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Porta Cipher

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

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Porta Cipher -

Tag(s) : Poly-Alphabetic Cipher

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Porta Cipher

Porta Decoder

 



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Alphabet fill: [Original (22 letters)] [Variant 26 letters (1)] [Variant 26 letters (2)]

Porta Encoder

 



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Alphabet fill: [Original (22 letters)] [Variant 26 letters (1)] [Variant 26 letters (2)]

Answers to Questions (FAQ)

What is the Porta cipher? (Definition)

The original Porta encryption is a polyalphabetic encryption using a key and a series of 11 reversible alphabets.

How to encrypt using Porta cipher?

Porta encryption uses a key and 11 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.

As Porta is Italian, its alphabet has only 22 letters: ABCDEFGHILMNOPQRSTVXYZ (missing J, K, U and W)

To use the Latin alphabet, the principle has been intrapolated to 26 letters and 13 alphabets.

Example: The alphabets used by default are:

ABNOPQRSTUVWXYZABCDEFGHIJKLM
CDZNOPQRSTUVWXYABCDEFGHIJKLM
EFYZNOPQRSTUVWXABCDEFGHIJKLM
GHXYZNOPQRSTUVWABCDEFGHIJKLM
IJWXYZNOPQRSTUVABCDEFGHIJKLM
KLVWXYZNOPQRSTUABCDEFGHIJKLM
MNUVWXYZNOPQRSTABCDEFGHIJKLM
OPTUVWXYZNOPQRSABCDEFGHIJKLM
QRSTUVWXYZNOPQRABCDEFGHIJKLM
STRSTUVWXYZNOPQABCDEFGHIJKLM
UVQRSTUVWXYZNOPABCDEFGHIJKLM
WXPQRSTUVWXYZNOABCDEFGHIJKLM
YZOPQRSTUVWXYZNABCDEFGHIJKLM

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 ABCDEFGHIJKLMTUVWXYZNOPQRS, which allows to substitute the letter D with W

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.

How to decrypt Porta cipher?

Decryption is the same as encryption if alphabets are reversible.

What are common Porta alphabets?

Since the original alphabet was Italian, it only had 22 letters, modernizing Porta to the current 26 letter alphabet is an interpolation.

Several sets of reversible alphabets are mentioned, including:

— (1) original version extrapolated (rotation of the second half of the alphabet to the right) :

NOPQRSTUVWXYZABCDEFGHIJKLM
ZNOPQRSTUVWXYABCDEFGHIJKLM
YZNOPQRSTUVWXABCDEFGHIJKLM
XYZNOPQRSTUVWABCDEFGHIJKLM
WXYZNOPQRSTUVABCDEFGHIJKLM
VWXYZNOPQRSTUABCDEFGHIJKLM
UVWXYZNOPQRSTABCDEFGHIJKLM
TUVWXYZNOPQRSABCDEFGHIJKLM
STUVWXYZNOPQRABCDEFGHIJKLM
RSTUVWXYZNOPQABCDEFGHIJKLM
QRSTUVWXYZNOPABCDEFGHIJKLM
PQRSTUVWXYZNOABCDEFGHIJKLM
OPQRSTUVWXYZNABCDEFGHIJKLM

— (2) variant suggested by ACA (American Cryptogram Association) (rotation of the second half of the alphabet to the left):

NOPQRSTUVWXYZABCDEFGHIJKLM
OPQRSTUVWXYZNABCDEFGHIJKLM
PQRSTUVWXYZNOABCDEFGHIJKLM
QRSTUVWXYZNOPABCDEFGHIJKLM
RSTUVWXYZNOPQABCDEFGHIJKLM
STUVWXYZNOPQRABCDEFGHIJKLM
TUVWXYZNOPQRSABCDEFGHIJKLM
UVWXYZNOPQRSTABCDEFGHIJKLM
VWXYZNOPQRSTUABCDEFGHIJKLM
WXYZNOPQRSTUVABCDEFGHIJKLM
XYZNOPQRSTUVWABCDEFGHIJKLM
YZNOPQRSTUVWXABCDEFGHIJKLM
ZNOPQRSTUVWXYABCDEFGHIJKLM

— (3) variant (double rotation of the alphabet) :

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

— (4) variant (double rotation of the alphabet)

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

To increase the security in the transmission of the encryption, it is recommended to use other alphabets.

How to recognize a Porta ciphertext?

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

How to decipher Porta without the key?

A Friedman test retrieves the key length probabilistically.

What are the variants of the Porta cipher?

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

When was the Porta cipher invented?

In the 16th century by Giambattista della Porta, a physicist who dedicated a book on messages encryption.

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Cite as source (bibliography):
Porta Cipher on dCode.fr [online website], retrieved on 2024-07-18, https://www.dcode.fr/porta-cipher

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