A Replacement for CAPTCHA

By Larry Seltzer  |  Posted 2008-12-09 Print this article Print


So Shapiro and I talked about what might replace the CAPTCHA. One possibility is a variation on the old PGP (Pretty Good Privacy) scheme. PGP advocates are anxious to point out that their system provides solid enforcement of identity, so that you can prove that messages are from whom they purport to be from and that they have not been tampered with. But PGP is, almost by design, difficult to work with.

Imagine a new class of e-mail account, a verified account, sort of like EV-SSL, which provides enhanced verification of identity for SSL applicants and a visual feedback, in the form of the green address bar, for Web sites. In this case, before getting one of these accounts, a user would have to get a digital certificate from a trusted certificate authority. There might be many of these, as there are with SSL certificates, or there might be just one: There was some talk years ago about the U.S. post office getting into this business. Anyway, stick with me on this.

To get one of these new accounts, you have to present your certificate to the e-mail provider, who might also act as a go-between to obtain it for you. Every message is signed by some hash of that certificate, which therefore must either reside on your computer or be network-accessible in a secure way.

The result is a PGP-like system where individual senders can be uniquely identified. If enough users adopt this system, then recipients can start to use it for reliable whitelists and blacklists.

I know I've ignored a ton of technical stuff, but assume I've solved the technical problems. There remain huge problems of privacy, ease of use and so on. What identity information should the certificate authority require from me, an ordinary Internet user? It must be strong enough to ensure identity, yet not onerous enough to violate privacy or be too troublesome to bother with. If I lose my certificate, I'll need to reprove my identity to the CA in order to get my certificate reissued.

To make sense, does the system require that all users get no more than one certificate? There's a good argument for this, although it ends some aspects of anonymity, since no matter what address I use I can be identified, if not by name, then as a particular unique individual whose public key may be identical to one used by other e-mail addresses.

I could go on with the problems that such a system presents; as with replacing SMTP, even if you could make it happen technically it won't happen because people would rather put up with the problem than deal with the solution.

Five years after CAN-SPAM, the spam problem is, in many ways, much worse. CAN-SPAM is not at fault, nor could a "better law" have done any better. As long as there is money to be made by sending e-mails, rules won't get in the way of the spammers.

Security Center Editor Larry Seltzer has worked in and written about the computer industry since 1983.

For insights on security coverage around the Web, take a look at eWEEK.com Security Center Editor Larry Seltzer's blog Cheap Hack.


Larry Seltzer has been writing software for and English about computers ever since—,much to his own amazement—,he graduated from the University of Pennsylvania in 1983.

He was one of the authors of NPL and NPL-R, fourth-generation languages for microcomputers by the now-defunct DeskTop Software Corporation. (Larry is sad to find absolutely no hits on any of these +products on Google.) His work at Desktop Software included programming the UCSD p-System, a virtual machine-based operating system with portable binaries that pre-dated Java by more than 10 years.

For several years, he wrote corporate software for Mathematica Policy Research (they're still in business!) and Chase Econometrics (not so lucky) before being forcibly thrown into the consulting market. He bummed around the Philadelphia consulting and contract-programming scenes for a year or two before taking a job at NSTL (National Software Testing Labs) developing product tests and managing contract testing for the computer industry, governments and publication.

In 1991 Larry moved to Massachusetts to become Technical Director of PC Week Labs (now eWeek Labs). He moved within Ziff Davis to New York in 1994 to run testing at Windows Sources. In 1995, he became Technical Director for Internet product testing at PC Magazine and stayed there till 1998.

Since then, he has been writing for numerous other publications, including Fortune Small Business, Windows 2000 Magazine (now Windows and .NET Magazine), ZDNet and Sam Whitmore's Media Survey.

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