Double Columnar Transposition

Because two is better than one. This was used by the U.S. Army during World War II.

A double transposition, also known as a double columnar transposition, was used by the U.S. Army in World War I, and it is very similar to the German's Übchi code. It is just two columnar transpositions in a row.

If you want to decode the third part of Kryptos with this tool, enter 21 "a" characters as the first key, 28 "a" as the second, check the "Number duplicate entries backwards", and paste in the text for K3, with or without the question mark.