Systems and method for authenticating users are presented. A system can send a passkey to a user interface of a known device. A user can then send a messaging service message with the passkey from a second device to the system. After receiving the message from the user, the system can extract the passkey from the message, and compare the received passkey against the passkey originally sent to the user. The known device and the second device can each have separate and unique device identifiers.