The process for electing a new Pope is amazingly baroque and weird. Bruce Schneier has posted a long and engrossing security analysis of how the election works and how its simple and ancient procedures (and the small number of participants) make it more secure than modern electronic voting systems:
How hard is this to hack? The first observation is that the system is entirely manual, making it immune to the sorts of technological attacks that make modern voting systems so risky. The second observation is that the small group of voters — all of whom know each other — makes it impossible for an outsider to affect the voting in any way. The chapel is cleared and locked before voting. No one is going to dress up as a cardinal and sneak into the Sistine Chapel. In effect, the voter verification process is about as perfect as you're ever going to find.
Eavesdropping on the process is certainly possible, although the rules explicitly state that the chapel is to be checked for recording and transmission devices "with the help of trustworthy individuals of proven technical ability." I read that the Vatican is worried about laser microphones, as there are windows near the chapel's roof.
That leaves us with insider attacks. Can a cardinal influence the election? Certainly the Scrutineers could potentially modify votes, but it's difficult. The counting is conducted in public, and there are multiple people checking every step. It's possible for the first Scrutineer, if he's good at sleight of hand, to swap one ballot paper for another before recording it. Or for the third Scrutineer to swap ballots during the counting process.
