Hackers at Defcon went to work on four different models of US voting machines, and cracked all of them within hours of seeing them for the first time.

Even if some model were totally secure against crackers, it can't be secure against the company that develops its proprietary software.

If, however, the software is free, and the local election authority can change it, then the machine can't be secure against the local election authority. There is always someone that can change the software, and we can't trust that someone.

For secure elections, we must vote on paper.