diff --git a/keyserver/src/responders/user-responders.js b/keyserver/src/responders/user-responders.js
--- a/keyserver/src/responders/user-responders.js
+++ b/keyserver/src/responders/user-responders.js
@@ -740,8 +740,11 @@
 
   const issuedAt = new Date().toISOString();
   const reservedUsernameMessage: ReservedUsernameMessage = {
-    statement: 'This user is the owner of the following username',
-    payload: username,
+    statement: 'This user is the owner of the following username and user ID',
+    payload: {
+      username,
+      userID: viewer.userID,
+    },
     issuedAt,
   };
   const message = JSON.stringify(reservedUsernameMessage);
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
@@ -61,8 +61,11 @@
       +issuedAt: string,
     }
   | {
-      +statement: 'This user is the owner of the following username',
-      +payload: string,
+      +statement: 'This user is the owner of the following username and user ID',
+      +payload: {
+        +username: string,
+        +userID: string,
+      },
       +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
@@ -36,9 +36,9 @@
 use crate::id::generate_uuid;
 use crate::nonce::generate_nonce_data;
 use crate::reserved_users::{
+  validate_account_ownership_message_and_get_user_id,
   validate_add_reserved_usernames_message,
   validate_remove_reserved_username_message,
-  validate_signed_account_ownership_message,
 };
 use crate::siwe::{is_valid_ethereum_address, parse_and_verify_siwe_message};
 use crate::token::{AccessTokenData, AuthType};
@@ -61,6 +61,7 @@
 pub struct UserRegistrationInfo {
   pub username: String,
   pub flattened_device_key_upload: FlattenedDeviceKeyUpload,
+  pub user_id: Option<String>,
 }
 
 #[derive(Clone)]
@@ -173,6 +174,7 @@
           device_type: DeviceType::try_from(DBDeviceTypeInt(device_type))
             .map_err(handle_db_error)?,
         },
+        user_id: None,
       };
       let session_id = generate_uuid();
       self
@@ -217,16 +219,16 @@
       .username_in_reserved_usernames_table(&message.username)
       .await
       .map_err(handle_db_error)?;
-    if username_in_reserved_usernames_table {
-      validate_signed_account_ownership_message(
-        &message.username,
-        &message.keyserver_message,
-        &message.keyserver_signature,
-      )?;
-    } else {
+    if !username_in_reserved_usernames_table {
       return Err(tonic::Status::permission_denied("username not reserved"));
     }
 
+    let user_id = validate_account_ownership_message_and_get_user_id(
+      &message.username,
+      &message.keyserver_message,
+      &message.keyserver_signature,
+    )?;
+
     if let client_proto::ReservedRegistrationStartRequest {
       opaque_registration_request: register_message,
       username,
@@ -276,6 +278,7 @@
           device_type: DeviceType::try_from(DBDeviceTypeInt(device_type))
             .map_err(handle_db_error)?,
         },
+        user_id: Some(user_id),
       };
 
       let session_id = generate_uuid();
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
@@ -144,6 +144,7 @@
         Some((registration_state.username, Blob::new(password_file))),
         None,
         None,
+        registration_state.user_id,
       )
       .await
   }
@@ -160,6 +161,7 @@
         None,
         Some(wallet_address),
         Some(social_proof),
+        None,
       )
       .await
   }
@@ -170,8 +172,9 @@
     username_and_password_file: Option<(String, Blob)>,
     wallet_address: Option<String>,
     social_proof: Option<String>,
+    user_id: Option<String>,
   ) -> Result<String, Error> {
-    let user_id = generate_uuid();
+    let user_id = user_id.unwrap_or_else(generate_uuid);
     let device_info =
       create_device_info(flattened_device_key_upload.clone(), social_proof);
     let devices = HashMap::from([(
@@ -212,6 +215,9 @@
       .put_item()
       .table_name(USERS_TABLE)
       .set_item(Some(user))
+      // make sure we don't accidentaly overwrite existing row
+      .condition_expression("attribute_not_exists(#pk)")
+      .expression_attribute_names("#pk", USERS_TABLE_PARTITION_KEY)
       .send()
       .await
       .map_err(|e| Error::AwsSdk(e.into()))?;
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
@@ -17,6 +17,16 @@
   issued_at: String,
 }
 
+// This type should not be changed without making equivalent changes to
+// `ReservedUsernameMessage` in lib/types/crypto-types.js
+#[derive(Deserialize)]
+#[serde(rename_all = "camelCase")]
+struct UsernameAndID {
+  username: String,
+  #[serde(rename = "userID")]
+  user_id: String,
+}
+
 fn validate_and_decode_message<T: serde::de::DeserializeOwned>(
   keyserver_message: &str,
   keyserver_signature: &str,
@@ -69,22 +79,25 @@
   Ok(deserialized_message)
 }
 
-pub fn validate_signed_account_ownership_message(
+pub fn validate_account_ownership_message_and_get_user_id(
   username: &str,
   keyserver_message: &str,
   keyserver_signature: &str,
-) -> Result<(), Status> {
-  let deserialized_message = validate_and_decode_message::<String>(
+) -> Result<String, Status> {
+  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>(
     keyserver_message,
     keyserver_signature,
-    b"This user is the owner of the following username",
+    EXPECTED_STATEMENT,
   )?;
 
-  if deserialized_message.payload != username {
+  if deserialized_message.payload.username != username {
     return Err(Status::invalid_argument("message invalid"));
   }
 
-  Ok(())
+  Ok(deserialized_message.payload.user_id)
 }
 
 pub fn validate_add_reserved_usernames_message(