If you're reading this article, you've probably stumbled on all the hubub about the app SnapHack, an app that enables you to save your received SnapChat photos. So how does it work?
SnapHack acts as an additional SnapChat app, connecting to the SnapChat data servers (just as the original app does) and showing you your SnapChats — only it saves the image without notifying the SnapChat server that it has done so.
So I won't get the little "Screenshot!" notification that my SnapChat was saved by the recipient?
Nope. That Screenshot notification originates from the SnapChat app (on your recipient's phone.) The app listens for a screenshot notification, and sends a message to the SnapChat server (which in turn, sends a message to your SnapChat app.)
What SnapChats of mine can someone using SnapHack get?
SnapHack users can only save new SnapChats that they haven't opened with the regular SnapChat app. SnapHack users will not be able to open and save old SnapChats that were opened with the regular SnapChat app prior to SnapHack's release.
So anything that says it was opened prior to SnapHack's release is safe. (Looks like SnapHack's release date was October 14th, 2013.)
How does SnapHack do it?
The creator of SnapHack reverse-engineered the communication protocol between SnapChat servers and the SnapChat app, so that they can mimic the same communication. Likely we'll see an update to SnapChat shortly that reflects a change in the server communication in order to break SnapHack.