01 What is the Polybios Square?

Polybius square – Wikipedia

The Polybios cipher is a monographic monoalphabetic substitution cipher. It is named after the ancient Greek historian Polybios.

For the cipher, a 5×5 square is formed from the alphabet. The letters J, C and W are optionally replaced by I, K or VV.

The key consisting of the digits 1- 5 is written over the columns and rows.

Then the plaintext is replaced letter by letter by the row and column number.