Ché Guevara used a number code when communicating with Fidel Castro. He transposed each letter in the text to a number […]. He then wrote those numbers one behind the other. Below that line he wrote a second line of numbers, known only to him and Castro, which was used only once. He then added both lines, number per number, and below each set of numbers he wrote (the last digit of) the sum. This give a third line of numbers. Only that row was transmitted. When Castro subtracted the second line from the third, he had the first line as the result.
The Ché method cannot be cracked because the key (the second line of numbers) is random, as long as the message, is only used once.
Here is a photo of the original manuscript:
More details: http://serdis.dis.ulpgc.es/~ii-cript/PAGINA%20WEB%20CLASICA/CRIPTPLOGIA/CIFRADO%20ONE-TIME%20PAD.htm (in Spanish)