Patristic Cipher

The Patristic Cipher is a letter-substitution format where the plaintext is first encrypted using a keyword-based substitution (like a keyed alphabet) and then formatted into uniform blocks (commonly 5 letters each). Spaces, punctuation, and other non-letter characters are removed so that the ciphertext appears as a continuous stream of letters, hiding word boundaries.

Patristic Cipher: Encoding

Plaintext message:

HELLO WORLD

Step 1: Clean the text

Remove spaces and non-alphabet characters:

HELLOWORLD

Step 2: Build the keyed alphabet using keyword "KEY"

The keyed alphabet is created by first writing the keyword (removing duplicates) and then appending the remaining letters of the alphabet in order:

Keyword: KEY
Deduplicated keyword: KEY
Remaining letters: ABCDFGHIJLMNOPQRSTUVWXZ
Keyed alphabet: K E Y A B C D F G H I J L M N O P Q R S T U V W X Z

Step 3: Encrypt each letter

Map each plaintext letter to the corresponding letter in the keyed alphabet at the same index as the standard alphabet:

Standard Alphabet:  A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Keyed Alphabet:     K E Y A B C D F G H I J L M N O P Q R S T U V W X Z

Plaintext: H E L L O W O R L D
Indexing:   7 4 11 11 14 22 14 17 11 3  (0-based)
Substitute: F B J J N V N Q J A

Step 4: Apply Patristic grouping

Group letters into blocks of 5 (common Patristic style). If needed, pad the final block with filler letters (here none are needed as length = 10):

Ciphertext:
FBJJN VNQJA

Patristic Cipher: Decoding

To decode, the recipient:

1. Removes any grouping spaces: FBJJNVNQJA
2. Uses the same keyword KEY to reconstruct the keyed alphabet
3. Maps each ciphertext letter back to the standard alphabet:

Ciphertext: F B J J N V N Q J A
Mapped back using keyed alphabet → H E L L O W O R L D
Plaintext: HELLOWORLD

This step-by-step shows how the keyed alphabet substitution combined with the Patristic grouping produces a visually uniform ciphertext while concealing the original word structure.