01 What is the Bifid Cipher?

Bifid cipher – Wikipedia

The bifid cipher, which was introduced by Félix Marie Delastelle (1840 – 1902) in 1895 under the name cryptographie nouvelle in the French journal Revue du Génie civil, uses the idea of fractionating (“breaking”) bigrams in addition to the classical methods of Polybios square and transposition.

It is thus a monoalphabetic monographic substitution cipher with subsequent transposition.