Transposition ciphers rearrange the positions of characters in the plaintext to create ciphertext, rather than substituting characters with others (as substitution ciphers do). Transposition ciphers can be keyed or keyless.
🔐 1. Keyless Transposition Cipher
A keyless transposition cipher uses a fixed rule for rearranging the characters, without using a specific key.
🧠 Logic:
Usually, the plaintext is written in rows of a fixed length, then read off in columns (or vice versa).
📘 Example:
Let’s use 4-column transposition:
Plaintext: HELLOTHISISSECRET
Step 1: Write in rows of 4:
H E L L
O T H I
S I S S
E C R E
T
Step 2: Pad last row (if needed):
H E L L
O T H I
S I S S
E C R E
T X X X
Step 3: Read column-wise:
Column 1: H O S E T
Column 2: E T I C X
Column 3: L H S R X
Column 4: L I S E X
Ciphertext: HOSETETICLHSRXLISX
🔑 2. Keyed Transposition Cipher (Columnar Transposition)
A keyed transposition cipher uses a keyword to determine the order in which columns are read.
🧠 Logic:
-
Write the plaintext in rows under columns labeled by the keyword.
-
Sort the columns alphabetically by the keyword.
-
Read off the columns in this new order.
📘 Example:
Plaintext: DEFENDTHEEASTWALL
Keyword: ZEBRAS
Step 1: Number the keyword alphabetically:
Z E B R A S
6 3 1 5 0 4 (0-indexed alphabetical order)
Step 2: Fill grid with plaintext:
Z E B R A S
D E F E N D
T H E E A S
T W A L L X
Step 3: Read columns in order: 0,1,3,4,5,6 → A,B,E,S,R,Z
Column A (4): N A L
Column B (2): F E A
Column E (1): E H W
Column S (5): D S X
Column R (3): E E L
Column Z (0): D T T
Ciphertext: NALFEAEHWD SXEELDTT
Summary
| Type | Uses Key? | Pattern | Example |
|---|---|---|---|
| Keyless | ❌ | Fixed row/column | HELLO → HOSETETIC... |
| Keyed | ✅ | Keyword determines column order | DEFEND → NALFEA... |