If you wish to transmit an absolutely secure message to someone, the best method is to use one time pad cryptography. It doesn’t matter how powerful the computers anyone trying to break the code have, it’s unbreakable. There are only two vulnerabilities: it’s important to protect the physical pieces of paper from being compromised and it doesn’t protect against the “Hit him with a $5 wrench until he talks” attack.
The codes must be manually generated using a pair of different colored 10 sided dice. Ensure the first digit always comes from the darker color (to prevent biasing the ordering of the digits) and that the numbers are written onto two duplicate pages. It does take about half an hour to fill a pair of pages and is best done while watching television. As the pages are filled, place them into a pair of duotang binders. When a satisfactory number of pages are filled (such as 10), pass one of the binders to the person you will be exchanging messages with. Keep the duotangs well hidden away from cursory searches.
It’s critical that the codes are only encoded and decoded manually directly on the paper outside of the duotang (to prevent writing impressions on the next page from being exploited) and never inside a computer. Computers and cell phones can be compromised without you knowing it. Never reuse the codes and burn the pages after use to ensure they cannot be examined.
The document is hyperlinked below. Print out all of the pages for each pair of duotangs and make many copies of the first page.