Perfectly possible. It's "proprietary" in contrast to "standardized", not in contrast to "open". The Signal protocol isn't a standard from any standards body (e

Perfectly possible. It's "proprietary" in contrast to "standardized", not in contrast to "open". The Signal protocol isn't a standard from any standards body (eg ISO, or the IETF), so it's proprietary. Its spec is freely accessible, and free to use, but it's still one company's protocol.