How can they guarantee E2EE when the message is converted to analog light waves? What about in your brain?

The end of end-to-end is the two messaging devices. That is all and in almost every case, it’s enough.

How can they guarantee end-to-end encryption when someone could be looking over your shoulder as you type your message?

Same answer. They don’t control the input, they just keep it safe once it’s in transit.

Yeah, fair point and largely the point of the article. Too many conditions exist for Signal to accidentally reveal information. I still wonder how many people assume the privacy offered is much more than just the data in transit. I’d bet a large percentage of users believe that.

No one can guarantee any thing. Not even most gods or a time traveler from 1000 years in the future. An omnipotent being could.

It makes sense to attack and work on problems that are actually happening and are solvable within some level of aspiration.

I’m not sure how things work on either mobile OS. Perhaps there can be more clear language and notifications on if everything is being passed to the virtual keyboard developer or not.

Unless you mean Apple, Google, and other Android manufacturers themselves. That’s beyond the scope of any E2E stuff.