The visual aid was a T-shirt.

To understand how wearing an article of clothing could make you an international arms dealer, it helps to take a look at the relevant laws and some illustrative incidents.

The Arms Control Export Act (ACEA) gives the president the authority to designate certain items (such as battleships and land mines) as defense articles or defense services. These designated items make up the United States Munitions List. The ACEA further authorizes the International Traffic in Arms Regulations (ITAR), which list and control the import and export of designated defense articles and services.

On the munitions list, sandwiched between laser targeting systems and particle beam weapons, is cryptographic software. Such software is defined as "components or software with the capability of maintaining secrecy or confidentiality of information or information systems."

The ITAR further prohibits exporting cryptographic software (or technical data about cryptographic software) without a license from the U.S. Department of State. The definition of "export" includes disclosing or transferring technical data to a foreign person, either in the U.S. or abroad.

PGP (Pretty Good Privacy) encryption software protects the privacy of information sent over unsecure communications channels, such as the Internet and online services. Despite governmental attempts to institute encryption technologies allowing for federal police surveillance, PGP has become the de facto world standard for personal encryption software.

Only a short time ago the Justice Department dropped an over-two-year grand jury investigation against the author of PGP, Philip Zimmermann. Because encryption software is on the munitions list, the grand jury investigation proceeded on the theory that by posting PGP to an Internet Usenet group, Zimmermann became an international arms dealer.

The conclusion of this investigation is hardly cause for relief. The Justice Department backed down only after Zimmerman, winner of a Chrysler Innovation in Design award for PGP, waged a protracted publicity battle against this assault on computing privacy.

Moreover, the law remains the same. Without a final adjudication of the Zimmerman case on the relevant issues, other users of encryption software, including users of PGP, remain exposed to the same charges underlying -- and the aggravation caused by -- the grand jury investigation.

Another case involves the book "Applied Cryptography," by Bruce Schneier. When Phil Karn applied for a license to export this book (readily available in bookstores and libraries), the Office of Defense Trade Controls told him that the contents of the book were in the "public domain" and not subject to State Department licensing restrictions.

A request to export a disk with the exact same source code as that printed in the book, however, was denied, appealed, and denied again. The State Department took the position that while the source code in the book is exportable, once that same code is put into a machine-readable form, it becomes a controlled munition. Karn's explanation of the relative ease of scanning and conversion of the printed source code into machine-readable form failed to move the State Department.

The arbitrary regime under ITAR is now the subject of a full-blown constitutional challenge in a California federal court, where Daniel Bernstein, a graduate student in mathematics, has challenged the ACEA and ITAR as vague, overbroad and in violation of the First and Fifth Amendments.

Bernstein contends that these regulations not only prohibit publication of his work in cryptography but also restrain discussion in situations where he cannot ascertain the nationality of all possible audience members and obtain a license for any foreigners. Bernstein alleges that the regulations constitute an impermissible prior-restraint on his First Amendment rights.

Which brings us to the T-shirts ...

Billed by promoters as a "classic example of civil disobedience," the shirt has some computer code printed on it. The code is an implementation of the "RSA" algorithm published by three M.I.T. professors.

It is the same algorithm used in Philip Zimmermann's PGP software.

To ensure the shirt will qualify as a non-exportable munition, the shirt even has machine-readable bar-code rendition of the software printed on it. To demonstrate the arbitrariness of the arms control regulations, only U.S. or Canadian citizens can order the shirt from the U.S. address, but since the algorithm is widely available, non-U.S. citizens can order the shirts from an address in England.

Along with the sales pitch ("Now you, too, can become an international arms dealer for the price of a T-shirt") come warnings that if a non-U.S. citizen sees you wearing the shirt you may be classified as a criminal. (If you wear it inside-out, is it a concealed weapon?) If you are arrested, the promoters will refund the purchase price of the shirt.

T-shirts are not the only item that might get you in trouble with the Feds. If you wish to have private communications over online services or wireless communications, you might have non-exempt encryption software on your laptop computer. But if you take that computer on an international flight without a temporary export license, you risk arrest as an arms dealer. Fortunately, bills have been recently intoduced in Congress to loosen this law.

Cryptographic software is essential to development of commerce in information. If the U.S. restrains free development of such software, U.S. technology companies will be faced with a state-induced disadvantage.

The World Trade Center bombers were caught when they tried to recover a deposit on the rental van they blew up along with the building. And the government has devices that can read the contents of your computer screen from outside your office. Given these realities, is it really necessary to prohibit discussing higher math with foreigners?