Codebusters Test Curriculum Framework 3 — Questions and Answers
Question 1: In a columnar transposition cipher, what role does the keyword play in the decryption process?
- It determines the Caesar shift for each column
- It provides the order in which columns are read to reconstruct the plaintext (Correct answer)
- It identifies which letters to skip during decryption
- It sets the number of rails used in the transposition
Correct answer: It provides the order in which columns are read to reconstruct the plaintext
In a columnar transposition cipher, the keyword's alphabetical order determines the sequence in which columns are read during decryption.
Question 2: Which cipher is based on encoding letters as pairs of numbers using a Polybius square?
- Porta cipher
- Polybius checkerboard cipher (Correct answer)
- Beaufort cipher
- ADFGVX cipher
Correct answer: Polybius checkerboard cipher
The Polybius checkerboard cipher encodes each letter as its row and column coordinates within a 5x5 grid.
Question 3: In the context of Codebusters, what does 'index of coincidence' measure?
- The number of repeated letters in the ciphertext
- The probability that two randomly chosen letters in a text are identical (Correct answer)
- The average shift value of a Caesar cipher
- The key length of a Vigenère cipher in absolute terms
Correct answer: The probability that two randomly chosen letters in a text are identical
The index of coincidence measures how likely two randomly selected letters from a text are the same, helping distinguish monoalphabetic from polyalphabetic ciphers.
Question 4: The Kasiski test is used in Codebusters to:
- Verify that a Caesar shift has been applied correctly
- Estimate the keyword length of a Vigenère cipher by finding repeated sequences (Correct answer)
- Count digraph frequencies in a substitution cipher
- Determine whether a transposition or substitution cipher was used
Correct answer: Estimate the keyword length of a Vigenère cipher by finding repeated sequences
The Kasiski test finds repeated trigrams or longer sequences in the ciphertext; the distances between repetitions often share a common factor equal to the keyword length.
Question 5: A Codebusters team encounters a cipher where letters are replaced by symbols from a pigpen-style grid. This is best described as:
- A transposition cipher
- A steganographic cipher
- A symbolic substitution cipher (Correct answer)
- A running key cipher
Correct answer: A symbolic substitution cipher
Pigpen and similar grid-based ciphers are symbolic substitution ciphers that replace plaintext letters with geometric shapes or symbols.
Question 6: In a Baconian cipher, each plaintext letter is represented by:
- A pair of Morse code symbols
- A 5-character sequence of A's and B's (or binary) (Correct answer)
- Its position in a Polybius square
- A two-digit number from 01 to 26
Correct answer: A 5-character sequence of A's and B's (or binary)
The Baconian cipher represents each letter as a unique 5-character sequence of two symbols (A and B), creating 32 possible combinations for 26 letters.
Question 7: In Codebusters, which property makes a one-time pad (OTP) theoretically unbreakable?
- It uses an extremely long repeating keyword
- The key is truly random, at least as long as the message, and used only once (Correct answer)
- It applies multiple layers of transposition before substitution
- It encrypts digraphs instead of individual letters
Correct answer: The key is truly random, at least as long as the message, and used only once
A one-time pad is information-theoretically secure because its random key is as long as the plaintext and is never reused, leaving no statistical patterns to exploit.
Codebusters Test Curriculum Framework 3
In a columnar transposition cipher, what role does the keyword play in the decryption process?