This was one of the most secure hand ciphers used in the Second World War. It was used by both the Allies and the Axis, and served both well. Its main weakness was that if the attacker intercepted ...
Let's recap the basic method for column addition with this video. Start from the ones column: 3 + 9 = 12. Write a 2 in the ones column and carry 10 to the tens column by writing a 1 below.