Recent trends in the security of home WiFi networks are driven by two phenomena: ordinary users often struggle with the security setup of their home networks, and, as a result, some of them end up skipping security activation. Simultaneously, there is a proliferation of WiFi gadgets and sensors that do not support an interface for entering a key. These include WiFi sound systems, medical sensors, USB keys, light and temperature sensors, motion detectors and surveillance sensors, home appliances, and game consoles. Even new models of these devices are unlikely to support a keypad because of limitations on their form factor, style, cost, or functionality. Responding to these two requirements—easing security setup for home users, and securing devices that do not have an interface for entering a key—the WiFi Alliance has introduced the Push Button Configuration (PBC) mechanism. To establish a secure connection between two WiFi devices, the user pushes a button on each device, and the devices broadcast their Diffie-Hellman public keys, which they then use to protect all future communication. PBC is a mandatory part of the new WiFi Protected Setup certification program. It is already adopted by the major WiFi manufacturers (e.g., Cisco, NetGear, HP, Microsoft, Sony) and implemented in about 2,000 new products from 117 different companies.
Unfortunately, the PBC approach taken by the WiFi Alliance does not fully address WiFi security. Diffie-Hellman’s key-exchange protocol protects against only passive adversaries that snoop on the wireless medium to obtain key exchange messages. Since the key exchange messages are not authenticated in any way, the protocol is vulnerable to an active man-in-the-middle (MITM) attack. That is, an adversary can impersonate each device to the other, convincing both devices to establish a secure connection via the adversary. With WiFi increasingly used in medical sensors that transmit a patient’s vital signals and surveillance sensors that protect one’s home, there is a concern that, being vulnerable to MITM attacks, PBC may give users a false sense of security.
One may wonder why the WiFi Alliance did not adopt a user-friendly solution that also protects against MITM attacks. We believe the reason is that existing user-friendly solutions to MITM attacks require devices to support an out-of-band communication channel. For example, devices can exchange keys over a visual channel between an LCD and a camera, an audio channel, an infrared channel, a dedicated wireless channel allocated exclusively for key exchange, etc. Given the cost, size, and capability constraints imposed on many WiFi products, it is difficult for the industry to adopt a solution that requires an out-of-band communication channel.
TEP: A MITM Resistant Pairing Protocol
In our work, we present Tamper-Evident Pairing (TEP), the first wireless pairing protocol that works in-band, with no pre-shared keys, and protects against MITM attacks. TEP relies on a Tamper-Evident Announcement (TEA) mechanism, which guarantees that an adversary cannot tamper with either the payload in a transmitted message, or with the fact that the message was sent. We formally proved that the design protects from MITM attacks. Further, we implemented a prototype of TEA and TEP for the 802.11 wireless protocol using off-the-shelf WiFi devices, and showed that TEP is practical on real-world 802.11 networks and devices.