Message Across Islands

There are two islands. On one of the islands, a man has a message he wants to send his friend on the neighboring island. The man with the message has a chest, a lock, and one key to that lock. He knows that his friend has a different lock and one key to his lock. There is a pirate who sails back and forth between the two islands: if the pirate sees a chest on the beach, he will ferry the chest over to the other island. However, if the chest is not locked, the pirate will steal everything that is inside. In addition, if the pirate sees anything other than a locked chest on the beach, he will steal it, and he will kill anyone he sees on the beach. How does the man with the message get his message to his friend?