Nihilist cipher

There are two keys. The square key builds the 5x5 grid, and the key word is added to every number. Type a message to get numbers, or paste numbers in and switch direction to get the message back.

The sample is the example from Wikipedia. With the square ZEBRAS and the key RUSSIAN, "dynamite winter palace" begins 37 106 62.

Fill a 5x5 square with the square key, skipping repeated letters, then the rest of the alphabet. I and J share a cell. Each letter is now a two-digit number, its row then its column, so the letter in the top-left corner is 11.

Turn the key word into numbers the same way. Write those numbers over the message numbers, repeating the key, and add each pair. D is 23 in the ZEBRAS square and R is 14, so the first number is 37.

The sums are ordinary sums, so they can go over 100. To decode, take the key numbers away and read each result as a row and a column.

The cipher takes its name from the Russian Nihilists, who used it in the 1880s in their campaign against the tsarist regime.

Stronger versions lasted well into the 20th century. Max Clausen used one in Richard Sorge's spy network in Japan, and it was also used by the Lucy spy ring in Switzerland. Later Soviet versions added the digits without carrying, and the VIC cipher used by Reino Häyhänen in the 1950s grew out of the same idea.

The Polybius square is the first half of this cipher on its own. The bifid cipher uses the same square but splits the row and column digits apart instead of adding to them.