Codebusters Test Codebusters Cipher Decryption 2 — Questions and Answers
Question 1: In a Caesar cipher with shift 13 (ROT13), what does the letter 'N' encrypt to?
- A (Correct answer)
- N
- Z
- M
Correct answer: A
N is at index 13; adding 13 gives 26, which wraps to 0, the letter A.
Question 2: If the ciphertext 'KHOOR' was encrypted with a Caesar shift of 3, what is the plaintext?
- HELLO (Correct answer)
- WORLD
- CIPHER
- CODED
Correct answer: HELLO
Shifting each letter back 3 gives K→H, H→E, O→L, O→L, R→O, spelling HELLO.
Question 3: The most frequent letter in English text is 'E'. In a Caesar-encrypted ciphertext, what does the most frequent ciphertext letter most likely represent?
- The encryption of E (Correct answer)
- Always the letter Z
- The shift value itself
- A space character
Correct answer: The encryption of E
Because Caesar shifts all letters by the same amount, E's high frequency is preserved in the ciphertext.
Question 4: An Atbash cipher maps A→Z, B→Y, C→X, etc. What does the word 'ZOO' encode to under Atbash?
- ALL (Correct answer)
- ZOO
- DOG
- CAT
Correct answer: ALL
Z→A, O→L, O→L, so ZOO becomes ALL.
Question 5: A Caesar cipher uses a shift of 25. What is the equivalent negative (backward) shift?
- -1 (Correct answer)
- -2
- -3
- -25
Correct answer: -1
Shifting forward 25 is the same as shifting backward 1 in a 26-letter alphabet.
Question 6: Why is ROT13 considered self-reciprocal — meaning applying it twice returns the original plaintext?
- The shift of 13 is exactly half of 26 (Correct answer)
- The letters loop back to the start randomly
- It uses a symmetric private key
- It has no repeated letters in its key
Correct answer: The shift of 13 is exactly half of 26
Since 13 + 13 = 26, a second ROT13 shift brings every letter back to its original position.
Question 7: In a Codebusters event, a Caesar ciphertext letter 'M' decrypts to plaintext 'D'. What shift was used?
- 9 (Correct answer)
- 3
- 12
- 17
Correct answer: 9
M is at index 12 and D is at index 3; 12 − 3 = 9, so the shift is 9.
Codebusters Test Codebusters Cipher Decryption 2
In a Caesar cipher with shift 13 (ROT13), what does the letter 'N' encrypt to?