Porta Cipher (History and Example)

Porta cipher is a polyalphabetic cipher that uses a key and a series of 13 reversible alphabets each associated with 2 letters of the alphabet.

For example, A and B for the first letter, C and D for the second, and so on.

Since the 13 cipher alphabets are reciprocal, encrypting and decrypting are the same.

Porta cipher is very similar to Vigenere cipher.

Difference between porta cipher and vigenere cipher:

Vigenere cipher is a polyalphabetic cipher with 26 alphabets while the porta cipher is the same except that is only uses 13 alphabets.

A brief history of porta cipher:

It was invented by Giovanni Battista della Porta in the 16th century.

Are you interested in finding out more about ciphers and codes?

The Codebreakers – The Story of Secret Writing book by David Kahn is what I would start with.

Porta Cipher

In this guide I will be discussing the following:

  • How to solve Porta cipher.

1) How to solve Porta cipher

How to encrypt using the porta cipher:

Here are the alphabets used by default:

AB ABCDEFGHIJKLM
NOPQRSTUVWXYZ
NOPQRSTUVWXYZABCDEFGHIJKLM
CD ABCDEFGHIJKLM
ZNOPQRSTUVWXY
ZNOPQRSTUVWXYBCDEFGHIJKLMA
EF ABCDEFGHIJKLM
YZNOPQRSTUVWX
YZNOPQRSTUVWXCDEFGHIJKLMAB
GH ABCDEFGHIJKLM
XYZNOPQRSTUVW
XYZNOPQRSTUVWDEFGHIJKLMABC
IJ ABCDEFGHIJKLM
WXYZNOPQRSTUV
WXYZNOPQRSTUVEFGHIJKLMABCD
KL ABCDEFGHIJKLM
VWXYZNOPQRSTU
VWXYZNOPQRSTUFGHIJKLMABCDE
MN ABCDEFGHIJKLM
UVWXYZNOPQRST
UVWXYZNOPQRSTGHIJKLMABCDEF
OP ABCDEFGHIJKLM
TUVWXYZNOPQRS
TUVWXYZNOPQRSHIJKLMABCDEFG
QR ABCDEFGHIJKLM
STUVWXYZNOPQR
STUVWXYZNOPQRIJKLMABCDEFGH
ST ABCDEFGHIJKLM
RSTUVWXYZNOPQ
RSTUVWXYZNOPQJKLMABCDEFGHI
UV ABCDEFGHIJKLM
QRSTUVWXYZNOP
QRSTUVWXYZNOPKLMABCDEFGHIJ
WX ABCDEFGHIJKLM
PQRSTUVWXYZNO
PQRSTUVWXYZNOLMABCDEFGHIJK
YZ ABCDEFGHIJKLM
OPQRSTUVWXYZN
OPQRSTUVWXYZNMABCDEFGHIJKL

 

To encode, take the first letter of the key, then find the corresponding alphabet. Finally make a substitution of the first letter of the plaintext with this alphabet.

Here is an example:

Plaintext: Kifanga.

Key: Porta.

The first letter of the key P corresponds to the alphabet TUVWXYZNOPQRSHIJKLMABCDEFG, so I substitute the letter K with Q.

Then I take the second letter of the key O and the corresponding alphabet TUVWXYZNOPQRSHIJKLMABCDEFG, I substitute the letter I with O.

Thus, ciphertext: QOXRAZT.

How to decrypt Porta cipher:

Decryption is the same as encryption since alphabets used are reversible.

What are common Porta alphabets:

Here are sets of reversible alphabets:

NOPQRSTUVWXYZABCDEFGHIJKLM
ZNOPQRSTUVWXYBCDEFGHIJKLMA
YZNOPQRSTUVWXCDEFGHIJKLMAB
XYZNOPQRSTUVWDEFGHIJKLMABC
WXYZNOPQRSTUVEFGHIJKLMABCD
VWXYZNOPQRSTUFGHIJKLMABCDE
UVWXYZNOPQRSTGHIJKLMABCDEF
TUVWXYZNOPQRSHIJKLMABCDEFG
STUVWXYZNOPQRIJKLMABCDEFGH
RSTUVWXYZNOPQJKLMABCDEFGHI
QRSTUVWXYZNOPKLMABCDEFGHIJ
PQRSTUVWXYZNOLMABCDEFGHIJK
OPQRSTUVWXYZNMABCDEFGHIJKL

or

NOPQRSTUVWXYZABCDEFGHIJKLM
OPQRSTUVWXYZNMABCDEFGHIJKL
PQRSTUVWXYZNOLMABCDEFGHIJK
QRSTUVWXYZNOPKLMABCDEFGHIJ
RSTUVWXYZNOPQJKLMABCDEFGHI
STUVWXYZNOPQRIJKLMABCDEFGH
TUVWXYZNOPQRSHIJKLMABCDEFG
UVWXYZNOPQRSTGHIJKLMABCDEF
VWXYZNOPQRSTUFGHIJKLMABCDE
WXYZNOPQRSTUVEFGHIJKLMABCD
XYZNOPQRSTUVWDEFGHIJKLMABC
YZNOPQRSTUVWXCDEFGHIJKLMAB
ZNOPQRSTUVWXYBCDEFGHIJKLMA

 

How to recognize a Porta ciphertext:

The encrypted message has a low index of coincidence due to the use of several alphabets.

How to decipher porta without key:

Use Friedman test to get the key length using probability techniques.

What are the variants of the Porta cipher:

Porta is similar to Bellaso cipher, which also uses several alphabets.

Are you interested in learning how to break codes?

The Elementary Cryptanalysis – A Mathematical Approach book by Abraham Sinkov is what I would recommend.

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