$[9$ points$]($Same question as $3\mathrm{.}5$ in "Classic Encryption Techniques" handout$)$

$3\mathrm{.}5$ One way to solve the key distribution problem is to use a line from a book that both

the sender and the receiver possess. Typically, at least in spy novels, the first sentence

of a book serves as the key. The particular scheme discussed in this problem is from

one of the best suspense novels involving secret codes, Talking to Strange Men, by

Ruth Rendell. Work this problem without consulting that book!

Consider the following message:

SIDKHKDM AF HCRKIABIE SHIMC KD LFEAILA

This ciphertext was produced using the first sentence of The Other Side of Silence

$($a book about the spy Kim Philby$)$:

The snow lay thick on the steps and the snowflakes driven by the wind looked black in

the headlights of the cars.

A simple substitution cipher was used.

a$.$ What is the encryption algorithm?

b$.$ How secure is it$?$

c$.$ To make the key distribution problem simple, both parties can agree to use the

first or last sentence of a book as the key. To change the key, they simply need

to agree on a new book. The use of the first sentence would be preferable to the

use of the last. Why?