Is there any tool which allows me to encrypt /decrypt file names in ubuntu?
e.g. I create a directory, all the file names there are encrypt. When I mount it with a password, all the file names there will be decrypted.
Is there such a thing?
ecryptfs would be apt for your use if you just need to encrypt a directory. Other options would be dm-crypt, Luks, truecrypt though they tend to work on whole filesystem (needs a block or loop device)
See https://help.ubuntu.com/community/EncryptedPrivateDirectory for installation instruction