This app is intended as a tool for understanding a method of encrypting messages known as Public Key Cryptography (RSA). Invented in the 1970s this method still (in 2023) underlies much of the secure electronic transfer of information including communication with a bank (either online or from ATM machines), ordering goods online, transfering medical records and much more. The app is NOT designed for actually sending secure messages; the version of the method implemented here is intensionally a toy version which is more easily understood than the full version. You will see how messages are encoded and decoded and their authenticity verified by using the app to exchange messages with colleagues and friends. The extensive help pages explain most of the mathematics behind this method. It involves concepts often presented to 2nd or 3rd year university students but is accessible to smart and curious middle-school students; the original version of this app was in fact written to accompany a series of sessions of the author with the seventh-grade math class of one of his grandchildren.