Schneier on Security

Fax Signatures

Aren’t fax signatures the weirdest thing? It’s trivial to cut and paste—with real scissors and glue—anyone’s signature onto a document so that it’ll look real when faxed. There is so little security in fax signatures that it’s mind-boggling that anyone accepts them.

Yet people do, all the time. I’ve signed book contracts, credit card authorizations, nondisclosure agreements and all sorts of financial documents—all by fax. I even have a scanned file of my signature on my computer, so I can virtually cut and paste it into documents and fax them directly from my computer without ever having to print them out. What in the world is going on here?

And, more importantly, why are fax signatures still being used after years of experience? Why aren’t there many stories of signatures forged through the use of fax machines?

The answer comes from looking at fax signatures not as an isolated security measure, but in the context of the larger system. Fax signatures work because signed faxes exist within a broader communications context.

In a 2003 paper, “Economics, Psychology, and Sociology of Security,” Professor Andrew Odlyzko looks at fax signatures and concludes:

Although fax signatures have become widespread, their usage is restricted. They are not used for final contracts of substantial value, such as home purchases. That means that the insecurity of fax communications is not easy to exploit for large gain. Additional protection against abuse of fax insecurity is provided by the context in which faxes are used. There are records of phone calls that carry the faxes, paper trails inside enterprises and so on. Furthermore, unexpected large financial transfers trigger scrutiny. As a result, successful frauds are not easy to carry out by purely technical means.

He’s right. Thinking back, there really aren’t ways in which a criminal could use a forged document sent by fax to defraud me. I suppose an unscrupulous consulting client could forge my signature on an non-disclosure agreement and then sue me, but that hardly seems worth the effort. And if my broker received a fax document from me authorizing a money transfer to a Nigerian bank account, he would certainly call me before completing it.

Credit card signatures aren’t verified in person, either—and I can already buy things over the phone with a credit card—so there are no new risks there, and Visa knows how to monitor transactions for fraud. Lots of companies accept purchase orders via fax, even for large amounts of stuff, but there’s a physical audit trail, and the goods are shipped to a physical address—probably one the seller has shipped to before. Signatures are kind of a business lubricant: mostly, they help move things along smoothly.

Except when they don’t.

On October 30, 2004, Tristian Wilson was released from a Memphis jail on the authority of a forged fax message. It wasn’t even a particularly good forgery. It wasn’t on the standard letterhead of the West Memphis Police Department. The name of the policeman who signed the fax was misspelled. And the time stamp on the top of the fax clearly showed that it was sent from a local McDonald’s.

The success of this hack has nothing to do with the fact that it was sent over by fax. It worked because the jail had lousy verification procedures. They didn’t notice any discrepancies in the fax. They didn’t notice the phone number from which the fax was sent. They didn’t call and verify that it was official. The jail was accustomed to getting release orders via fax, and just acted on this one without thinking. Would it have been any different had the forged release form been sent by mail or courier?

Yes, fax signatures always exist in context, but sometimes they are the linchpin within that context. If you can mimic enough of the context, or if those on the receiving end become complacent, you can get away with mischief.

Arguably, this is part of the security process. Signatures themselves are poorly defined. Sometimes a document is valid even if not signed: A person with both hands in a cast can still buy a house. Sometimes a document is invalid even if signed: The signer might be drunk, or have a gun pointed at his head. Or he might be a minor. Sometimes a valid signature isn’t enough; in the United States there is an entire infrastructure of “notary publics” who officially witness signed documents. When I started filing my tax returns electronically, I had to sign a document stating that I wouldn’t be signing my income tax documents. And banks don’t even bother verifying signatures on checks less than $30,000; it’s cheaper to deal with fraud after the fact than prevent it.

Over the course of centuries, business and legal systems have slowly sorted out what types of additional controls are required around signatures, and in which circumstances.

Those same systems will be able to sort out fax signatures, too, but it’ll be slow. And that’s where there will be potential problems. Already fax is a declining technology. In a few years it’ll be largely obsolete, replaced by PDFs sent over e-mail and other forms of electronic documentation. In the past, we’ve had time to figure out how to deal with new technologies. Now, by the time we institutionalize these measures, the technologies are likely to be obsolete.

What that means is people are likely to treat fax signatures—or whatever replaces them—exactly the same way as paper signatures. And sometimes that assumption will get them into trouble.

But it won’t cause social havoc. Wilson’s story is remarkable mostly because it’s so exceptional. And even he was rearrested at his home less than a week later. Fax signatures may be new, but fake signatures have always been a possibility. Our legal and business systems need to deal with the underlying problem—false authentication—rather than focus on the technology of the moment. Systems need to defend themselves against the possibility of fake signatures, regardless of how they arrive.

This essay previously appeared on Wired.com.

EDITED TO ADD (6/3): 2005 story, “Federal Jury Convicts N.Y. Attorney of Faking Judge’s Order.”

Posted on June 3, 2008 at 7:01 AM • View Comments