
   bool tokenValid = 1;
 }
+
+// CompareUsers
+
+message CompareUsersRequest {
+  repeated string users = 1;
+}
+
+message CompareUsersResponse {
+  repeated string usersMissingFromKeyserver = 1;
+  repeated string usersMissingFromIdentity = 2;
+}