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.

(source: https://themathematicaltourist.wordpress.com/page/2/)

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)

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If he only wrote the last digit of the sum at 3rd line, how was the first line reproduced?

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You have to subtract the third line minus the second (this 2nd line is the key) to get the original message (first line). If you cannot perform the operation with positive numbers, then add ten to the digit of the third line. So, for example 5-7 must be transformed into 15-7, which is 8.

There is a really comprehensive discussion in http://m4t.es (in Spanish but you can easily follow the operations)

Here is the link:

http://m4t.es/index.php/curiosidades/125-el-cifrado-de-vernam-y-el-cuaderno-de-un-solo-uso-de-che-guevara

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