← ACARA Digital Technologies Years 9–10

Transposition cipher: rail fence

A cipher that keeps the same letters but changes their order, like writing a message in a zigzag across rails and then reading each rail in turn.

Part of: ACARA Digital Technologies Years 9–10 — Cyber Security Threats and Vulnerabilities — Module 112

Understanding Transposition cipher: rail fence in ACARA Digital Technologies Years 9–10

What it does: Sorting both texts gives the same list of letters, which shows the ciphertext is just a reordering of the plaintext. No letters were swapped for different ones. A transposition cipher rearranges the letters. Swapping letters for other letters is called substitution, like the Caesar cipher. Zigzag the letters: H, L, O land on rail 1 and E, L on rail 2. Read rail 1 first, then rail 2, to get HLOEL. Transposition never changes the letters themselves. It only moves them around. Rail 1 gets A and C. Rail 2 gets T, A and K. Rail 3 gets T. Read the rails in order to get ACTAKT.

Common mistakes: In a two-rail fence the bottom rail holds the letters at positions 1, 3, 5 and so on. Starting at 2 repeats letters from the top rail and drops others. It should be text[1::2]. The message is text, so it needs quotes. Without them Python treats MEETATNOON as a variable name and raises an error. It should be text = 'MEETATNOON'.

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This module's key terms are also in a printable crossword (Modules 860–875) and word search (Modules 860–944).

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Learning

Duo

Which describes a transposition cipher?

It swaps each letter for a different letterIt keeps the same letters but changes their order

Matches

Rail fence of HELLO (2 rails) = IFMMP

↓

IFMMP replaces each letter with the next letter, which is a substitution (Caesar) cipher. A rail fence only reorders the original letters, so the answer is HLOEL.

Applying

Meanings

Term

transposition cipher

Meaning

A cipher that keeps the same letters but changes their order, like writing a message in a zigzag across rails and then reading each rail in turn.

Practice

message = "HELLOWORLD"

Testing

Gaps

top = text[0::]

Check Answer

Spots

2top = text[0::2]

3bottom = text[2::2]

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Quick Questions

1. Which describes a transposition cipher?

A. It swaps each letter for a different letter

B. It keeps the same letters but changes their order ✓

A transposition cipher rearranges the letters. Swapping letters for other letters is called substitution, like the Caesar cipher.

2. Two-rail fence: what does HELLO become?

A. ELHLO

B. HLOEL ✓

Zigzag the letters: H, L, O land on rail 1 and E, L on rail 2. Read rail 1 first, then rail 2, to get HLOEL.

3. Which is true about a rail fence ciphertext?

A. It has exactly the same letters as the message, in a new order ✓

B. It has new letters that were not in the message

Transposition never changes the letters themselves. It only moves them around.

4. Three-rail fence: what does ATTACK become?

A. ACTAKT ✓

B. ACTKAT

Rail 1 gets A and C. Rail 2 gets T, A and K. Rail 3 gets T. Read the rails in order to get ACTAKT.

5. What is the key in a rail fence cipher?

A. The number of rails ✓

B. The number of places each letter is shifted

A shift amount is the key for a Caesar cipher. A rail fence key is how many rails the zigzag uses.

Match the Pairs

rail_fence("HELLO", 2) -> "HLOEL" → Writes HELLO in a zigzag over 2 rails, then reads the top rail (HLO) followed by the bottom rail (EL)

rail_fence("ATTACK", 3) -> "ACTAKT" → Zigzags ATTACK over 3 rails, then reads the top rail (AC), the middle rail (TAK) and the bottom rail (T)

A transposition cipher keeps exactly the same letters and only changes their order → sorted("HELLO") == sorted("HLOEL")

Columnar transposition writes the message in rows, then reads it down each column → ATT / ACK -> read columns: AATCTK

Rail fence of HELLO (2 rails) = IFMMP → IFMMP replaces each letter with the next letter, which is a substitution (Caesar) cipher. A rail fence only reorders the original letters, so the answer is HLOEL.

Decrypt HLOEL on 2 rails: top rail = HL, bottom rail = OEL → The top rail of a 5-letter message holds 3 letters, not 2. The correct split is top HLO and bottom EL, which rebuilds HELLO.

Key Term

transposition cipher

A cipher that keeps the same letters but changes their order, like writing a message in a zigzag across rails and then reading each rail in turn.

Fill in the Blanks

1. top = text[0::2]

Hint: Type the step size that picks every other letter for the top rail of a two-rail zigzag.

2. bottom = text[1::2]

Hint: Type the starting index of the second letter, where the bottom rail begins.

3. top = text[0::2] bottom = text[1::2] cipher = top + bottom

Hint: Type the variable name for the rail that is read after the top rail.

4. rail1 = text[0::4]

Hint: Type the gap between top-rail letters in a three-rail zigzag. The pattern repeats every 2 x 3 - 2 letters.

5. top = cipher[:(len(cipher) + 1) // 2]

Hint: Type the number to divide by to find how many letters the top rail holds when rebuilding a two-rail zigzag.

Spot the Error

Question 1

A text = 'MEETATNOON'

B top = text[0::2]

C bottom = text[2::2]

D print(top + bottom)

Line C — In a two-rail fence the bottom rail holds the letters at positions 1, 3, 5 and so on. Starting at 2 repeats letters from the top rail and drops others. It should be text[1::2].

Question 2

A text = MEETATNOON

B top = text[0::2]

C bottom = text[1::2]

D print(top + bottom)

Line A — The message is text, so it needs quotes. Without them Python treats MEETATNOON as a variable name and raises an error. It should be text = 'MEETATNOON'.

Question 3

A msg = 'WEAREDISCOVERED'

B rail1 = msg[0::4]

C rail2 = msg[1::4]

D rail3 = msg[2::4]

E print(rail1 + rail2 + rail3)

Line C — In a three-rail zigzag the middle rail is touched on the way down and on the way up. It gets every second letter (positions 1, 3, 5...). A step of 4 skips half of them. It should be msg[1::2].

Question 4

A cipher = 'MEANOETTON'

B half = len(cipher) / 2

C top = cipher[:half]

D bottom = cipher[half:]

E print(top, bottom)

Line B — To rebuild the zigzag we split the cipher text into the top rail and bottom rail. A single slash gives a decimal (5.0), and slice positions must be whole numbers. It should be // 2.

Question 5

A key = '3'

B text = 'MEETATNOON'

C for i in range(key):

D print(text[i::key])

Line A — In columnar transposition the key is the number of columns, so it must be an integer. Quotes make it a string and range() fails. It should be key = 3.

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