Porta Cipher
Encode and decode using the Porta cipher. A reciprocal polyalphabetic cipher from the 16th century.
Reciprocal cipher: The same operation encrypts and decrypts. If you encrypt "HELLO" you get ciphertext; encrypt that ciphertext with the same key and you get "HELLO" back.
Porta Tableau
| Key | A | B | C | D | E | F | G | H | I | J | K | L | M |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| AB | N | O | P | Q | R | S | T | U | V | W | X | Y | Z |
| CD | O | P | Q | R | S | T | U | V | W | X | Y | Z | N |
| EF | P | Q | R | S | T | U | V | W | X | Y | Z | N | O |
| GH | Q | R | S | T | U | V | W | X | Y | Z | N | O | P |
| IJ | R | S | T | U | V | W | X | Y | Z | N | O | P | Q |
| KL | S | T | U | V | W | X | Y | Z | N | O | P | Q | R |
| MN | T | U | V | W | X | Y | Z | N | O | P | Q | R | S |
| OP | U | V | W | X | Y | Z | N | O | P | Q | R | S | T |
| QR | V | W | X | Y | Z | N | O | P | Q | R | S | T | U |
| ST | W | X | Y | Z | N | O | P | Q | R | S | T | U | V |
| UV | X | Y | Z | N | O | P | Q | R | S | T | U | V | W |
| WX | Y | Z | N | O | P | Q | R | S | T | U | V | W | X |
| YZ | Z | N | O | P | Q | R | S | T | U | V | W | X | Y |
Letters N-Z map back reciprocally. The same key letter pair works for both halves.
How Porta Works
- • Alphabet split into two halves: A-M and N-Z
- • Key letters pair up: AB, CD, EF... each selects a tableau row
- • First half letters map to second half (and vice versa)
- • The mapping is reciprocal: encrypt and decrypt are identical
What is the Porta Cipher?
The Porta cipher is a polyalphabetic substitution cipher invented by Giovanni Battista della Porta in 1563. It is notable for being reciprocal - the same operation both encrypts and decrypts the message.
How It Works
Alphabet Halves
The alphabet is divided into two halves:
- First half: A B C D E F G H I J K L M
- Second half: N O P Q R S T U V W X Y Z
Key Pairs
Key letters are grouped in pairs: AB, CD, EF, GH, IJ, KL, MN, OP, QR, ST, UV, WX, YZ. Both letters in a pair use the same tableau row.
The Tableau
Each key pair defines a different mapping between the two halves of the alphabet. The mapping is reciprocal: if A maps to N, then N maps to A.
Reciprocal Property
The key feature of Porta is that encryption equals decryption. To decrypt a message, simply "encrypt" it again with the same keyword.
Porta in Geocaching
This cipher appears in puzzles because:
- Historical: 16th century cryptography
- Self-inverse: Interesting mathematical property
- Visual tableau: Makes an interesting puzzle image
- Less common: Not as well-known as Vigenere
vs Vigenere Cipher
| Porta | Vigenere |
|---|---|
| 13 tableau rows | 26 tableau rows |
| Key pairs (AB, CD...) | Each key letter unique |
| Reciprocal | Different encrypt/decrypt |
| Half-alphabet swap | Full alphabet shift |
Breaking the Cipher
- Kasiski examination: Find repeating patterns to determine key length
- Frequency analysis: After determining key length
- Known plaintext: Helps determine the keyword
- Short keys: Can be brute-forced