How Vigenère Works
Each letter in the keyword determines the shift for the corresponding plaintext letter:
Example with keyword "KEY":
Plaintext: H E L L O
Keyword: K E Y K E
Shifts: 10 4 24 10 4
Ciphertext: R I J V S
The keyword repeats to match the message length. Each letter shifts by its position in the alphabet (A=0, B=1, ... K=10, etc.).
Why Use Vigenère?
- Two-part puzzle — Find the keyword AND decode the message
- Thematic keywords — The key can relate to your cache theme
- Harder to brute force — Unlike Caesar, you can't just try 26 shifts
- Historical appeal — Called "le chiffre indéchiffrable" for 300 years
Step 1: Choose Your Keyword
The keyword is the heart of your puzzle. Choose something meaningful:
Thematic Keywords
Related to your cache location, history, or theme
Example: "CASTLE" for a cache near a castle
Research Keywords
Answer to a trivia question or historical fact
Example: "Who designed this building?" → "WREN"
Field Keywords
Something visible at a location
Example: "Read the name on the memorial"
Tip: Shorter keywords (4-6 letters) are easier to guess or crack. Longer keywords (8+) are much harder without hints.
Step 2: Encode Your Message
Use our Vigenère Cipher tool:
Plaintext:
THE CACHE IS UNDER THE BRIDGE
Keyword:
SECRET
Ciphertext:
LLI GEGLX MK YRUXJ XLI FJMHKI
Step 3: Create Keyword Hints
The solver needs to discover the keyword. Methods include:
Direct Research
"The keyword is the surname of the architect who designed this church."
Hidden in Plain Sight
Embed the keyword using first letters of sentences, bold letters, or other steganography.
Multi-Stage Reveal
First stage gives the keyword, second stage has the encrypted message.
Trivia/Quiz
Answer a series of questions—first letters spell the keyword.
Difficulty Variations
Easy (D1.5-D2)
- • State it's a Vigenère cipher
- • Short, obvious keyword (3-4 letters)
- • Keyword directly stated or easily found
Medium (D2.5-D3)
- • Hint at polyalphabetic cipher without naming it
- • Keyword requires research or observation
- • Medium keyword length (5-7 letters)
Hard (D3.5+)
- • No hints about cipher type
- • Keyword must be deduced from multiple clues
- • Long keyword or keyphrase
- • Combined with another cipher layer
Complete Example Puzzle
Cache Title: "The Diplomat's Cipher"
Cache description:
"In 1586, Blaise de Vigenère presented his cipher to the French court. For three centuries, it was considered unbreakable.
This intercepted diplomatic message uses his method. The ambassador was known to use the name of his home city as his key. He was posted from Paris to London in 1853.
Decode the message:"
CTDGJ MLKAG KAR RNG MVDKE TRNA
Solution:
1. Recognise Vigenère from the historical hint
2. Keyword = PARIS (his home city)
3. Decode with key PARIS
4. Result: NORTH FIFTY ONE THE CLOCK TOWER
Pro Tips
- Test the keyword. Make sure your keyword hints lead unambiguously to the right answer.
- Watch for repeating patterns. If your message has repeated words and keyword length divides evenly, patterns emerge that help crackers.
- Consider case sensitivity. Decide if your keyword should be uppercase, lowercase, or exact case—and make this clear.
- Provide a checker. Vigenère has many failure modes— a coordinate checker helps frustrated solvers.