12 Digraph substitution
In his 1563 book De Furtivis Literarum Notis, Italian polymath and cryptologist Giambattista della Porta (1535–1615) proposed an interesting encryption method. Instead of replacing single letters, his system substituted letter pairs (also known as digraphs). In the centuries that followed, different variants of this digraph substitution were developed—including the so-called Playfair cipher. In this chapter, we will cover both methods: Playfair and Porta’s original system.
How general digraph substitution works
The method Giambattista della Porta suggested is based on a digraph substitution table with 400 unique entries. ...
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