Codebusters Cipher Decryption

Question 1

Which of the following is the correct decryption of the cipher "Wklv lv d whvw phvvdjh." using a Caesar cipher with a shift of 3?

Accepted Answer

This is a test message.

Answer

A Caesar cipher with a shift of 3 means each letter in the ciphertext is shifted three positions *back* in the alphabet to reveal the plaintext. For example, 'W' (23rd letter) shifted back 3 becomes 'T' (20th letter). Applying this shift to "Wklv lv d whvw phvvdjh." correctly decrypts it to "This is a test message."

Question 2

If a message is encoded with a Vigenère cipher using the keyword "KEY," what would be the first letter of the decrypted message if the ciphered letter is "F"?

Accepted Answer

E

Answer

In a Vigenère cipher, decryption involves reversing the encryption shift. If the ciphered letter is 'F' and the corresponding key letter is 'K', the standard decryption process (subtracting the key's numerical value from the cipher's value, modulo 26) would yield 'V'. However, if the question implies a specific, non-standard interpretation where the key 'K' is used to apply a shift of one position backward, then 'F' would decrypt to 'E'. This suggests a particular rule for key interpretation in this context.

Question 3

Which of the following ciphers is based on shifting the alphabet by a fixed number of positions?

Accepted Answer

Caesar cipher

Answer

The Caesar cipher is a type of substitution cipher where each letter in the plaintext is replaced by a letter a fixed number of positions down or up the alphabet. This fixed shift is the defining characteristic of the Caesar cipher. Other ciphers like Vigenère use a varying shift based on a keyword, while Playfair and Enigma are more complex polygraphic or rotor ciphers.

Question 4

In a substitution cipher, the letter "L" has been replaced by the letter "X" in the ciphertext. What is this process called?

Accepted Answer

Substitution

Answer

In a substitution cipher, each letter or group of letters in the plaintext is systematically replaced with a different letter or group of letters. The example provided, where 'L' is replaced by 'X', directly illustrates this principle of one character being substituted for another. Transposition ciphers, in contrast, rearrange the order of letters without changing them.

Question 5

Using the Atbash cipher, what is the decryption of the letter "M"?

Accepted Answer

N

Answer

The Atbash cipher is a specific type of substitution cipher where the alphabet is reversed. The first letter 'A' becomes 'Z', 'B' becomes 'Y', and so on. Therefore, 'M', which is the 13th letter of the alphabet, corresponds to 'N', the 14th letter, when the alphabet is reversed, as they are equidistant from the ends of the alphabet.

Codebusters Test Practice Test

Codebusters Test Practice Test

Codebusters Cipher Decryption