FRACTIONATED MORSE

Morse code + trigraph substitution

Overview

A two-step cipher invented by M.E. Ohaver
Step 1: Convert plaintext to Morse code (dots, dashes, separators)
Step 2: Split the Morse stream into trigraphs and substitute each with a letter using a key alphabet
Appears as Cryptanalysis (C) at all levels

Morse Code Quick Reference

Letter	Morse	Letter	Morse	Letter	Morse
A	·–	J	·–––	S	···
B	–···	K	–·–	T	–
C	–·–·	L	·–··	U	··–
D	–··	M	––	V	···–
E	·	N	–·	W	·––
F	··–·	O	–––	X	–··–
G	––·	P	·––·	Y	–·––
H	····	Q	––·–	Z	––··
I	··	R	·–·

The Fractionation Step

Convert plaintext letters to Morse, separated by X between letters
Place XX between words
The key alphabet maps 26 trigraphs of {·, –, X} to letters
Split the long Morse string into groups of 3 and look each up in the key

Example: plain "CAT"

C = –·–· X A = ·– X T = –

–·–· X ·– X –

Split into 3s: –·– ·X· –X–

The Key Alphabet

There are 27 possible trigraphs (· – X)³ minus one — 26 map to letters A–Z
The key alphabet is usually derived from a keyword
For Cryptanalysis: the key is not given — you find the trigraph-to-letter mapping by working backwards

On the test, the key alphabet or keyword is always provided — this is a C problem, but the key IS given in the standard form of this cipher in competitions.

Decryption Process

Step 1: Using the key, convert each ciphertext letter back to its Morse trigraph
Step 2: Concatenate all trigraphs into one long Morse string
Step 3: Split on X (letter separator) and XX (word separator)
Step 4: Convert each Morse group back to a letter

Work backwards: cipher → trigraph stream → Morse groups → plaintext letters.

Common Pitfalls

The X separator is part of the fractionation — don't confuse it with the letter X's Morse code (–··–)
If a Morse group doesn't match any letter, you may have miscounted — recheck boundaries
Some versions use different separators — read the problem statement carefully

Key Takeaways

Fractionated Morse = convert to Morse → regroup into trigraphs → substitute
Reverse: substitute back to trigraphs → reassemble Morse → convert to letters
The Morse code table is always provided on the test
Appears as Cryptanalysis (C) at all levels
Next class: Cryptarithm — substituting digits for letters in arithmetic

Homework: memorize Morse code for A–Z. This is non-negotiable.