Codebusters Test Codebusters Transposition Ciphers 2 — Questions and Answers
Question 1: In a keyword columnar transposition with the keyword BAND, in what order are the columns extracted?
- 1, 2, 3, 4 (left to right)
- 2, 1, 4, 3 (alphabetical order: A, B, D, N) (Correct answer)
- 4, 1, 3, 2
- 3, 4, 1, 2
Correct answer: 2, 1, 4, 3 (alphabetical order: A, B, D, N)
Alphabetizing BAND gives A (column 2, rank 1), B (column 1, rank 2), D (column 4, rank 3), N (column 3, rank 4), so columns are extracted in order 2, 1, 4, 3.
Question 2: What is a double transposition cipher?
- A cipher that applies one transposition and one substitution step
- A cipher that applies two separate transposition operations to the plaintext in sequence (Correct answer)
- A transposition cipher that uses two different keywords simultaneously
- A cipher that transposes the ciphertext back to produce the final output
Correct answer: A cipher that applies two separate transposition operations to the plaintext in sequence
A double transposition cipher applies columnar (or another) transposition twice in succession, making it far more resistant to cryptanalysis than single transposition alone.
Question 3: When cryptanalyzing an unknown transposition cipher, what is a productive first step after confirming high IC?
- Apply the Vigenère decryption algorithm with trial keys
- Test different column counts by arranging ciphertext into grids and examining column-pair digraph frequencies (Correct answer)
- Search for the keyword by comparing ciphertext to a dictionary
- Apply the Caesar shift for each of the 26 possible values
Correct answer: Test different column counts by arranging ciphertext into grids and examining column-pair digraph frequencies
Trying different grid widths (column counts) and checking whether the resulting columns pair into plausible English digraphs is the standard attack on columnar transposition.
Question 4: With a 3-rail fence cipher, what is the pattern of rail assignments for successive letters?
- 1, 2, 3, 1, 2, 3, 1, 2, 3…
- 1, 2, 3, 2, 1, 2, 3, 2, 1… (Correct answer)
- 1, 1, 2, 2, 3, 3, 1, 1, 2…
- 1, 3, 2, 1, 3, 2, 1, 3, 2…
Correct answer: 1, 2, 3, 2, 1, 2, 3, 2, 1…
The rail fence zigzag with 3 rails follows the pattern 1→2→3→2→1→2→3…, bouncing between the outermost rails and reversing direction each time it hits rail 1 or rail 3.
Question 5: What does it mean when the last row of a columnar transposition grid is incomplete?
- The cipher is automatically padded with the letter X
- Some columns contain one fewer letter than others, which constrains how many letters can be in each column (Correct answer)
- The keyword length must equal the message length
- Decryption is impossible without additional information
Correct answer: Some columns contain one fewer letter than others, which constrains how many letters can be in each column
When the message length is not a multiple of the keyword length, the last row is partially filled, meaning some columns have one extra letter and others do not — this structure is a clue for determining the grid dimensions.
Question 6: Which cipher did the Union Army use extensively during the American Civil War as a military field cipher?
- The Vigenère cipher
- The route transposition cipher (Correct answer)
- The Caesar cipher
- The Playfair cipher
Correct answer: The route transposition cipher
The Union Army used a route transposition cipher called the 'Union Route Cipher,' filling messages into grids and reading them out in pre-arranged routes, which was challenging for Confederates to break without the route key.
Question 7: In a rail fence cipher, how do you decrypt a ciphertext if you know there are 2 rails?
- Reverse the ciphertext and apply frequency analysis
- Calculate the length of each rail, split the ciphertext accordingly, and interleave the two groups in zigzag order (Correct answer)
- XOR the ciphertext with a binary key derived from the rail count
- Apply the Caesar shift of 2 to every other letter
Correct answer: Calculate the length of each rail, split the ciphertext accordingly, and interleave the two groups in zigzag order
To decrypt a 2-rail fence cipher, determine how many letters fall on each rail (ceiling and floor of n/2), split the ciphertext into those two groups, then interleave them in the original zigzag pattern to recover the plaintext.
Codebusters Test Codebusters Transposition Ciphers 2
In a keyword columnar transposition with the keyword BAND, in what order are the columns extracted?