--- a/handouts/ho09.tex	Thu Dec 11 23:51:08 2014 +0000
+++ b/handouts/ho09.tex	Fri Dec 12 15:41:37 2014 +0000
@@ -62,13 +62,13 @@
 Our starting point is a small, idealised programming language.
 It is idealised because we cut several corners in comparison
 with real programming languages. The language we will study
-contains, amongst other things, variables holding integers. We
-want to find out what the sign of these integers (positive or
-negative) will be when the program runs. This sign-analysis
-seems like a very simple problem, but it will turn out even
-such simple problems, if approached naively, are in general
-undecidable, just like Turing's halting problem. I let you
-think why?
+contains, amongst other things, variables holding integers.
+Using static analysis, we want to find out what the sign of
+these integers (positive or negative) will be when the program
+runs. This sign-analysis seems like a very simple problem. But
+it will turn out even such simple problems, if approached
+naively, are in general undecidable, just like Turing's
+halting problem. I let you think why?
 
 
 Is sign-analysis of variables an interesting problem? Well,
@@ -108,11 +108,13 @@
 sequences of statements. Statements are either labels,
 variable assignments, conditional jumps (\pcode{jmp?}) and
 unconditional jumps (\pcode{goto}). Labels are just strings,
-which can be used as the target of a jump. The conditional
-jumps and variable assignments involve (arithmetic)
-expressions. Expressions are either numbers, variables or
-compound expressions built up from \pcode{+}, \pcode{*} and
-\emph{=} (for simplicity reasons we do not consider any other
+which can be used as the target of a jump. We assume that in
+every program the labels are unique---otherwise if there is a
+clash we do not know where to jump to. The conditional jumps
+and variable assignments involve (arithmetic) expressions.
+Expressions are either numbers, variables or compound
+expressions built up from \pcode{+}, \pcode{*} and \emph{=}
+(for simplicity reasons we do not consider any other
 operations). We assume we have negative and positive numbers,
 \ldots \pcode{-2}, \pcode{-1}, \pcode{0}, \pcode{1},
 \pcode{2}\ldots{} An example program that calculates the
@@ -173,20 +175,58 @@
 complexity is really beside the point here. Furthermore, real
 machine code has many instructions for manipulating memory. We
 do not have this at all. This is actually a more serious
-simplification because we assume numbers to be arbitrary
-precision, which is not the case with real machine code. In
+simplification because we assume numbers to be arbitrary small
+or large, which is not the case with real machine code. In
 real code basic number formats have a range and might
-over-flow or under-flow from this range. Also the numbers of
-variables in our programs is unlimited, while memory, of
-course, is always limited somehow on any actual machine. To
-sum up, our language might look very simple, but it is not
-completely removed from practically relevant issues.
+over-flow or under-flow from this range. Also the number of
+variables in our programs is potentially unlimited, while
+memory in an actual computer, of course, is always limited
+somehow on any actual. To sum up, our language might look very
+simple, but it is not completely removed from practically
+relevant issues.
 
 
 \subsubsection*{An Interpreter}
 
-Designing a language is like being god: you can say what 
-each part of the program should mean. 
+Designing a language is like playing god: you can say what
+names for variables you allow; what programs should look like;
+most importantly you can decide what each part of the program
+should mean and do. While our language is rather simple and
+the meaning is rather straightforward, there are still places
+where we need to make a real choice. For example with
+conditional jumps, say the one in the factorial program: 
+
+\begin{center}
+\code{jmp? n = 0 done}
+\end{center}
+
+\noindent How should they work? We could introduce Booleans
+(\pcode{true} and \pcode{false}) and then jump only when the
+condition is \pcode{true}. However, since we have numbers in
+our language anyway, why not just encoding \emph{true} as
+zero, and \pcode{false} as anything else? In this way we can
+dispense with the additional concept of Booleans, but also we
+could replace the jump above by
+
+\begin{center}
+\code{jmp? n done}
+\end{center}
+
+\noindent which behaves exactly the same. But what does it
+mean that two jumps behave the same?
+
+I hope the above discussion makes it already clear we need to
+be a bit more careful with our programs. Below we shall
+describe an interpreter for our programs, which specifies
+exactly how programs are supposed to be run\ldots{}at least we
+will specify this for all \emph{good} programs. By good
+programs we mean where for example all variables are
+initialised. Our interpreter will just crash if it cannot find
+out the value for a variable, because it is not initialised.
+
+First we will pre-process our programs. This will simplify 
+our definition of our interpreter later on. We will transform
+programs into \emph{snippets}. 
 
