It’s not natively supported by the base RCS standard, in the section at the end of the paper in the section titled “Third Party RCS Clients” Google explains that they’ve built the e2ee their Messages app themselves, (on top of standard RCS).

A developer has to use Google’s implementation specifically in order to send and recieve e2ee messages to Google’s Messages app (and Samsung Messages who also implemented this recently)

Although the e2ee implementation is using the Signal protocol under the hood, it’s for message content only - this is what is transmitted in cleartext (taken from the paper)

• Phone numbers of senders and recipients
• Timestamps of the messages
• IP addresses or other connection information
• Sender and recipient’s mobile carriers
• SIP, MSRP, or CPIM headers, such as User-Agent strings which may contain device manufacturers and models
• Whether the message has an attachment
• The URL on content server where the attachment is stored
• Approximated size of messages, or exact size of attachments

Without using this implementation of the Signal protocol on top of RCS, the message will deliver to the contact’s phone, but shows up as unencrypted garbled text

That is a very useful resource though, never knew there was a paper available on the implementation. Saving 😁