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Lesson 4
PATRISTOCRAT
Monoalphabetic substitution — word spaces removed
Patristocrat vs. Aristocrat
- Same monoalphabetic substitution as Aristocrat
- Key difference: all spaces removed, ciphertext split into 5-letter groups
- This removes the word-boundary clue — much harder to crack!
- Still appears as DC at all competition levels
Aristocrat vs. Patristocrat
Aristo: LPM HDTZX YQJVB
Patri: LPMHD TZXYQ JVBEJ
Why 5-Letter Groups?
- Standard cryptographic practice — hides word lengths
- Padding: a random letter fills the last group if text length isn't a multiple of 5
- You may see a note like "K = null" (K is the padding letter)
- Don't try to decode the null letter — ignore it
Breaking a Patristocrat
- Frequency analysis still works — letter frequencies don't change
- Without word spaces, use digraph and trigraph frequencies
- Most common English digraphs: TH, HE, IN, ER, AN, RE, ON, EN
- Most common trigraphs: THE, AND, ING, ION, ENT
- Look for repeated 3-letter sequences — same trigraph often = THE
Index of Coincidence
- A statistical tool: measures how often two randomly chosen letters match
- English text IC ≈ 0.065
- Random (polyalphabetic) IC ≈ 0.038
- If IC is near 0.065 → monoalphabetic → use frequency analysis
You won't need to calculate IC by hand on the test, but knowing it confirms you're dealing with a monoalphabetic cipher.
Strategy for Patristocrats
- Step 1: Count all 26 cipher letters, rank by frequency
- Step 2: Map top frequencies to ETAOIN…
- Step 3: Look for repeating bigrams/trigrams in the ciphertext
- Step 4: Try substitutions and check if real words appear
- Step 5: Extend confirmed letters; iterate
Write your current mapping on a separate line — update it as you go.
Practice Problem
Cryptanalysis — no key. Null = X
Ciphertext (5-letter groups)
ACOSK PVBER HACKS OCBEP AOCSK X
- Count letters. Find the most frequent. Look for repeating trigrams.
Plaintext (click to reveal)
SOLVE THE CODE BEFORE TIME RUNS OUT
Key Takeaways
- Patristocrat = Aristocrat without word spaces, in 5-letter blocks
- Harder than Aristocrat — rely on digraphs, trigraphs, and frequency
- Null letter may pad the final group — problem will note it
- Appears as DC at all levels
- Next class: Baconian Cipher — hiding letters in patterns of A and B
Homework: solve one Patristocrat from a sample test using a frequency tally.