The Problem with Simple Substitution

• In a standard monoalphabetic cipher, E always maps to the same symbol
• Frequency analysis immediately exposes E, T, A, O…
• Homophonic substitution fixes this by giving multiple cipher symbols to common letters
• Rare letters get fewer symbols; common letters get many

How it Works

• Each plaintext letter is assigned 1 or more cipher equivalents (homophones)
• When encrypting, pick any one of that letter's homophones at random
• The more frequent a letter, the more homophones it gets — flattening frequencies

Cipher Symbol Sets

• Symbols are often two-digit numbers (00–99 gives 100 possible symbols)
• Total symbols used = 100 (or however many) distributed proportionally to letter frequencies
• On the test, the cipher might use numbers, symbols, or a combination

Decryption (D-type — key given)

• The key table maps each cipher symbol back to a plaintext letter
• Look up each ciphertext symbol in the table → write the plaintext letter
• Multiple different symbols all map to the same letter — that's fine

Cryptanalysis (DC — no key)

• Count how often each individual symbol appears
• Group symbols by frequency — all homophones of E will be frequent
• The total symbol frequencies should match English letter frequencies
• Try to group symbols into sets that together match E, T, A, O… frequencies

Breaking Strategy

• Step 1: Tally all 100 possible symbols; find which are used
• Step 2: Sort by frequency; the most frequent symbols are likely E-homophones
• Step 3: Look for single-symbol words → A or I
• Step 4: Look for 3-symbol sequences that repeat → likely THE
• Step 5: Assign letters, check against word boundaries, iterate

Practice (D-type)

Key Takeaways

• Homophonic = multiple cipher symbols per plaintext letter, distributed by frequency
• Defeats simple frequency analysis — all symbol frequencies appear roughly equal
• D-type: look up each symbol in the provided key
• C-type: group symbols by joint frequency, then use word patterns
• Appears as DC at all levels