In cryptography, the Polybius square, also known as the Polybius checkerboard, is a device invented by the Ancient Greek historian and scholar Polybius, for fractionating plain-text characters so that they can be represented by a smaller set of symbols.
The original square used the Greek alphabet, but can be used with any alphabet. In fact, it has also been used with Japanese hiragana. With the modern English alphabet, in typical form, it appears thus:
The original square used the Greek alphabet, but can be used with any alphabet. In fact, it has also been used with Japanese hiragana. With the modern English alphabet, in typical form, it appears thus:
Each letter is then represented by its coordinates in the grid. For example, "BAT" becomes "12 11 44". Because 26 characters do not quite fit in a square, it is rounded down to the next lowest square number by combining two letters (usually I and J). (Polybius had no such problem because the Greek alphabet he was using had 24 letters). Alternatively, the ten digits could be added and 36 characters would be put into a 6 × 6 grid. Such a larger grid might also be used for Cyrillic script (of which the most common alphabet variant has 33 letters, though some have fewer, and some up to 37). This encryption is standard, and so far has no key, and thus is easily broken. We need to introduce the idea of a key to the Polybius Square to make it more secure (Kirchhoff's Principle). This is done in a very simple way. We reorder the alphabet in the same way as we did for the mixed alphabet cipher before we put it in the grid. That is, we use the letters of the keyword first, ignoring any repeat So using a keyword of polybius we get the mixed square below.