One way to solve the key distribution problem is to use a line from a book that
ID: 3664075 • Letter: O
Question
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?
Explanation / Answer
a. The first letter t corresponds to A, the second letter h corresponds to B, e is C, s is D, and so on. Second and subsequent occurrences of a letter in the key sentence are ignored. The result
ciphertext: SIDKHKDM AF HCRKIABIE SHIMC KD LFEAILA
plaintext: basilisk to leviathan blake is contact
b. It is a monalphabetic cipher and so easily breakable.
c. The last sentence may not contain all the letters of the alphabet. If the first sentence is used, the second and subsequent sentences may also be used until all 26 letters are encountered.
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