ETA ON IRSH and digrams. (Mnemonic: before the Notre Dame game, the Michigan Wolverines asked, What's the ETA ON the IRiSH?)
Speaking of Irish, one of the great unsolved ciphers is also one of the great unknown ciphers. Very little work has been done on deciphering the markings in the ancient Irish Cave of Writing (Pluais na Scrioba). If you are interested in trying your hand at it, please let me know.
OIT. To connect, open Terminal (Macintosh) or something else (Windows). At the prompt, type
replacing your username where appropriate.
Hit <return>. You will be prompted for your password. Type exit to get out. Here are some basic commands.
Handbook on Applied Cryptography (free).