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  Bifid Cipher
Period/Block Size:

In classical cryptography, the bifid cipher combines the Polybius square with transposition, and uses fractionation to achieve diffusion. This makes Bifid fairly difficult to crack. It was invented around 1901 by Felix Delastelle.

The key should have 25 unique letters from the alphabet - generally "J "is ommitted

The period/block size needs to be larger than 1 (typically 5 to 10) and smaller than the message size.