Porta — Thirteen reciprocal alphabets, one per pair of key letters. No letter ever encrypts to itself, and the cipher is its own inverse.
Reciprocal polyalphabetic over 13 half-alphabet tables. Same period finding as Vigenere, 13 shifts per column.
A worked example§
Encrypting a sample with the cipher itself, at build time:
MEET ME BY THE OLD CLOCK TOWER AT DAWNKey: LEMON
produces the ciphertext
RTXM SW QF MNW MRX VQMVR AJHXK TB STCHThis example is generated by running the cipher when the page is built, and the round trip is checked, so it always matches what the solver does.
How it is broken§
Period finding as usual, then thirteen possibilities per column rather than twenty-six — a Porta column is easier than a Vigenere one.
History and context§
Giambattista della Porta, 1563. His De Furtivis Literarum Notis also described the first known digraphic cipher, three centuries before Playfair made the idea practical.
See also§
Vigenère · Beaufort · Variant Beaufort · Gronsfeld · Quagmire III (keyed alphabet) · Sum-clock (additive wheels)
Browse every cipher in the Polyalphabetic ciphers family, read the history of codebreaking, or return to the wiki main page.
Frequently asked questions§
How is the Porta broken?
Period finding as usual, then thirteen possibilities per column rather than twenty-six — a Porta column is easier than a Vigenere one.
What key does the Porta use?
keyword. The keyspace is unbounded in practice.
How much ciphertext do I need?
At least 24 characters for this solver to attempt it; short messages can be readable and still not be proof.
Can I break a Porta on this page?
Yes — the browser solver on this page handles it directly. Paste the ciphertext into the solver and press the button.