diff --git a/native/native_rust_library/src/lib.rs b/native/native_rust_library/src/lib.rs
--- a/native/native_rust_library/src/lib.rs
+++ b/native/native_rust_library/src/lib.rs
@@ -20,8 +20,8 @@
   UploadOneTimeKeysRequest,
 };
 use grpc_clients::identity::protos::unauth::{
-  AuthResponse, DeviceKeyUpload, DeviceType, Empty, IdentityKeyInfo,
-  OpaqueLoginFinishRequest, OpaqueLoginStartRequest, Prekey,
+  AuthResponse, DeviceKeyUpload, DeviceType, Empty, ExistingDeviceLoginRequest,
+  IdentityKeyInfo, OpaqueLoginFinishRequest, OpaqueLoginStartRequest, Prekey,
   RegistrationFinishRequest, RegistrationStartRequest,
   SecondaryDeviceKeysUploadRequest, WalletAuthRequest,
 };
@@ -240,6 +240,14 @@
       promise_id: u32,
     );
 
+    #[cxx_name = "identityLogInExistingDevice"]
+    fn log_in_existing_device(
+      user_id: String,
+      device_id: String,
+      challenge_response: String,
+      promise_id: u32,
+    );
+
     #[cxx_name = "identityFindUserIDForWalletAddress"]
     fn find_user_id_for_wallet_address(wallet_address: String, promise_id: u32);
 
@@ -1502,6 +1510,47 @@
   Ok(serde_json::to_string(&user_id_and_access_token)?)
 }
 
+fn log_in_existing_device(
+  user_id: String,
+  device_id: String,
+  challenge_response: String,
+  promise_id: u32,
+) {
+  RUNTIME.spawn(async move {
+    let result =
+      log_in_existing_device_helper(user_id, device_id, challenge_response)
+        .await;
+    handle_string_result_as_callback(result, promise_id);
+  });
+}
+
+async fn log_in_existing_device_helper(
+  user_id: String,
+  device_id: String,
+  challenge_response: String,
+) -> Result<String, Error> {
+  let mut identity_client = get_unauthenticated_client(
+    IDENTITY_SOCKET_ADDR,
+    CODE_VERSION,
+    DEVICE_TYPE.as_str_name().to_lowercase(),
+  )
+  .await?;
+
+  let request = ExistingDeviceLoginRequest {
+    user_id,
+    device_id,
+    challenge_response,
+  };
+
+  let response = identity_client
+    .log_in_existing_device(request)
+    .await?
+    .into_inner();
+
+  let user_id_and_access_token = UserIDAndDeviceAccessToken::from(response);
+  Ok(serde_json::to_string(&user_id_and_access_token)?)
+}
+
 #[derive(
   Debug, derive_more::Display, derive_more::From, derive_more::Error,
 )]