PORTA CIPHER

13-row keyword-driven substitution

Background

Invented by Giovanni Battista della Porta (~1563)
A polyalphabetic substitution using a 13-row tableau and a repeating keyword
At Invitationals/Regionals: Decrypt (key given)
At State/Nationals: Decrypt or Cryptanalysis
The Porta tableau is always provided on the test

13 rows labeled by key-letter pairs: AB, CD, EF, GH, IJ, KL, MN, OP, QR, ST, UV, WX, YZ
Each row contains 13 letters — half the alphabet
The remaining 13 letters of the alphabet form the other half
The cipher is its own inverse — encrypt and decrypt use the same row

Porta is self-reciprocal: encrypting with the same key decrypts the message.

Step 1: Write the keyword above the plaintext, repeating as needed
Step 2: For each plaintext letter, look up the row corresponding to the current key letter's pair
Step 3: Find the plaintext letter's position (top half or bottom half) and swap it with the corresponding letter in the other half

Key: KEY Plain: HELLO

Key letters: K E Y K E

Plain: H E L L O

Row for K: IJ row → look up H and E …

Self-reciprocal property makes Porta easier to work with — one procedure for both directions.

For Cryptanalysis: determine the keyword length using the Kasiski test or IC analysis
Once key length is known, group letters by key position and solve each group independently
The 13 rows are fixed — once you know which row a group uses, there are only 13 options

In competition, DC problems at State may test your ability to determine the keyword length.

D-type problem — key given

Key: CODE Ciphertext: XYZAB

Repeat key: C O D E C

Apply Porta tableau row for each key letter to each cipher letter

Use the Porta tableau (provided on test) — look up cipher letter in the key letter's row

Homework: using the Porta tableau, encrypt "SCIENCE" with keyword "OLY".