The Porta cipher was developed by the Neapolitan doctor Giovan Battista della Porta (* 1535 in Vico Equense; † 1615 in Naples).
The cipher is similar to the Vigenère cipher in some aspects. Depending on the current character of the key, a different alphabet for encoding the message is used. The translation table for the Porta cipher is shown in tab. 1.
Let the key be "KEY" and the message be "GEHEIMNIS". The G would be encoded by a K. The corresponding alphabet is found in the respective column of the translation table. According to the translation table, the G becomes an O. Next, the E is encoded by an E. So the E is mapped to a P.
Since the key "KEY" is shorter than the plaintextit is repeated until it covers the whole length of the plaintext. Finally, the ciphertext would be: "OPVZTNFTE".
Tab. 1: Porta cipher
The security of the Porta cipher is comparable to that of the Vigenère-Chiffre, but there are only half times as many possible keys.
Since several plaintext characters can be mapped to the same character in the ciphertext, this cipher belongs to the class of polyalphabetic substitutions.