You can save this HTML file for use on an offline computer, then save two shared keys as a text files on two USB drives: one set you give to the recipient (in person!), one you keep, then the ciphertexts can be shared electronically. Use one shared key (and a separate tab of this webpage) for each direction of communication. Never use part of a shared key that's already encrypted one message to encrypt another: the separate keys for each direction of communication are to avoid this happening with crossed send/recieve.
"Pad" or shared key (hex): Auto-crop