Running Key Cipher

What is the Running Key Cipher?

The Running Key cipher is a variant of the Vigenere cipher that uses a long passage of text (such as from a book) as the key instead of a short keyword. This makes frequency analysis much more difficult because the key doesn't repeat in the same predictable pattern.

How It Works

Encryption

Each letter of the plaintext is shifted by the corresponding letter in the key text. The key text should ideally be at least as long as the message being encrypted.

Key Selection

The key is typically a passage from:

• Books: A specific page or chapter
• Documents: Constitution, Declaration of Independence
• Religious texts: Bible verses, prayers
• Poetry: Famous poems or lyrics

Running Key in Geocaching

This cipher appears in puzzles because:

• Literary themes: Can incorporate book references
• Multiple clues: Both cipher and key source need to be found
• More secure: Can't be solved with just frequency analysis
• Historical: Used in real espionage

Cryptanalysis

The running key cipher can be attacked by:

• Dictionary attack: Try common texts as keys
• Probable word: Guess words in the plaintext
• Statistical analysis: Analyze digraph frequencies
• Known key source: If the book is known, try different starting points

vs Vigenere Cipher

The main differences from standard Vigenere:

• Key length: Running key uses very long keys
• Repetition: Vigenere keys repeat; running keys typically don't
• Security: Running key is much harder to break without the key source
• Key exchange: Only need to share the key source reference