Skip / Scytale Cipher

What is the Skip Cipher?

The skip cipher (also called interval cipher or Scytale cipher) is a transposition cipher that rearranges letters by reading every nth character. The ancient Greeks used a version called the Scytale, wrapping a strip of parchment around a rod.

How It Works

Encoding

Write the message in rows with a specified width, then read column by column:

• Original: HELLO WORLD
• Skip 3: HEL | LOW | ORL | D
• Read down columns: HLOD, ELRL, OLOW
• Result: HLODELRLLOW

Decoding

Read every nth character starting from position 1, then 2, etc.:

• Ciphertext: HLODELRLLOW
• Every 3rd starting at 0: H, D, R, L
• Every 3rd starting at 1: L, E, L, O
• And so on...

The Scytale

The ancient Scytale was a physical encryption device:

• Wrap a strip of leather or parchment around a rod
• Write the message along the length of the rod
• Unwrap to reveal scrambled text
• Same diameter rod required to decrypt

Skip Cipher in Geocaching

This cipher is common in geocaching because:

• Historical: Ancient cipher with interesting history
• Simple concept: Easy to understand and implement
• Brute-forceable: Try all skip values until readable
• Physical puzzles: Can use actual Scytale devices

Breaking the Cipher

Without knowing the skip value:

• Brute force: Try all possible skip values
• Message length: Skip value usually divides evenly
• Pattern recognition: Look for common letter combinations
• Frequency: Letters should match normal distribution

Variations

• Multiple passes: Apply skip cipher multiple times
• Variable skip: Change skip value through message
• Combined ciphers: Use with substitution for more security