Classical Cipher used in Classical Cipher used in World War I

Playfair Cipher

Polygraphic substitution is a cipher in which a uniform substitution is performed on blocks of letters

Playfair Cipher encrypts pairs of letters (bigrams or digrams), instead of single letters. This makes Frequency Analysis much harder, since there are possible bigrams rather than the possible monograms.

a considerably larger cipher text is required in order to be useful. The playfair cipher