Messages and keys work for a mountain rescue team that sends short encrypted words between two radio stations. A new volunteer suggests using the same Caesa...
Question 1 Report
Messages and keys work for a mountain rescue team that sends short encrypted words between two radio stations. A new volunteer suggests using the same Caesar cipher key of 1 for every message because it is easy to remember. The coordinator tests the plaintext word HELP and receives ciphertext IFMQ. The coordinator also wants a brief check of whether this encryption method is suitable for confidential rescue data.
(a) Identify the ciphertext letter produced from plaintext H. [1]
(b) State the Caesar shift used by the volunteer. [1]
(c) Write the plaintext word obtained by decrypting IFMQ. [4]
(d) Give one reason why a fixed Caesar key of 1 is weak encryption. [1]
Answer Details
(a) H encrypts to I [1].
(b) The Caesar shift is 1 [1].
(c) Decrypt by shifting each letter back one: I to H [1], F to E [1], M to L [1], and Q to P [1]. Thus IFMQ decrypts to HELP [4].
(d) A fixed key of 1 is weak because there are very few possible keys, so an attacker can quickly try every shift [1]. Repeated letter patterns also remain predictable.
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