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Porta cipher

Thirteen alphabets, each pairing the first half of the alphabet with the second, picked in turn by a key.

Conversion

Porta is its own inverse. Run the cipher text through with the same key and the message comes back.

26 characters
Updates as you type
Key: porta

Show the steps
  1. Write the key PORTA over the message, repeating it. Each key letter picks one of 13 rows, one for each pair AB, CD and so on to YZ.
  2. The rows your key uses: OP: ABCDEFGHIJKLM ↔ UVWXYZNOPQRST; QR: ABCDEFGHIJKLM ↔ VWXYZNOPQRSTU; ST: ABCDEFGHIJKLM ↔ WXYZNOPQRSTUV; AB: ABCDEFGHIJKLM ↔ NOPQRSTUVWXYZ.
  3. Each row pairs every letter in A–M with one in N–Z. Find the message letter in the row and swap it for its partner.
  4. E under key P (row OP) → Y; N under key O (row OP) → G; C under key R (row QR) → X; I under key T (row ST) → R; P under key A (row AB) → C…
  5. The swap works both ways, so the same key turns the cipher text back.

Using the Porta tool

Type a key and your message to get the cipher text. Decoding is the same step with the same key, so paste cipher text in and the message comes out. Only letters are carried, in capitals.

The sample is the American Cryptogram Association's own example. With the key PORTA, "encipherment is reciprocal" becomes YGXRC OYJVR GMQJE YWQGE HWVU.

How the 13 alphabets work

The key letters come in pairs. A and B pick the first row, C and D the second, and so on to Y and Z, which gives 13 rows. In each row the letters A to M are paired with the letters N to Z.

In the AB row, A pairs with N, B with O and so on, the same swap as ROT13. Each later row slides the second half one place further along, so in the CD row A pairs with O.

Write the key over the message, repeating it, and swap each message letter for its partner in the row its key letter picks. A letter from the first half always becomes one from the second half, and the other way round. That is why the cipher undoes itself, and also why no letter can ever stand for itself.

Named after della Porta

The cipher is named after Giambattista della Porta, who was born near Naples in 1535 and died in 1615. In 1563 he published De Furtivis Literarum Notis, a work about cryptography that described the first known cipher to swap letters two at a time.

Charles J. Mendelsohn called him "the outstanding cryptographer of the Renaissance".

Which way the rows turn

Not every site lays out the table the same way. This tool follows the American Cryptogram Association, where the CD row sends A to O. Some sites slide the rows the other way, so the CD row sends A to Z. If a Porta message from somewhere else won't decode, that is the likely reason.

Questions

How do I decode a Porta cipher?

Type the cipher text in with the same key. Each row of the table is a set of swaps, so the step that encoded the message also decodes it.

Why are there only 13 alphabets?

Each row covers two key letters, A and B, then C and D, and so on. Twenty-six letters make 13 pairs.

Can a letter stand for itself in Porta?

No. Letters A to M always become letters N to Z, and N to Z always become A to M.

Sources

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