There is one called tap code, also known as knock code. It was used by AmericanPOW's in the Vietnam war to communicate deceptive stories when tortured for information. It is a 5x5 polybius square. read more
There is one called tap code, also known as knock code. It was used by AmericanPOW’s in the Vietnam war to communicate deceptive stories when tortured for information. It is a 5x5 polybius square. read more