Porta Cipher

Summary: A polyalphabetic substitution cipher that uses a tableau with 13 rows (one for each letter pair AB, CD, EF, etc.). Each key letter determines which row to use for encryption. The cipher is reciprocal (encryption = decryption).

Porta Tableau:

Key Pair | Cipher Alphabet (substitutes for ABCDEFGHIJKLM)
AB       | NOPQRSTUVWXYZABCDEFGHIJKLM
CD       | OPQRSTUVWXYZNMABCDEFGHIJKL
EF       | PQRSTUVWXYZMNABCDEFGHIJKLO
GH       | QRSTUVWXYZNOPABCDEFGHIJKLM
IJ       | RSTUVWXYZNOPQABCDEFGHIJKLM
KL       | STUVWXYZNOPQRABCDEFGHIJKLM
MN       | TUVWXYZNOPQRSABCDEFGHIJKLM
OP       | UVWXYZNOPQRSTABCDEFGHIJKLM
QR       | VWXYZNOPQRSTUABCDEFGHIJKLM
ST       | WXYZNOPQRSTUVABCDEFGHIJKLM
UV       | XYZNOPQRSTUVWABCDEFGHIJKLM
WX       | YZNOPQRSTUVWXABCDEFGHIJKLM
YZ       | ZNOPQRSTUVWXYABCDEFGHIJKLM

Example:

• Plaintext: MEET AT DAWN
• Key: FORTIFICATION (repeating)
• Process:
  • M with F (key EF): Use row EF, M->X
  • E with O (key OP): Use row OP, E->S
  • E with R (key QR): Use row QR, E->W
  • T with T (key ST): Use row ST, T->M
  • (Continue for all letters)
• Ciphertext: XSWM KH OBRW

Decryption (given key FORTIFICATION): Apply the same process—Porta is reciprocal.

Additional Example with Detailed Steps:

• Plaintext: ATTACK
• Key: DEFEND (repeating)

Step-by-step encryption:

Plaintext:  A  T  T  A  C  K
Key:        D  E  F  E  N  D
Key pair:   CD EF EF EF MN CD

1. A with key D (pair CD):

   • A is in first half (A-M), position 0
   • Row CD alphabet: OPQRSTUVWXYZNMABCDEFGHIJKL
   • A (position 0) -> O

2. T with key E (pair EF):

   • T is in second half (N-Z), position 19
   • In Porta, second half uses same row
   • Row EF alphabet: PQRSTUVWXYZMNABCDEFGHIJKLO
   • T (position 19) -> G

3. T with key F (pair EF):

   • Same as above: T -> G

4. A with key E (pair EF):

   • A (position 0) -> P

5. C with key N (pair MN):

   • Row MN alphabet: TUVWXYZNOPQRSABCDEFGHIJKLM
   • C (position 2) -> V

6. K with key D (pair CD):

   • Row CD alphabet: OPQRSTUVWXYZNMABCDEFGHIJKL
   • K (position 10) -> Y

Ciphertext: OGGPVY

To decrypt OGGPVY with key DEFEND: Apply the exact same process (Porta is self-reciprocal):

• O with D -> A
• G with E -> T
• G with F -> T
• P with E -> A
• V with N -> C
• Y with D -> K Plaintext: ATTACK