In simple words:
Messages are stored in transaction extra field, like PayLink (Payment ID), but encrypted. Sender encrypt his message with recipient private key (only recipient know his key), message is sent like any other DarkNote/CryptoNote transaction, so it is anonymous, untraceable and even the fact that message was send is unknown.
Recipient "looks" at encrypted extra field and decrypt only his message.
So DarkNote messages are:
-anonymous
-untraceable
-"invisible"
-Mixed with other messages
-Transferred via p2p PoW ASIC resistant decentralized DarkNote network
Thanks a lot for the simple explanation!
By what method is the message "invisible"? Does this mean that the tx_extra field from a transaction without a message is indistinguishable from one with a message in it? Do all transactions have a filled tx_extra field? So, when it's mixed with other messages through the ring signature, you wouldn't know if one had a message or not? Also, how long can the total message be?