A historian has recovered a paper note containing the ciphertext YMJ VZNHP GWTBS KTC. The note is believed to have been written using a Caesar cipher. A pen...
Question 1 Report
A historian has recovered a paper note containing the ciphertext YMJ VZNHP GWTBS KTC. The note is believed to have been written using a Caesar cipher. A pencilled instruction says that each ciphertext letter is three positions after its plaintext letter. The historian enters the message and key into a decryption program, then saves the correct plaintext in an archive file.
(a) Identify the cipher described in the note. [1]
(b) State the number used as the encryption key. [1]
(c) Use the key to write the plaintext for ciphertext YMJ. [3]
(d) Give one reason why a person who knows the key can read the ciphertext. [1]
Answer Details
(a) The method is a Caesar cipher [1].
(b) The encryption key is 3 [1].
(c) Decryption reverses the encryption shift, so move each ciphertext letter back three places: Y becomes T [1], M becomes H [1], and J becomes E [1]. Therefore YMJ decrypts to THE [3].
(d) Someone who knows the key can read the ciphertext because they can reverse the shift, moving every letter back by 3 [1].
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