[vwuon]

>The simplest possible way to implement this: local hash matching. In this situation, there’s a full CEI hash database inside every client device. The image that’s about to be sent is hashed using the same algorithm that hashed the known CEI images, then the client checks to see if that hash is inside this database. If the hash is in the database, the client will refuse to send the message (or forward it to law enforcement authorities).

The image could be scanned when it's received, and not when it's sent. That way you can't use hacked clients to send forbidden images.

[amarshall]

But then you just use a modified client to receive them. I’ve no idea how often the recipient isn’t wanting to receive the message in this context, but I’d expect it’s not often.