Rail Fence Cipher

What is the Rail Fence Cipher?

The Rail Fence cipher (also called zigzag cipher) is a transposition cipher that rearranges letters by writing them in a zigzag pattern across a number of "rails" (rows), then reading off each rail in order.

Unlike substitution ciphers that replace letters, the Rail Fence cipher keeps the original letters but changes their positions.

How It Works

Encryption

1. Write the message in a zigzag pattern down and up across the rails
2. Read off each rail from top to bottom
3. Concatenate to form the ciphertext

Example with 3 Rails

Message: "WEAREDISCOVERED"

W . . . E . . . C . . . R . .
. E . R . D . S . O . E . E .
. . A . . . I . . . V . . . D

Reading rails: WEC R + ERDSOEE + AIVD = "WECRLTEERDSOAIE"

Decryption

1. Calculate how many letters go on each rail
2. Distribute the ciphertext across the rails
3. Read off in zigzag order to recover the plaintext

Rail Fence in Geocaching

This cipher is popular in geocaching puzzles because:

• Visual: The zigzag pattern can be drawn or visualized
• Simple concept: Easy to understand and execute
• Variable security: More rails = more scrambling
• Hint-friendly: "Zigzag" or "fence" references in puzzle titles

Recognizing Rail Fence Cipher

Look for these signs:

• Same letters: Original letters are preserved, just reordered
• References to rails, fences, zigzags: Common in puzzle hints
• Patterns: Some letter groupings may be recognizable
• Length: Ciphertext is same length as plaintext

Number of Rails

The number of rails affects security:

• 2 rails: Simple, easy to break
• 3 rails: Most common, moderate scrambling
• 4-5 rails: Good for longer messages
• More rails: Diminishing returns for short messages

If the number of rails equals or exceeds the message length, the cipher becomes ineffective (no scrambling occurs).

Breaking the Cipher

Without knowing the number of rails:

• Brute force: Try each rail count (usually 2-10)
• Pattern matching: Look for common words in each attempt
• Message length: Longer messages are easier to crack

Variations

• Offset start: Begin partway through the zigzag
• Double encryption: Apply with different rail counts
• Combined ciphers: Use with substitution for more security

History

The Rail Fence cipher dates back to ancient Greece and was used in various forms throughout history. Its simplicity made it popular for quick field encryption, though it's now considered cryptographically weak.