Transposition Cipher

A transposition cipher changes the order of characters to obscure a message without altering the characters themselves.
An early example involved wrapping paper around a stick to encrypt messages. When unwrapped, it became unreadable until rewrapped around the same-size stick.

A contemporary method uses a grid to reorder characters.
Example: Encrypting "Meet at 3:00 P.M. Today at the usual location" using rows of six characters:
- Create a table with six columns, filling rows left to right:
  - M e e t a t
  - 3 : 0 0 P
  - M . T o d
  - a y a t
  - t h e u
  - s u a l l
  - o c a t i
  - o n .
From this table, read vertically to create the encoded message, thus obscuring spaces in the final output to keep the cipher's key hidden.

To decrypt a message encrypted with a tabular transposition cipher:
- Identify the number of characters and divide by the number of characters per row to ascertain the number of rows.
- For example, with a 32-character message split into four columns, this will yield eight rows.
Example decryption steps:
- Write down characters in the respective columns from the encrypted text, then read off row by row.
- Original Message Example: "At 11, surveillance on front lines."

More complex versions specify column reading order after writing the message.
For instance, with a specified order of columns, the encryption becomes harder to crack. For example:
- Keyword-based approach for ordering columns can simplify remembering the method.
Example: Using "MATH" as a keyword designating rows of four due to four letters enables structured reading based on alphabetical order of letters:
- Order columns as 2, 5, 1, 3, 4 based on sorted letters.

Encrypting: "At 4, surveillance on enemy camp" using keyword "MAINE" (5 letters):
- Rows of five characters:
  - Each filled with applicable letters and padded (e.g., with 'A').
- Read columns based on keyword alphabetical order to form encrypted message.
- When finished, reorganize letters and eliminate spaces.

Decrypting an example using "PLAN":
- Convert message into four columns per rows of 20 characters.
- Determine column completion order via alphabetized key letters.
- Decrypt by reading rows to retrieve the original message: "At noon, attack base camp."

Transposition ciphers maintain letter frequency, making them susceptible to frequency analysis.
While they conceal message structure, simplistic frequency patterns can still expose information from letter pairs.

Transposition ciphers are useful for obscuring messages by reorganizing characters within a defined structure. The effectiveness can decrease with simpler and more predictable ciphers.