 \end{document}
 

Binary file slides/slides11.pdf has changed

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/slides/slides11.tex	Fri Dec 12 15:41:37 2014 +0000
@@ -0,0 +1,138 @@
+\documentclass[dvipsnames,14pt,t]{beamer}
+\usepackage{../slides}
+\usepackage{../langs}
+\usepackage{../graphics}
+\usepackage{../data}
+\usepackage{../grammar}
+
+% beamer stuff 
+\renewcommand{\slidecaption}{APP 11, King's College London}
+\newcommand{\bl}[1]{\textcolor{blue}{#1}}
+
+\begin{document}
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+\begin{frame}[t]
+\frametitle{%
+  \begin{tabular}{@ {}c@ {}}
+  \\
+  \LARGE Access Control and \\[-3mm] 
+  \LARGE Privacy Policies (11)\\[-6mm] 
+  \end{tabular}}\bigskip\bigskip\bigskip
+
+  \normalsize
+  \begin{center}
+  \begin{tabular}{ll}
+  Email:  & christian.urban at kcl.ac.uk\\
+  Office: & S1.27 (1st floor Strand Building)\\
+  Slides: & KEATS (also homework is there)\\
+  \end{tabular}
+  \end{center}
+
+\end{frame}
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%     
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+\begin{frame}[c]
+
+\begin{itemize}
+\item you can still send me your homework\bigskip 
+\item Unix AC question: use a terminal-based editor (vm, 
+  vim)\bigskip 
+\item exams: 2 out of 3 questions, 5 or so subquestions 
+  each, you can fill in your answers on the question sheet
+\end{itemize}
+
+\end{frame}
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%  
+
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+\begin{frame}[c]
+\frametitle{Interlock Protocol}
+
+The interlock protocol (``best bet'' against MITM):
+
+\begin{center}
+\begin{tabular}{ll@{\hspace{2mm}}l}
+1. & \bl{$A \to B :$} & \bl{$K^{pub}_A$}\\
+2. & \bl{$B \to A :$} & \bl{$K^{pub}_B$}\\
+3. & & \bl{$\{A,m\}_{K^{pub}_B} \;\mapsto\; H_1,H_2$}\\
+   & & \bl{$\{B,m'\}_{K^{pub}_A} \;\mapsto\; M_1,M_2$}\\
+4. & \bl{$A \to B :$} & \bl{$H_1$}\\
+5. & \bl{$B \to A :$} & \bl{$\{H_1, M_1\}_{K^{pub}_A}$}\\
+6. & \bl{$A \to B :$} & \bl{$\{H_2, M_1\}_{K^{pub}_B}$}\\
+7. & \bl{$B \to A :$} & \bl{$M_2$}
+\end{tabular}
+\end{center}\pause
+
+\footnotesize
+\bl{$m$} = How is your grandmother? \bl{$m'$} = How is the
+weather today in London?
+
+\end{frame}
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%  
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+\begin{frame}[c]
+
+\begin{center}
+\begin{tabular}{l@{\hspace{9mm}}l}
+\begin{tabular}[t]{@{}l@{}}
+\bl{$A \to C : K^{pub}_A$}\\
+\bl{$C \to B : K^{pub}_C$}\\
+\bl{$B \to C : K^{pub}_B$}\\
+\bl{$C \to A : K^{pub}_C$}\medskip\\
+\bl{$\{A,m\}_{K^{pub}_C} \;\mapsto\; H_1,H_2$}\\
+\bl{$\{B,n\}_{K^{pub}_C} \;\mapsto\; M_1,M_2$}\bigskip\\
+\bl{$\{C,a\}_{K^{pub}_B} \;\mapsto\; C_1,C_2$}\\
+\bl{$\{C,b\}_{K^{pub}_A} \;\mapsto\; D_1,D_2$}
+\end{tabular} &
+\begin{tabular}[t]{@{}l@{}}
+\bl{$A \to C : H_1$}\\
+\bl{$C \to B : C_1$}\\
+\bl{$B \to C : \{C_1, M_1\}_{K^{pub}_C}$}\\
+\bl{$C \to A : \{H_1, D_1\}_{K^{pub}_A}$}\\
+\bl{$A \to C : \{H_2, D_1\}_{K^{pub}_C}$}\\
+\bl{$C \to B : \{C_2, M_1\}_{K^{pub}_B}$}\\
+\bl{$B \to C : M_2$}\\
+\bl{$C \to A : D_2$}
+\end{tabular}
+\end{tabular}
+\end{center}
+
+\end{frame}
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%  
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+\begin{frame}[c]
+
+\begin{itemize}
+\item you have to ask something that cannot imitated 
+  (requires \bl{$A$} and \bl{$B$} know each other)
+\item what happens if \bl{$m$} and \bl{$n$} are voice
+  messages?\bigskip  
+
+\item the moral: establishing a secure connection from ``zero'' is
+almost impossible---you need to rely on some established 
+trust\medskip
+
+\item that is why we rely on certificates, which however are
+badly, badly realised (just today a POODLE attack against SSL)
+
+\end{itemize}
+
+\end{frame}
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%  
+
+
+
+
+\end{document}
+
+
+%%% Local Variables:  
+%%% mode: latex
+%%% TeX-master: t
+%%% End: 
+