diff --git a/keyserver/src/creators/account-creator.js b/keyserver/src/creators/account-creator.js
--- a/keyserver/src/creators/account-creator.js
+++ b/keyserver/src/creators/account-creator.js
@@ -13,6 +13,7 @@
   RegisterRequest,
 } from 'lib/types/account-types.js';
 import type {
+  UserDetail,
   ReservedUsernameMessage,
   SignedIdentityKeysBlob,
 } from 'lib/types/crypto-types.js';
@@ -190,7 +191,9 @@
   ];
 
   ignorePromiseRejections(
-    createAndSendReservedUsernameMessage([request.username]),
+    createAndSendReservedUsernameMessage([
+      { username: request.username, userID: id },
+    ]),
   );
 
   return {
@@ -248,7 +251,9 @@
   await processAccountCreationCommon(viewer);
 
   ignorePromiseRejections(
-    createAndSendReservedUsernameMessage([request.address]),
+    createAndSendReservedUsernameMessage([
+      { username: request.address, userID: id },
+    ]),
   );
 
   return id;
@@ -349,7 +354,7 @@
 }
 
 async function createAndSendReservedUsernameMessage(
-  payload: $ReadOnlyArray<string>,
+  payload: $ReadOnlyArray<UserDetail>,
 ) {
   const issuedAt = new Date().toISOString();
   const reservedUsernameMessage: ReservedUsernameMessage = {
diff --git a/keyserver/src/cron/update-identity-reserved-usernames.js b/keyserver/src/cron/update-identity-reserved-usernames.js
--- a/keyserver/src/cron/update-identity-reserved-usernames.js
+++ b/keyserver/src/cron/update-identity-reserved-usernames.js
@@ -4,19 +4,19 @@
 
 import type { ReservedUsernameMessage } from 'lib/types/crypto-types.js';
 
-import { fetchAllUsernames } from '../fetchers/user-fetchers.js';
+import { fetchAllUserDetails } from '../fetchers/user-fetchers.js';
 import { fetchOlmAccount } from '../updaters/olm-account-updater.js';
 
 async function updateIdentityReservedUsernames(): Promise<void> {
-  const [usernames, rustAPI, accountInfo] = await Promise.all([
-    fetchAllUsernames(),
+  const [userDetails, rustAPI, accountInfo] = await Promise.all([
+    fetchAllUserDetails(),
     getRustAPI(),
     fetchOlmAccount('content'),
   ]);
   const issuedAt = new Date().toISOString();
   const reservedUsernameMessage: ReservedUsernameMessage = {
     statement: 'Add the following usernames to reserved list',
-    payload: usernames,
+    payload: userDetails,
     issuedAt,
   };
   const stringifiedMessage = JSON.stringify(reservedUsernameMessage);
diff --git a/keyserver/src/fetchers/user-fetchers.js b/keyserver/src/fetchers/user-fetchers.js
--- a/keyserver/src/fetchers/user-fetchers.js
+++ b/keyserver/src/fetchers/user-fetchers.js
@@ -7,6 +7,7 @@
   FUTURE_CODE_VERSION,
 } from 'lib/shared/version-utils.js';
 import type { AvatarDBContent, ClientAvatar } from 'lib/types/avatar-types.js';
+import type { UserDetail } from 'lib/types/crypto-types.js';
 import {
   undirectedStatus,
   directedStatus,
@@ -448,6 +449,16 @@
   return result.map(row => row.username);
 }
 
