How to build large-scale end-to-end encrypted group video calls

Signal released end-to-end encrypted group calls a year ago, and since then we’ve scaled from support for 5 participants all the way to 40. There is no off the shelf software that would allow us to support calls of that size while ensuring that all communication is end-to-end encrypted, so we built our own open source Signal Calling Service to do the job. This post will describe how it works in more detail.