Nihilist Cipher
Encode and decode using the Nihilist cipher. A Russian cipher combining Polybius square with keyword addition.
Polybius Square
| 1 | 2 | 3 | 4 | 5 | |
|---|---|---|---|---|---|
| 1 | A | B | C | D | E |
| 2 | F | G | H | I | K |
| 3 | L | M | N | O | P |
| 4 | Q | R | S | T | U |
| 5 | V | W | X | Y | Z |
How Nihilist Works
- Convert plaintext to Polybius numbers (e.g., A=11, B=12)
- Convert keyword to numbers the same way
- Add keyword numbers to plaintext numbers (repeating key)
- Result is two-digit sums (e.g., 11+22=33)
History
The Nihilist cipher was used by Russian revolutionaries in the 1880s to communicate secretly. It was named after the Nihilist movement that opposed the Tsarist government.
What is the Nihilist Cipher?
The Nihilist cipher is a superencipherment cipher used by Russian Nihilists in the 1880s. It combines a Polybius square substitution with addition of a keyword, similar to how Vigenere adds a keyword to plaintext.
How It Works
Step 1: Polybius Square
Create a 5×5 Polybius square (optionally keyed). Each letter is represented by a two-digit number (row, column), e.g., A=11, B=12, F=21, etc.
Step 2: Convert to Numbers
Convert both the plaintext and keyword to their Polybius numbers.
Step 3: Addition
Add the keyword numbers to the plaintext numbers, repeating the keyword as needed. The sums typically range from 22 to 110.
Example
With keyword "KEY" and plaintext "HELLO":
- Plaintext: H(23) E(15) L(31) L(31) O(34)
- Keyword: K(25) E(15) Y(54) K(25) E(15)
- Ciphertext: 48 30 85 56 49
Nihilist Cipher in Geocaching
This cipher appears in puzzles because:
- Russian history: Revolutionary and spy themes
- Numeric output: Works well for coordinate hints
- Two keys: Grid keyword and cipher keyword add complexity
- Less common: More challenging to identify
Cryptanalysis
The Nihilist cipher can be attacked by:
- Kasiski examination: Find key length from pattern repetition
- Frequency analysis: The sums have predictable distributions
- Known plaintext: Reveals keyword numbers directly
- Brute force: Try common keywords
Weaknesses
Despite adding numbers, the cipher has vulnerabilities:
- Sum patterns: Same plaintext+key pairs give same sums
- Limited range: Sums only go from 22 to 110
- Carries reveal info: Large sums indicate large components