+async function fetchAllUserDetails(): Promise<UserDetail[]> {
+  const query = SQL`SELECT username, id FROM users`;
+  const [result] = await dbQuery(query);
+
+  return result.map(row => ({
+    username: row.username,
+    userID: row.id,
+  }));
+}
+
 async function fetchKeyserverAdminID(): Promise<?string> {
   const changeRoleExtractString = `$.${threadPermissions.CHANGE_ROLE}`;
   const query = SQL`
@@ -490,6 +501,7 @@
   fetchAllUserIDs,
   fetchUsername,
   fetchAllUsernames,
+  fetchAllUserDetails,
   fetchKnownUserInfos,
   fetchKeyserverAdminID,
   fetchUserIDForEthereumAddress,
diff --git a/lib/types/crypto-types.js b/lib/types/crypto-types.js
--- a/lib/types/crypto-types.js
+++ b/lib/types/crypto-types.js
@@ -67,12 +67,17 @@
     signature: t.String,
   });
 
+export type UserDetail = {
+  +username: string,
+  +userID: string,
+};
+
 // This type should not be changed without making equivalent changes to
 // `Message` in Identity service's `reserved_users` module
 export type ReservedUsernameMessage =
   | {
       +statement: 'Add the following usernames to reserved list',
-      +payload: $ReadOnlyArray<string>,
+      +payload: $ReadOnlyArray<UserDetail>,
       +issuedAt: string,
     }
   | {
@@ -82,10 +87,7 @@
     }
   | {
       +statement: 'This user is the owner of the following username and user ID',
-      +payload: {
-        +username: string,
-        +userID: string,
-      },
+      +payload: UserDetail,
       +issuedAt: string,
     };
 
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
@@ -627,20 +627,20 @@
   ) -> Result<tonic::Response<Empty>, tonic::Status> {
     let message = request.into_inner();
 
-    let usernames = validate_add_reserved_usernames_message(
+    let user_details = validate_add_reserved_usernames_message(
       &message.message,
       &message.signature,
     )?;
 
-    let filtered_usernames = self
+    let filtered_user_details = self
       .client
-      .filter_out_taken_usernames(usernames)
+      .filter_out_taken_usernames(user_details)
       .await
       .map_err(handle_db_error)?;
 
     self
       .client
-      .add_usernames_to_reserved_usernames_table(filtered_usernames)
+      .add_usernames_to_reserved_usernames_table(filtered_user_details)
       .await
       .map_err(handle_db_error)?;
 
diff --git a/services/identity/src/constants.rs b/services/identity/src/constants.rs
--- a/services/identity/src/constants.rs
+++ b/services/identity/src/constants.rs
@@ -89,6 +89,7 @@
 // Usernames reserved because they exist in Ashoat's keyserver already
 pub const RESERVED_USERNAMES_TABLE: &str = "identity-reserved-usernames";
 pub const RESERVED_USERNAMES_TABLE_PARTITION_KEY: &str = "username";
+pub const RESERVED_USERNAMES_TABLE_USER_ID_ATTRIBUTE: &str = "userID";
 
 pub mod devices_table {
   /// table name
diff --git a/services/identity/src/database.rs b/services/identity/src/database.rs
--- a/services/identity/src/database.rs
+++ b/services/identity/src/database.rs
@@ -21,6 +21,7 @@
   OlmAccountType,
 };
 use crate::error::{consume_error, Error};
+use crate::reserved_users::UserDetail;
 use chrono::{DateTime, Utc};
 use serde::{Deserialize, Serialize};
 use tracing::{debug, error, info, warn};
@@ -35,7 +36,8 @@
   NONCE_TABLE_CREATED_ATTRIBUTE, NONCE_TABLE_EXPIRATION_TIME_ATTRIBUTE,
   NONCE_TABLE_EXPIRATION_TIME_UNIX_ATTRIBUTE, NONCE_TABLE_PARTITION_KEY,
   NOTIF_ONE_TIME_KEY, RESERVED_USERNAMES_TABLE,
-  RESERVED_USERNAMES_TABLE_PARTITION_KEY, USERS_TABLE,
+  RESERVED_USERNAMES_TABLE_PARTITION_KEY,
+  RESERVED_USERNAMES_TABLE_USER_ID_ATTRIBUTE, USERS_TABLE,
   USERS_TABLE_DEVICES_ATTRIBUTE,
   USERS_TABLE_DEVICES_MAP_CONTENT_ONE_TIME_KEYS_ATTRIBUTE_NAME,
   USERS_TABLE_DEVICES_MAP_CONTENT_PREKEY_ATTRIBUTE_NAME,
@@ -1036,19 +1038,18 @@
 
