Codes, stegonography, cryptography, programs

Need to be more clear about what you're needing. https://en.wikipedia.org/wiki/Code https://en.wikipedia.org/wiki/Cryptography https://en.wikipedia.org/wiki/Cipher https://en.wikipedia.org/wiki/Steganography (usually cracked pretty easily) https://en.wikipedia.org/wiki/VeraCrypt (my preferred encryption program, adored by millions) ..and as Manstuprator linked, PGP is a reliable old favourite for emailing back and forth. It used to be just the encryption but became a full-fledged email program with encryption at it's heart. It's not uncrackable completely but will stop the casual snoop from seeing anything between yourself and the other end. The short: you have a public key and a private key and so does your friend. Your public key is for your friend to encrypt for you before sending email, and the private key is for you to decrypt when receiving the email (you don't reveal your private key but you share your public key). Same for your friend, you encrypt using his public key and he decrypts using his private key. In-between the two of you, only the National Security Agency will have a chance of cracking it after a bit of time hammering at it with their supercomputers.