For encrypting a single file there are many options such as:
PGP encryption softwares
scrypt and bcrypt (slow and secure hash algo)
ccrypt (not sure what it's using)

Encrypting a single file is useless if it stays in memory or on the disk after you've opened it. Make sure it's not cached, replicated in /tmp, this happens a lot when you open it with vim, etc.. Otherwise a good reverse engineer in data carving can fetch it back.

Moreover, if you really want to erase that file check the `shred` utility or consider using full disk encryption.