Tool to decrypt / encrypt with Porta cipher. Invented by an Italian physicist Della Porta, this poly alphabetical cipher changes of alphabet every letter.

Porta Cipher - dCode

Tag(s) : Poly-Alphabetic Cipher

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Tool to decrypt / encrypt with Porta cipher. Invented by an Italian physicist Della Porta, this poly alphabetical cipher changes of alphabet every letter.

The encryption by Porta uses a key and a series of 13 reversible alphabets each associated with 2 letters of the alphabet: A and B for the first, C and D for the second, and so on.

Example: The alphabets used by default are:

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 |

The Porta algorithm consists of taking the first letter of the key, finding the corresponding alphabet, and making a substitution of the first letter of the plain text with this alphabet.

Example: Encrypt the message DCODE with the key PORTA. The first letter of the key P corresponds to the alphabet TUVWXYZNOPQRSHIJKLMABCDEFG, which allows to substitute the letter D with P

The encryption is continued by taking the second letter of the key and the corresponding alphabet and substituting the second letter of the plain text, with this second alphabet and so on.

Example: DCODE is then encrypted WVJUR.

Decryption is the same as encryption if alphabets are reversible.

Several sets of reversible alphabets are mentioned, including:

NOPQRSTUVWXYZABCDEFGHIJKLM |

ZNOPQRSTUVWXYBCDEFGHIJKLMA |

YZNOPQRSTUVWXCDEFGHIJKLMAB |

XYZNOPQRSTUVWDEFGHIJKLMABC |

WXYZNOPQRSTUVEFGHIJKLMABCD |

VWXYZNOPQRSTUFGHIJKLMABCDE |

UVWXYZNOPQRSTGHIJKLMABCDEF |

TUVWXYZNOPQRSHIJKLMABCDEFG |

STUVWXYZNOPQRIJKLMABCDEFGH |

RSTUVWXYZNOPQJKLMABCDEFGHI |

QRSTUVWXYZNOPKLMABCDEFGHIJ |

PQRSTUVWXYZNOLMABCDEFGHIJK |

OPQRSTUVWXYZNMABCDEFGHIJKL |

NOPQRSTUVWXYZABCDEFGHIJKLM |

OPQRSTUVWXYZNMABCDEFGHIJKL |

PQRSTUVWXYZNOLMABCDEFGHIJK |

QRSTUVWXYZNOPKLMABCDEFGHIJ |

RSTUVWXYZNOPQJKLMABCDEFGHI |

STUVWXYZNOPQRIJKLMABCDEFGH |

TUVWXYZNOPQRSHIJKLMABCDEFG |

UVWXYZNOPQRSTGHIJKLMABCDEF |

VWXYZNOPQRSTUFGHIJKLMABCDE |

WXYZNOPQRSTUVEFGHIJKLMABCD |

XYZNOPQRSTUVWDEFGHIJKLMABC |

YZNOPQRSTUVWXCDEFGHIJKLMAB |

ZNOPQRSTUVWXYBCDEFGHIJKLMA |

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

A Friedman test retrieves the key length probabilistically.

This is not a variant but Porta is similar to the Bellaso Cipher, another Italian, who uses several alphabets.

In the 16th century

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Source : https://www.dcode.fr/porta-cipher

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