The UP Messenger is built on top of the Matrix Standard Protocol with additional enhancements. The Matrix Standard is an open standard for communication. Matrix provides state-of-the-art end-to-end-encryption via the Olm and Megolm cryptographic ratchets.
The UP Messenger uses an open standard as it allows the community to review and validate it. Another important security aspect is that, in contrast to other messengers, where all communications flow through a server, and may be retained and stored on those servers, none of UP Messengers communications flow through servers. There are servers, but their sole purpose is key validation, so if Party A wants to call Party B, the server will force the system of Party A to generate a new private key, then will do the same with Party B, and then the server will validate the newly created private keys and will connect Party A and B directly. So none of your communications ever flow through servers or are retained on servers. Another nice byproduct of this security measure is that, since there is no bandwidth wasted on servers, the connection quality is visibly and audibly better than with other messengers.