To prevent fraudulent voting it's obvious that all democracies need to identify voters, prevent counting errors and eliminate manipulation of results.
Using modern technology for voting requires a secure system of ID. Just think of an online bank account to see that this can be done. An evolution of the electoral roll into an electronic database incorporating user names, passwords and pins would be required to facilitate electronic voting. All of the security systems employed by the banks would be needed in the fight against fraud and miss-counting. Crime against the system, as with any other crime, would be a police matter.
Self checking by voters can provide another security measure. Imagine that each voter has an account where votes are stored. These votes may be changed at any time (by the voter) up to the time that the poll is taken. They may then be viewed and compared with a separate register of votes on that particular issue, to ensure that they were cast correctly.
The register of votes should not identify each voter by name, but by a reference number, in order to ensure confidentiality.
Coercive access to an individuals voting account or identity theft would both be matters for the police.
Before decrying the complexities and costs of the above system it is worth remembering that the current paper system has its flaws. For example members of the armed forces have had problems voting and some students have been issued two votes, one at their parental home and another at their term time residence. Also, remember that the banks have employed similar security systems for some time now and have actually cut their administrative costs in the process.
More on bank security from the BBC.
More on how the police tackle computer crime.
e-democracy from wikipedia.