A simple loophole in iPhone 5s's TouchID lets you add more than five fingerprints to the system. In fact, it's so smart/stupid that you can add all five fingers to just one fingerprint entry. And when it's time to use your fingerprint to unlock the iPhone 5s, all the fingers that you added work just fine.
Apple hasn't issued any acknowledgment for this “feature” but there's a lot of commentary going around about it anyway.
If you're interested in this sort of a thing, here's what you need to do:
The process of adding a fingerprint to TouchID is simple. You head over to Settings → General → Passcode & Fingerprint → Fingerprints and tap on Add Fingerprint…
The normal method is to use one finger, tap on the home button repeatedly (adjusting the position ever so lightly) till your iPhone 5s records all parts of the fingerprint.
But for tapping the loophole that exists in the TouchID system, what you do is use as many fingers as you want to finish the fingerprint scan. This means you press the thumb once, then the index, may be your pinky next and so on. In the video show below, he uses all five fingers while setting up one fingerprint.
Video Tutorial of Adding More than Five Fingerprints to iPhone 5s TouchID
One sensible comment over at iMore goes something like “Wouldn't it make sense to add 2 fingers per slot; this way, you get all your fingers indexed” – this results in you being able to authenticate your iPhone 5s with any finger!
A Word on Accuracy
As far as I've seen and heard, when you add multiple fingerprints to one entry, the accuracy is still intact. Don't ask me how but it's a little insane that Apple's TouchID works even with multiple fingerprints in one slot. May be it's because of the resolution that the TouchID sensors have.
Nevertheless, setting it up might take a little while. Once you're done, if things aren't so very accurate, you might just go back to the normalcy of having five fingerprints in five entries. Simple.