https://support.google.com/messages/answer/10262381?hl=en
E2EE White paper (technical specifications) is listed on this site (pdf)
If you mean this link: that’s a high level description of the protocol, but it leaves out important details.
For example, Google uses MLS for group chats, but the document only mentions the Signal protocol. In other words, E2EE for group chats is broken even if you manage to implement the protocol exactly as they describe.
For example, they say the client “registers with the key server” and “uploads the public key parts”. What server is that? What protocol do we use? HTTPS POST? Do we use form/multipart? Do we encode the key in PEM or do we submit they bytes directly?
Another example: “Key material, digest, and some metadata are encrypted using the Signal session”. Whay do you mean “some”? What algorithm is used to generate the digest?
The document is a nice high level overview, but worthless if you want to implement their protocol. It basically says “we put signal, and send the signal messages over RCS, with out own key servers. Here’s how the Signal protocol works”. If, for example, Ubuntu Touch would like to implement this into their messenger, they’ll need to reverse engineer Google’s Messages app, guided by the description in their whitepaper.
