Even though voting by standard mail has its own problems, the report, from the U.S. National Institute of Standards and Technology (NIST), says that electronic transmission of completed ballots, also including telephone and fax, "present significant challenges to the integrity of the election."

The U.S. Department of Defense experimented with Internet voting in 2003, but dropped a pilot program the next year after security concerns surfaced. A handful of states have experimented with Internet voting, including Florida and Alabama in 2008, prompted by concerns about military mail ballots not being delivered in time to be counted.

The Help America Vote Act of 2002 (HAVA) requires the U.S. Election Assistance Commission (EAC) to study methods of overseas voting, and the NIST report is part of that effort. "IT security is an important aspect of this issue, so EAC asked NIST to conduct a study that would explore the security threats associated with potential electronic technologies for overseas voting, and identify possible ways of mitigating the threats," said Nelson Hastings, co-author of the report.

The says it's relatively safe to transmit unfilled ballots by fax or e-mail or put them on the Web, but sending filled-in ballots by those methods present problems with security or privacy.

Voting by telephone would require PINs, and PINs can be lost or stolen, the report says. In addition, telephone calls can be tapped, especially VoIP (voice over Internet Protocol) calls, the report says.