A great talk. I had no idea you could create a prompt for the user to input their password that wouldn't be visible to Javascript (really cool!).
One thing that was really glossed over in the beginning was switching from an arbitrary data format to an ArrayBuffer. Is there are built-in way to do that operation or will we need to write our own conversion functions?
For non-binary data in general you would have to create your own serialization mechanism. For everything that can be represented textually you can however use the TextEncoder/Decoder API: "var buffer = new TextEncoder("utf-8").encode(text)".