I love Siri, but I'm not so keen on being able to access it from the lock screen. After all, if I can pull up contact info, turn off settings, and so on without having to unlock my device, so can any random person. With the iPhone 5s, though, there's a way to configure things so that only your fingerprints can invoke Apple's voice assistant from the lock screen. You'll set this up by going to Settings> General> Passcode & Fingerprint, and then toggle off Siri's "Allow Access When Locked" button.
Assuming you set up Touch ID when you got your phone, only registered fingerprints will now be able to use Siri at the lock screen. The great thing is that for you, it'll seem as if Siri works as normal when the phone is locked and the Home button is held down; for everyone else, this is what your iPhone will show when that happens:
Just another bit of security for the paranoid—and I count myself among that number. In fact, I may just be the mayor of that particular demographic.