The following encrypted message from Alice to Bob is intercepted by Eve.
\begin{align*}
\amp\mathtt{tddsztmdsakswanawxwsaflgsl}\cspace\mathtt{jxxshtjlksgfxsgyso}\cspace\\
\amp\mathtt{av}\cspace\mathtt{sl}\cspace\mathtt{xsuxdztxsaf}\cspace\mathtt{tualsl}\cspace\mathtt{xstimaltfastfgl}\cspace\mathtt{xjsl}\cspace\\
\amp\mathtt{gkxso}\cspace\mathtt{gsafsl}\cspace\mathtt{xajsgofsdtfzmtzxstjxsvtddxwsvxdlk}\\
\amp\mathtt{safsgmjksztmdksl}\cspace\mathtt{xsl}\cspace\mathtt{ajwstddsl}\cspace\mathtt{xkxswayyxjsyjge}\\
\amp\mathtt{sxtv}\cspace\mathtt{sgl}\cspace\mathtt{xjsafsdtfzmtzxsvmklgekstfwsdtok}
\end{align*}
Eve knows that Alice and Bob are using a Caesar cipher. Decipher the message for Eve.
Solution.
Counting the number of occurrences of the characters in the cipher text we get:
| character | count |
| \(\cspace\) | \({15}\) |
| \(\mathtt{a}\) | \({ 15 }\) |
| \(\mathtt{d}\) | \({ 13 }\) |
| \(\mathtt{e}\) | \({2}\) |
| \(\mathtt{f}\) | \({ 12 }\) |
| \(\mathtt{g}\) | \({ 11 }\) |
| \(\mathtt{h}\) | \({1}\) |
| \(\mathtt{i}\) | \({1}\) |
| \(\mathtt{j}\) | \({ 10 }\) |
| \(\mathtt{k}\) | \({ 10 }\) |
| \(\mathtt{l}\) | \({ 16 }\) |
| \(\mathtt{m}\) | \({7}\) |
| \(\mathtt{n}\) | \({1}\) |
| \(\mathtt{o}\) | \({4}\) |
| \(\mathtt{s}\) | \({ 40 }\) |
| \(\mathtt{t}\) | \({ 19 }\) |
| \(\mathtt{u}\) | \({2}\) |
| \(\mathtt{v}\) | \({5}\) |
| \(\mathtt{w}\) | \({7}\) |
| \(\mathtt{x}\) | \({ 22 }\) |
| \(\mathtt{y}\) | \({4}\) |
| \(\mathtt{z}\) | \({ 7}\) |
According to the data in Figure 11.4.7 the character space which we represent by \(\cspace\) is the most common character in English language texts. Since \(\mathtt{s}\) is the character in the cipher text with the highest number of occurrences, Eve tries decrypting the cipher text with the Caesar cipher that replaces \(\cspace\) with \(\mathtt{s}\text{.}\)
| \(x\) | \(J(x)\) |
| \(\cspace\) | \(\mathtt{s}\) |
| \(\mathtt{a}\) | \(\mathtt{t}\) |
| \(\mathtt{b}\) | \(\mathtt{u}\) |
| \(\mathtt{c}\) | \(\mathtt{v}\) |
| \(\mathtt{d}\) | \(\mathtt{w}\) |
| \(\mathtt{e}\) | \(\mathtt{x}\) |
| \(\mathtt{f}\) | \(\mathtt{y}\) |
| \(\mathtt{g}\) | \(\mathtt{z}\) |
| \(\mathtt{h}\) | \(\cspace\) |
| \(\mathtt{i}\) | \(\mathtt{a}\) |
| \(\mathtt{j}\) | \(\mathtt{b}\) |
| \(\mathtt{k}\) | \(\mathtt{c}\) |
| \(\mathtt{l}\) | \(\mathtt{d}\) |
| \(\mathtt{m}\) | \(\mathtt{e}\) |
| \(\mathtt{n}\) | \(\mathtt{f}\) |
| \(\mathtt{o}\) | \(\mathtt{g}\) |
| \(\mathtt{p}\) | \(\mathtt{h}\) |
| \(\mathtt{q}\) | \(\mathtt{i}\) |
| \(\mathtt{r}\) | \(\mathtt{j}\) |
| \(\mathtt{s}\) | \(\mathtt{k}\) |
| \(\mathtt{t}\) | \(\mathtt{l}\) |
| \(\mathtt{u}\) | \(\mathtt{m}\) |
| \(\mathtt{v}\) | \(\mathtt{n}\) |
| \(\mathtt{w}\) | \(\mathtt{o}\) |
| \(\mathtt{x}\) | \(\mathtt{p}\) |
| \(\mathtt{y}\) | \(\mathtt{q}\) |
| \(\mathtt{z}\) | \(\mathtt{r}\) |
When decrypting Eve reads the table from right to left. So Eve decrypts \(\mathtt{t}\) to \(\mathtt{a}\text{,}\) \(\mathtt{d}\) to \(\mathtt{l}\text{,}\) \(\mathtt{s}\) to \(\cspace\text{,}\) and so on. Eve obtains the following message :
1
from the English translation of Julius Caesar’s De Bello Gallico (The Gallic Wars) by W. A. McDevitte and W. S. Bohn.
\begin{align*}
\amp\mathtt{all}\cspace\mathtt{gaul}\cspace\mathtt{is}\cspace\mathtt{divided}\cspace\mathtt{into}\cspace\mathtt{three}\cspace\mathtt{parts}\cspace\mathtt{one}\cspace\mathtt{of}\cspace\\
\amp\mathtt{which}\cspace\mathtt{the}\cspace\mathtt{belgae}\cspace\mathtt{inhabit}\cspace\mathtt{the}\cspace\mathtt{aquitani}\cspace\mathtt{another}\\
\amp\mathtt{}\cspace\mathtt{those}\cspace\mathtt{who}\cspace\mathtt{in}\cspace\mathtt{their}\cspace\mathtt{own}\cspace\mathtt{language}\cspace\mathtt{are}\cspace\mathtt{called}\cspace\\
\amp\mathtt{celts}\cspace\mathtt{in}\cspace\mathtt{ours}\cspace\mathtt{gauls}\cspace\mathtt{the}\cspace\mathtt{third}\cspace\mathtt{all}\cspace\mathtt{these}\cspace\mathtt{differ}\\
\amp\mathtt{}\cspace\mathtt{from}\cspace\mathtt{each}\cspace\mathtt{other}\cspace\mathtt{in}\cspace\mathtt{language}\cspace\mathtt{customs}\cspace\mathtt{and}\cspace\mathtt{laws}
\end{align*}
