Bootstrapping trust between wireless devices without entering or preloading secrets is a fundamental security problem in many applications, including home networking, mobile device tethering, and the Internet-of-Things. This is because many new wireless devices lack the necessary interfaces (keyboard, screen, etc.) to manually enter passwords, or are often preloaded with default keys that are easily leaked. Alternatively, two devices can establish a common secret by executing key agreement protocols. However, the latter are vulnerable to Man-in-the-Middle (MitM) attacks. In the wireless domain, MitM attacks can be launched by manipulating the over-the-air transmissions. The strongest form of manipulation is signal cancellation, which completely annihilates the signal at a targeted receiver. Recently, cancellation attacks were shown to be practical under predictable channel conditions, without an effective defense mechanism. In this paper, we propose HELP, a helper-assisted message integrity verification primitive that detects message manipulation and signal cancellation over the wireless channel (rather than prevent it). By leveraging transmissions from a helper device which has already established trust with one of the devices (e.g., the hub), we enable signal tampering detection with high probability. We then use HELP to build a device pairing protocol, which securely introduces new devices to the network without requiring them to share any secret keys with the existing devices beforehand. We carry out extensive analysis and real-world experiments to validate the security and performance of our proposed protocol.