you both need tutanota.com email accounts and then send emails to eachother. there is a Tutanota app. they have secure in-house emailling so yalls coms would be secure unto your email passwords and device, same as signal i assume. to outside tutanota addresses not sure if it is pgp because it has you give a password to the receiver and that person would enter that in a tutanota webpage, i belive. i dont think i ever used this style of mail. they do good at data respecting and deletion and logs. it's german. my only complaint so far is there is no multi delete for the inbox and after a few years you need to pay or delete old inbox messages. the search tool works good. you just gotta cache your inbox to your computer. another idea is VPN and message boards. you should have some alternate vocabulary though. if you want to be very secure you can both have the same book and use the page number to reference the com incidence number(message from 3feb13:00,3feb15:00,5feb03:00) and for each character(in the com) it can be incremented up or down based on a value of a letter beginning at the bottom of the page and skipping every other(or every two) letter(you do this so people cant guess the key). So if the letter is a c in the coded message and y is the coresponding letter [in the book] then 25(y) needs to be wound backward on the c(3),, so E would be the decoded letter. for every letter sent there will be a different key. you can even cite reference material external to the message to get more info in less words. like i could have this post referenced in a message and the listener will know the new protocol.