I read about a technique for a secure seal in a book, "The Codebreakers", by Kahn. In the mid 20th century diplomats would use wax seals to seal envelopes. Spy agencies would intercept the letters, make very good molds of the seals, open the envelopes, and replace the seals with duplicates made from the molds.
The countermeasure was to mix two colors of wax, and not mix them too well, so there would be splotches and swirls of different colors. They would take a color picture of the finished seal and send the picture by a different method.
So we could go back to the same method, and keep a color picture of the seal in our journals. |
