+----------+             +----------+             +----------+             +----------+
|  User A  |             | Backend  |             | Database |             |  User B  |
+----------+             +----------+             +----------+             +----------+
     |                        |                        |                        |
     |--- Get B's PubKey ---->|                        |                        |
     |<-- Return B's PubKey --|                        |                        |
     |                        |                        |                        |
     |--[ Derives AES Key ]--|                        |                        |
     |--[ Encrypts Message ]--|                        |                        |
     |                        |                        |                        |
     |---- Send Encrypted --->|                        |                        |
     |   {ct, iv, tag, to:B}  |---- Persist Record --->|                        |
     |                        |    {ct, iv, tag, to:B} |                        |
     |                        |                        |                        |
     |                        |---------------- Forward Encrypted ------------->|
     |                        |                 {ct, iv, tag, from:A}           |
     |                        |                        |                        |
     |                        |                        |                  [ Derives Key ]
     |                        |                        |                  [ Decrypts & ]
     |                        |                        |                  [ Verifies ]