Ensemble is a system that uses a collection of trusted personal devices to provide proximity-based authentication in pervasive environments. Users are able to securely pair their personal devices with previously unknown devices by simply placing them close to each other (e.g., users can pair their phones by just bringing them into proximity). Ensemble leverages a user's growing collection of trusted devices, such as phones, music players, computers and personal sensors to observe transmissions made by pairing devices. These devices analyze variations in received signal strength (RSS) in order to determine whether the pairing devices are in physical proximity to each other. We show that, while individual trusted devices can not properly distinguish proximity in all cases, a collection of trusted devices can do so reliably. Our Ensemble prototype extends Diffie-Hellman key exchange with proximity-based authentication. Our experiments show that an Ensemble-enabled collection of Nokia N800 Internet Tablets can detect devices in close proximity and can reliably detect attackers as close as two meters away.