Answer:
O Moon Letters.
Step-by-step explanation:
When the dwarves along with Bilbo and Gandalf went to the elves for shelter, they approached Elrond to help him read the 'map'. This map was to lead them to the secret entrance to the kingdom under the mountains that has been 'captured' by the dragon Smaug.
When Elrond saw the map, he recognized it to be written in a kind of letter called "Moon Letters". These letters are a kind of runes that can only be read in the moonlight. This form of letters was invented by the Dwarves and help 'hide' information from outsiders or enemies by presenting a blank page. This makes it impossible for the information to get into the wrong hands and thus, keeping the secret.