In Mimblewimble there are no addresses, and transactions need to be built interactively by the participating parties. This requires that both parties set direct socket connection for every transaction, which can be impractical, and doesn’t allow offline transactions.
A Secure Bulletin Board (SBBS) system solves this problem, enabling offline transactions, which makes the user experience the same as other cryptocurrencies such as Bitcoin. With SBBS, client wallets can exchange messages by store-and-forward nodes. The messages are encrypted with public key cryptography, over elliptic curve cryptography (ECC) algorithms.
Originally, a bulletin board system is a computer server running software that allows users to connect with a client. Once logged, the user can upload or download messages through public message boards or private email, as well as file downloads (often text files), and if the client allowed, direct chatting and even text-based multiplayer games. This is the ecosystem where public key cryptography, such as Phil Zimmerman’s PGP implementation originally spread.
The architecture of a BBS is replicated with Beam’s nodes and wallets, as described in the GitHub repository, and a Medium article. With SBBS, messages are private when addressed to an individual, such messages are encrypted with a public key before they are sent, without having to trust relaying nodes. Other messages can be set to public and be unencrypted or sent to specific channels.