Porta
The Porta cipher is a poly-alphabetic substitution cipher where each letter in the plaintext is shifted based on a keyword, using a fixed tabula.
Explanation
The Porta cipher, also known as the Porta square or Portax cipher, is a variant of the Vigenere cipher and shares some similarities with it.
In the Porta cipher, encryption and decryption are performed using a keyword and a tabula recta or Porta square. The Porta square is a table that consists of 26 rows and 26 columns, where each row represents the alphabet shifted by a certain number of positions.
The Porta square is constructed by shifting the letters of the alphabet by an amount determined by the keyword.
Each row of the square represents a different shift of the alphabet, with the first row being shifted by 0 positions, the second row by 1 position, the third row by 2 positions, and so on until the last row, which is shifted by 25 positions.
The Porta cipher provides stronger encryption than simple substitution ciphers. However, it is still susceptible to frequency analysis and other cryptanalysis techniques.
Facts
It was invented by Giovanni Battista della Porta in the 16th century.
It is one of the earliest known poly-alphabetic ciphers, preceding the Vigenere cipher by several centuries.
It was one of the first ciphers to use a keyword to determine the alphabetic shift.