Porta Cipher — Bench tool
Published 8 Oct 2026 ·
Giovanni Battista della Porta published this tableau in 1563 and solved a problem that had annoyed cryptographers for a century: he built a polyalphabetic cipher that is its own inverse. Encipher and decipher use the same procedure with the same key, so a field clerk could not get the direction wrong.
- Era
- Renaissance
- Type
- Polyalphabetic
- Key space
- 26^k, 13 rows
- Level
- Field
IWhat this tool does
The tableau has thirteen rows, and each row is keyed by a pair of letters: row one by A or B, row two by C or D, and so on to row thirteen by Y or Z. This halves the key alphabet and makes each row an involution.
Each row works by splitting the alphabet in half. Letters A-M are the first half, N-Z the second. In row one, A maps to N, B to O, and so on; N maps back to A, O back to B. Applying the row twice returns the original letter, which is the property that makes the cipher reciprocal.
| Key pair | A maps to | N maps to | Cipher of A then N |
|---|---|---|---|
| AB (row 1) | N | A | round trip |
| CD (row 2) | O | M | round trip |
| EF (row 3) | P | L | round trip |
A worked example with key PORTA on the plaintext LOOM:
| Pos | Plain | Key | Row | Cipher |
|---|---|---|---|---|
| 1 | L | P | 8 | A |
| 2 | O | O | 8 | Y |
| 3 | O | R | 9 | C |
| 4 | M | T | 10 | D |
Note the first two letters used the same row because P and O are both in row eight's key pair range; the last two rows differ. That is the mechanism: a key of length k gives 13^k distinct row sequences rather than 26^k, but buys reciprocity in exchange.
IIThe key
For a key of length k the row-sequence space is 13^k. A six-letter key gives about 4.8 million row sequences — a factor of 64 smaller than Vigenere's 26^6, and the price paid for reciprocity.
That is a real reduction but not a fatal one for its time. Porta's genuine weakness was the same as Vigenere's: the key repeats, and a repeating key is measurable with Kasiski examination.
Reciprocity is an operational feature, not a security feature. Making a cipher easier to use does not make it harder to break; it only means the users will make fewer mistakes.
In fairness, fewer mistakes is worth something. A cipher used correctly with a 13^6 key space is more useful to an army than one used incorrectly with 26^6, and Porta evidently made that judgement deliberately.
IIIWhat it will not protect
Historical ciphers are for study and recreation and are not suitable for protecting real information. The full analysis is in the cipher entry.
Read the full entry, with deciphering steps and weakness analysis →