Bifid cipher
Bifid cipher
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Bifid cipher

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Bifid cipher

In classical cryptography, the bifid cipher is a cipher which combines the Polybius square with transposition, and uses fractionation to achieve diffusion. It was invented around 1901 by Felix Delastelle.

First, a mixed alphabet Polybius square is drawn up, where the I and the J share their position:

The message is converted to its coordinates in the usual manner, but they are written vertically beneath:

They are then read out in rows:

Then divided up into pairs again, and the pairs turned back into letters using the square:

In this way, each ciphertext character depends on two plaintext characters, so the bifid is a digraphic cipher, like the Playfair cipher.

To decrypt, use the same Polybius square. Convert the ciphertext to its coordinates in the usual manner, but keep it written in a single row.

Find the midpoint of the row.

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