Skip to main content Contents Index
Prev Up Next \(\newcommand{\longdivision}[2]{#1\big)\!\!\overline{\;#2}}
\newcommand{\mlongdivision}[2]{\longdivision{#1}{#2}}
\renewcommand{\emptyset}{\{\}}
\newcommand{\blanksp}{\underline{\hspace{.25in}}}
\newcommand{\set}[1]{\left\{#1\right\}}
\newcommand{\cspace}{-}
\newcommand{\Ta}{\mathtt{a}}
\newcommand{\Tb}{\mathtt{b}}
\newcommand{\Tc}{\mathtt{c}}
\newcommand{\Td}{\mathtt{d}}
\newcommand{\Te}{\mathtt{e}}
\newcommand{\Tf}{\mathtt{f}}
\newcommand{\Tg}{\mathtt{g}}
\newcommand{\Th}{\mathtt{h}}
\newcommand{\Ti}{\mathtt{i}}
\newcommand{\Tj}{\mathtt{j}}
\newcommand{\Tk}{\mathtt{k}}
\newcommand{\Tl}{\mathtt{l}}
\newcommand{\Tm}{\mathtt{m}}
\newcommand{\Tn}{\mathtt{n}}
\newcommand{\To}{\mathtt{o}}
\newcommand{\Tp}{\mathtt{p}}
\newcommand{\Tq}{\mathtt{q}}
\newcommand{\Tr}{\mathtt{r}}
\newcommand{\Ts}{\mathtt{s}}
\newcommand{\Tt}{\mathtt{t}}
\newcommand{\Tu}{\mathtt{u}}
\newcommand{\Tv}{\mathtt{v}}
\newcommand{\Tw}{\mathtt{w}}
\newcommand{\Tx}{\mathtt{x}}
\newcommand{\Ty}{\mathtt{y}}
\newcommand{\Tz}{\mathtt{z}}
\newcommand{\So}{\Tf}
\newcommand{\Sno}{\Tg}
\newcommand{\Si}{\Th}
\newcommand{\Sni}{\Tj}
\newcommand{\N}{\mathbb{N}}
\newcommand{\Z}{\mathbb{Z}}
\newcommand{\W}{\mathbb{W}}
\newcommand{\ZZ}{\Z}
\newcommand{\PP}{\mathbb{P}}
\newcommand{\Q}{\mathbb{Q}}
\newcommand{\R}{\mathbb{R}}
\newcommand{\RR}{\R}
\newcommand{\F}{\mathbb{F}}
\newcommand{\A}{\mathbb{A}}
\newcommand{\abs}[1]{|#1|}
\newcommand{\fmod}{\bmod}
\newcommand{\fdiv}{\,\mathrm{div}\,}
\newcommand{\lcm}{\mathrm{lcm}}
\newcommand{\id}{\mathrm{id}}
\newcommand{\nr}[1]{\##1}
\newcommand{\gexp}[3]{#1^{#2 #3}}
\newcommand{\gexpp}[3]{\displaystyle\left(#1\right)^{#2 #3}}
\newcommand{\glog}[3]{\log_{#1}^{#3}#2}
\newcommand{\sol}[1]{{\color{blue}\textit{#1}}}
\newcommand{\gro}[1]{{\color{gray}#1}}
\newcommand{\todo}[1]{{\color{purple}TO DO: #1}}
\newcommand{\fixme}[1]{{\color{red}FIX ME: #1}}
\newcommand{\checkme}[1]{{\color{green}CHECK ME: #1}}
\newcommand{\degre}{^\circ}
\newcommand{\vect}[1]{\overrightarrow{#1}}
\newcommand{\nix}{}
\newcommand{\cox}[1]{\fcolorbox[HTML]{000000}{#1}{\phantom{M}}}
\newcommand{\tox}[1]{\texttt{\##1} \amp \cox{#1}}
\newcommand{\ttx}[1]{\texttt{\##1}}
\newcommand{\mox}[1]{\mathtt{\##1}}
\newcommand{\xx}{\mathtt{\#}}
\newcommand{\lt}{<}
\newcommand{\gt}{>}
\newcommand{\amp}{&}
\definecolor{fillinmathshade}{gray}{0.9}
\newcommand{\fillinmath}[1]{\mathchoice{\colorbox{fillinmathshade}{$\displaystyle \phantom{\,#1\,}$}}{\colorbox{fillinmathshade}{$\textstyle \phantom{\,#1\,}$}}{\colorbox{fillinmathshade}{$\scriptstyle \phantom{\,#1\,}$}}{\colorbox{fillinmathshade}{$\scriptscriptstyle\phantom{\,#1\,}$}}}
\)
Chapter 8 Codes
Objectives
Convert text into a sequence of numbers.
Apply substitution ciphers to encrypt and decrypt texts.
Apply frequency analysis to break Caesar ciphers.
In this chapter we apply functions in the encoding and encryption of texts. We first introduce a function that we use to represent characters by numbers in
Section 8.1 . This is followed by a short introduction to symmetric key cryptography in
Section 8.2 . and the description of simple symmetric key crypto systems, namely Caesar ciphers in
Section 8.3 and other substitution ciphers in
Section 8.4 . We conclude this chapter with an attack on substitution ciphers called frequency analysis in
Section 8.5 .