I did my best to make all necessary information at least minimally available in-game. If any in-game explanations are lacking or unclear, that is unintended. All the tools for deciphering the texts should be available in-game as well, although a pen and paper or open notepad would be helpful to keep track of key letters as you work through them. I also did not do a thorough job of play testing each text's deciphering, so I may have accidentally made some of them arbitrarily difficult or tedious.
The intended gameplay loop is to begin by typing "scry" (without quotation marks, followed by enter) to get a list of commands, then "scry <command>" to get detailed information on a command. For example, "scry vc" will tell how to use the vc (Vigenère cipher) command, including an explanation of how the Vigenère cipher works. Repeat for each command and you should be able to deduce how to use them to begin cracking each ciphertext.