Transposition Cipher

A Transposition Cipher is a method of encryption by which the positions held by units of plaintext (which are commonly characters or groups of characters) are shifted according to a regular system, so that the ciphertext constitutes a permutation of the plaintext. This technique does not change the units themselves but rearranges their order.

History

The Transposition Cipher has a long history, dating back to ancient times. One of the earliest known uses was by the Spartans who employed the Scytale, a device used for reading messages written on a strip of parchment wrapped around a rod of a particular diameter. When unwrapped, the message was scrambled, and only when rewrapped on a rod of the same diameter would it become readable again. This was one of the earliest examples of a transposition cipher.

Types of Transposition Ciphers

• Columnar Transposition: Here, the plaintext is written out in rows of a fixed length, and then read off column by column. The key often dictates the order of reading the columns.
• Rail Fence Cipher: The plaintext is written in a zigzag pattern on an imaginary fence, then read off in rows. This method uses a simple transposition where the message is written in a serpentine manner.
• Route Cipher: In this method, the grid or geometric shape is used, and the message is written into it. The route for reading the message is pre-determined, such as following the edges of the grid or a specific pattern.
• Double Transposition: This involves applying transposition twice, often with different keys for each transposition, which can significantly increase security.

Application and Security

While transposition ciphers were historically popular due to their relative simplicity, they are not considered secure by modern standards when used alone. However, they are still used in conjunction with other cipher techniques to enhance security:

• In Cryptanalysis, transposition ciphers can be part of more complex cipher systems.
• They can be combined with Substitution Ciphers in systems like the Adfgvx Cipher.

The security of transposition ciphers largely depends on the complexity of the transposition pattern and the frequency analysis resistance. However, with modern computational power, many transposition ciphers can be broken if the method of transposition is known or guessed through analysis of letter frequency and common word patterns.

Examples in Use

Here are a couple of simple examples:

• With a rail fence cipher, the message "HELLO WORLD" written in a zigzag pattern with two rails might yield "HLOOLELWRD" as the ciphertext.
• In a columnar transposition with key "4312", the plaintext "SECRET MESSAGE" could be arranged as:
  • SECRET
  • MESSAG
  • E
  Reading by the key order would give "SMEG CEE TRSA".

External Links

• Wikipedia: Transposition Cipher
• CryptoTool: Transposition Ciphers
• Cipher Mysteries: Transposition Ciphers

Related Topics

• Substitution Cipher
• Cryptanalysis
• Permutation Cipher
• Caesar Cipher