After selecting game mode (absolutely any mode out of 1 available right now) and difficulty level you will be given an encoded message (green letters). There is no trickery, the encoding right now uses only character substitution, f.e. word TEST could be encoded into AGVA or POTP.
By pressing any letter or number on your keyboard you choose which character in the cypher you want to replace (it will be shown in the field marked INPUT), by pressing another key you choose the selected character should be replace with.
If there is character you chose used in the cypher they will be highlighted, when you choose replacement character it will be shown in white letters below. The cypher will be unsolved until every single character will be correctly decyphered - it will be signaled by change in field marked VALIDATOR.