One Time Pad
Resources
for both inbound and outbound traffic.

Not quite 'one time', but perfectly
adequate for amateur radio.

Download Dirk Rijmenant's   CT-46 Encrypt / Decrypt Utility

Contrary to the mandatory trade-craft practiced by real agents in the field, we do not burn the key page we have just used.  Instead we reuse it over and over,  but be sure to specify a different 5-digit key indicator (start group) every time as a nod to security.