   pub async fn filter_out_taken_usernames(
     &self,
-    usernames: Vec<String>,
-  ) -> Result<Vec<String>, Error> {
+    user_details: Vec<UserDetail>,
+  ) -> Result<Vec<UserDetail>, Error> {
     let db_usernames = self.get_all_usernames().await?;
 
     let db_usernames_set: HashSet<String> = db_usernames.into_iter().collect();
-    let usernames_set: HashSet<String> = usernames.into_iter().collect();
 
-    let available_usernames: Vec<String> = usernames_set
-      .difference(&db_usernames_set)
-      .cloned()
+    let available_user_details: Vec<UserDetail> = user_details
+      .into_iter()
+      .filter(|user_detail| !db_usernames_set.contains(&user_detail.username))
       .collect();
 
-    Ok(available_usernames)
+    Ok(available_user_details)
   }
 
   async fn get_user_from_user_info(
@@ -1395,17 +1396,21 @@
 
   pub async fn add_usernames_to_reserved_usernames_table(
     &self,
-    usernames: Vec<String>,
+    user_details: Vec<UserDetail>,
   ) -> Result<(), Error> {
     // A single call to BatchWriteItem can consist of up to 25 operations
-    for usernames_chunk in usernames.chunks(25) {
-      let write_requests = usernames_chunk
+    for user_chunk in user_details.chunks(25) {
+      let write_requests = user_chunk
         .iter()
-        .map(|username| {
+        .map(|user_detail| {
           let put_request = PutRequest::builder()
             .item(
               RESERVED_USERNAMES_TABLE_PARTITION_KEY,
-              AttributeValue::S(username.to_string()),
+              AttributeValue::S(user_detail.username.to_string()),
+            )
+            .item(
+              RESERVED_USERNAMES_TABLE_USER_ID_ATTRIBUTE,
+              AttributeValue::S(user_detail.user_id.to_string()),
             )
             .build();
 
diff --git a/services/identity/src/reserved_users.rs b/services/identity/src/reserved_users.rs
--- a/services/identity/src/reserved_users.rs
+++ b/services/identity/src/reserved_users.rs
@@ -21,10 +21,10 @@
 // `ReservedUsernameMessage` in lib/types/crypto-types.js
 #[derive(Deserialize)]
 #[serde(rename_all = "camelCase")]
-struct UsernameAndID {
-  username: String,
+pub struct UserDetail {
+  pub username: String,
   #[serde(rename = "userID")]
-  user_id: String,
+  pub user_id: String,
 }
 
 fn validate_and_decode_message<T: serde::de::DeserializeOwned>(
@@ -88,7 +88,7 @@
   const EXPECTED_STATEMENT: &[u8; 60] =
     b"This user is the owner of the following username and user ID";
 
-  let deserialized_message = validate_and_decode_message::<UsernameAndID>(
+  let deserialized_message = validate_and_decode_message::<UserDetail>(
     keyserver_message,
     keyserver_signature,
     EXPECTED_STATEMENT,
@@ -104,8 +104,8 @@
 pub fn validate_add_reserved_usernames_message(
   keyserver_message: &str,
   keyserver_signature: &str,
-) -> Result<Vec<String>, Status> {
-  let deserialized_message = validate_and_decode_message::<Vec<String>>(
+) -> Result<Vec<UserDetail>, Status> {
+  let deserialized_message = validate_and_decode_message::<Vec<UserDetail>>(
     keyserver_message,
     keyserver_signature,
     b"Add the following usernames to reserved list",