book name $($Understanding Cryptography

A Textbook for Students and Practitioners

Foreword by Bart Preneel$)$

$1.$ The ciphertext below was encrypted using a substitution cipher. Decrypt the ciphertext without knowledge of the key.

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a$.$ Compute the relative frequency of all letters A$...$Z in the ciphertext. You may want to use a tool such as the open$-$source program CrypTool $[50]$ for this task. However, a paper and pencil approach is also still doable.

b$.$ Decrypt the ciphertext with the help of the relative letter frequency of the English language $($see Table $1\mathrm{.}1$ in Sect. $1\mathrm{.}2\mathrm{.}2).$ Note that the text is relatively short and that the letter frequencies in it might not perfectly align with that of general English language from the table.

Table $1\mathrm{.}1$ Relative letter frequencies of the English language

$\backslash$table$[[$Letter$,$Frequency,Letter,Frequency$],[A,0\mathrm{.}0817,N,0\mathrm{.}0675],[B,0\mathrm{.}0150,O,0\mathrm{.}0751],[C,0\mathrm{.}0278,P,0\mathrm{.}0193],[D,0\mathrm{.}0425,Q,0\mathrm{.}0010],[E,0\mathrm{.}1270,R,0\mathrm{.}0599],[F,0\mathrm{.}0223,S,0\mathrm{.}0633],[G,0\mathrm{.}0202,T,0\mathrm{.}0906],[H,0\mathrm{.}0609,U,0\mathrm{.}0276],[I,0\mathrm{.}0697,V,0\mathrm{.}0098],[J,0\mathrm{.}0015,W,0\mathrm{.}0236],[K,0\mathrm{.}0077,x,0\mathrm{.}0015],[L,0\mathrm{.}0403,Y,0\mathrm{.}0197],[M,0\mathrm{.}0241,Z,0\mathrm{.}0007]]$

c$.$ Who wrote the text?$\text{'}$