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Lesson 3
ARISTOCRAT
Monoalphabetic Substitution — word spaces intact
What is an Aristocrat?
- A monoalphabetic substitution cipher — each plaintext letter maps to a unique cipher letter (scrambled alphabet)
- Word spacing and punctuation are preserved
- The key is a full 26-letter mixed alphabet — not just a shift
- At Invitationals/Regionals: may be Decrypt (key given) or Cryptanalysis (no key)
- At State/Nationals: always Cryptanalysis — you must break it
Types of Keys
- Random key — 26 letters scrambled arbitrarily
- Keyword key — write a keyword (removing duplicates), then fill remaining letters in order
Keyword: CIPHER
Plain: 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
Cipher: C I P H E R A B D F G J K L M N O Q S T U V W X Y Z
- C I P H E R fills first 6 slots, then remaining letters A B D F G … in order
Breaking It — Frequency Analysis
- In English, letter frequency is predictable: E T A O I N S H R are most common
- Count how many times each cipher letter appears
- The most frequent cipher letter is probably E
- Second and third most frequent → T or A
Memorize: ETAOIN SHRDLU — the 12 most common English letters in order.
Pattern Words
- Look for 1-letter words: A or I
- Look for 2-letter words: OF, TO, IN, IS, IT, BE, AS, AT, OR, AN, HE…
- Look for double letters: SS, LL, EE, TT, FF, RR, NN…
- 3-letter words starting with the same cipher letter twice: likely THE
THE is the single most common 3-letter word. Find it first.
Cryptanalysis Walkthrough
Ciphertext
LPM HDTZX YQJVB EJU SXNCO JKM GDBF AJW
- 3-letter word LPM — likely THE → L=T, P=H, M=E
- 3-letter word AJW ends in same pattern — try FOX or DOG
- Fill in confirmed letters and guess remaining short words
- Iterate — each confirmed letter constrains others
Decrypt Type — Key Given
- The problem provides the full cipher alphabet or states the keyword
- Build a lookup table: cipher letter → plain letter
- Substitute every letter mechanically
Given key (plain → cipher)
A→Q B→W C→E D→R E→T …
For D-type: focus on speed. Write cipher→plain above each letter immediately.
Competition Tips
- Always write guessed plain letters above the cipher letters — easy to erase
- Use a separate letter-tracking grid: 26 plain / 26 cipher slots
- Confirm a guess with at least two independent words before committing
- If stuck, look for -ING, -TION, -ED endings
- Leave blanks and come back — partial solves still earn points
Practice Problem
Cryptanalysis — no key given
Ciphertext
YMJ ULGZPJW HTRUJYJX NS YMJ HTIJGZXYJWX JAJSY
Plaintext (click to reveal)
THE STUDENT COMPETES IN THE CODEBUSTERS EVENT
- Hint: YMJ appears twice — it's THE. Key is Caesar shift 5 (keyword-free).
Key Takeaways
- Aristocrat = scrambled alphabet, word spaces preserved
- Break it with frequency analysis + pattern words
- ETAOIN SHRDLU and THE/AND/OF are your best friends
- Appears as DC (either D or C) at all levels
- Next class: Patristocrat — same cipher, but word spaces are removed
Homework: solve one Aristocrat puzzle from a sample test.