Software-Key Escrow Emerges

Peter Wayner

For the last year, the U.S. government tried to convince the country that it should embrace the Clipper encryption chip--the top-secret chip for protecting secrets that came with a trapdoor that allowed law-enforcement officials to listen in. Public reaction to the plan was largely negative, because many people believed that the technology was overly expensive, dangerously fragile, and an unwelcome invasion of privacy. Now, Trusted Information Systems (Glenwood, MD) is offering a new software-based proposal that keeps the trapdoor for law enforcement but may eliminate many objections to cost and flexibility.

Many of the objections to Clipper's cost emerged from the government's plan to fix the design in hardware. A solution using software can be distributed at little cost, incorporated into operating systems, and quickly updated in the event the system is compromised.

TIS's system still maintains the controversial ``escrow'' feature by using public-key encryption technology to attach an additional field called the LEAF (Law Enforcement Access Field) that contains the session key for the conversation encrypted with the government's public key. Only law-enforcement officials would have access to the matching private key that could decode this extra field. This key can be split into many parts and distributed to different parties if necessary.

Many companies proposed similar schemes, but TIS took the additional step of defending against ``rogue'' implementations that could fool law-enforcement officials. Matthew Blaze, a researcher for AT&T (Holmdel, NJ), discovered such a flaw in the first prototypes of the Tessera PCMCIA cards that adapted Clipper for PCs. These attacks work by binding in a false session key into the LEAF so that law-enforcement personnel can't decrypt the mes sage. The TIS implementation defeats this by requiring both ends of the conversation to compute the LEAF using the chosen session key. The receiving end compares its LEAF to the one sent and shuts down if they don't match. This will force hackers to tamper with the equipment on both ends of the conversation--a limitation that TIS hopes will be severe enough to keep many in line.

The TIS proposal is just a proposal, but many people expect that it will be seriously studied by government officials. Others think that the most important problem is still individual privacy. David Banisar, a lawyer for EPIC ( Electronic Privacy Information Center) in Washington, D.C., says, ``We are concerned that the cure for Clipper is worse than the disease. Key escrow in software or hardware is a bad idea and threatens the security and privacy of communications. The Fourth Amendment doesn't require that every man, woman, and child in the U.S. leave a copy of their keys at the local police station. It's unacceptable to hav e these requirements for our communications.''