WPA Password Cracking in the Cloud for $34

Following on from the recent results of a project on the feasibility of password cracking using cloud computing, a new cloud service for cracking WPA passwords has been announced by researcher Moxie Marlinspike, best known for his work on discovering subtle flaws in the SSL protocol. The cloud service attempts to crack uploaded passwords against a 135 million word WPA-optimized dictionary on a 400 CPU cluster.

The stated purpose of the service is to assist penetration testers and network auditors who need to test the strength of WPA passwords that use pre-shared keys (PSK or personal mode). In this mode, the PSK master key is used to derive a session key from several parameters in the initial wireless connection handshake, including the MAC addresses of the client and base station. In practice, the PSK is a password and is therefore exposed to a brute force or dictionary attack.

Verifying a guess for the PSK password is relatively costly, roughly the equivalent of processing a megabyte of data, since the session key is derived from 4096 iterated hash operations. This high iteration count, or spin, limits the number of password trials to several hundred per second on a standard desktop. Marlinspike's cloud service searches through a 135 million word dictionary in just 20 minutes, a computation that would otherwise take 5 days or so on a standard desktop.

And all this for just $34, after a client has uploaded a capture of the WPA handshake on the network of interest. If the cloud service does not find a password then a client can be confident that the submitted password is resistant to dictionary attacks – no refund, you still pay for the assurance!

The Church Of Wifi has a project to create public rainbow tables for WPA based on common passwords, but the resulting tables only encode one million passwords because the tables must be recomputed for each network identifier (ESSN), which acts as salt in the password derivation function. Marlinspike opted to create his own WPA-optimized dictionary list which includes word combinations, phrases, numbers, symbols, and “elite speak”.

In Security Muggles I posted about finding ways to connect with management about security. Robert Graham, CEO of penetration testing company Errata Security, has commented that "When I show this to management and say it would cost $34 to crack your WPA password, it's something they can understand," he said. "That helps me a lot."

PC World has somewhat dramatically reported the announcement as New Cloud-based Service Steals Wi-Fi Passwords, suggesting that the cloud is reaching into wireless networks and grabbing passwords. In fact, the service is designed to trade-off a 5-day desktop computation against a 20-minute cloud computation for $34. Nonetheless, a better name would be WPA Auditor rather than WPA Cracker.