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.