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\section*{Handout 6 (Zero-Knowledge Proofs)}

Zero-knowledge proofs (short ZKP) solve a paradoxical puzzle:
How to convince somebody else that one knows a secret, without
revealing what the secret actually is? This sounds like a
problem the Mad Hatter from Alice in Wonderland would occupy
himself with, but actually there some serious and not so
serious applications of it. For example, if you solve
crosswords with your friend, say Bob, you might want to
convince him that you found a solution for one question, but
of course you do not want to reveal the solution, as this
might give Bob an advantage somewhere else in the crossword.
So how to convince Bob that you know the answer (or a secret)?
One way would be to come up with the following protocol:
Suppose the answer is \textit{folio}. Then look up the
definition of \textit{folio} in a dictionary. Say you find:

\begin{quote}
``an \textit{individual} leaf of paper or parchment, either
loose as one of a series or forming part of a bound volume,
which is numbered on the recto or front side only.''
\end{quote}

\noindent
Take the first non-article word in this definition,
in this case \textit{individual}, and look up the definition
of this word, say

\begin{quote}
``a single \textit{human} being as distinct from a group''
\end{quote}

\noindent 
In this definition take the second non-article word, that
is \textit{human}, and again look up the definition of this 
word. This will yield

\begin{quote}
``relating to \textit{or} characteristic of humankind''
\end{quote}

\noindent 
You could go on to look up the definition of the third
non-article in this definition and so on. But let us assume
you agreed with Bob to stop after three iterations with the
third non-article word in the last definition, that is
\textbf{or}. Now, instead of sending to Bob the solution
\textit{folio}, you send to him \textit{or}. 

How can Bob verify that you know the solution? Well, once he
solved it himself, he can use the dictionary and follow the
same ``trail'' as you did. If the final word agrees with what
you send him, he must infer you knew the solution earlier than
him. This protocol works like a one-way hash function because
\textit{or} does not give any hint as to what was the first
word was. I leave you to think why this protocol avoids
article words. 

After Bob found his solution and verified that according to
the protocol it ``maps'' also to \textit{or}, can he be
entirely sure no cheating is going on? Not with 100\%
certainty. It could have been entirely possible that he was
given \textit{or} as the word, but it derived from an entirely
different word. This might seem very unlikely, but at
least theoretical is a possibility. Protocols based on
zero-knowledge proofs will produce a similar result---they
give an answer that might be erroneous in a very small number
of cases. The point is to iterate them long enough so that
the theoretical possibility of cheating is negligibly small. 

By the way, the authors of the paper ``Dismantling Megamos
Crypto: Wirelessly Lockpicking a Vehicle Immobilizer'' who
were barred from publishing their results used also a hash to
prove they did the work and (presumably) managed to get into
cars without a key; see Figure~\ref{paper}. This is very
similar to the method about crosswords: They like to prove
that they did the work, but not giving out the ``solution''.
But this also shows what the problem with such a method is:
yes, we can hide the secret temporarily, but if somebody else
wants to verify it, then the secret has to be made public. Bob
needs to know that \textit{folio} is the solution before he
can verify the claim that somebody else had the solution
first. Similarly with the paper: we need to wait until the
authors are finally allowed to publish their findings in order
to verify the hash. This might happen at some point, but
equally it might never happen (what for example happens if the
authors lose their copy of the paper because of a disk
failure?). Zero-knowledge proofs, in contrast, can be
immediately be checked, even if the secret is not public yet
and never will be.

\begin{figure}
\begin{center}
\addtolength{\fboxsep}{4mm}
\fbox{\includegraphics[scale=0.4]{../pics/Dismantling_Megamos_Crypto.png}}
\end{center}
\caption{The authors of this paper used a hash in order to prove
later that they have managed to break into cars.\label{paper}}
\end{figure}


\subsubsection*{ZKP: An Illustrative Example}

The idea behind zero-knowledge proofs is not very obvious and
will surely take some time for you to digest. Therefore let us
start with a simple illustrative example. The example will not
be perfect, but hopefully explain the gist of the idea. The
example is called Alibaba's cave, which graphically looks as 
follows:

\begin{center}
\begin{tabular}{c@{\hspace{8mm}}c@{\hspace{8mm}}c}
\includegraphics[scale=0.1]{../pics/alibaba1.png} &
\includegraphics[scale=0.1]{../pics/alibaba2.png} &
\includegraphics[scale=0.1]{../pics/alibaba3.png} \\
Step 1 & Step 2 & Step 3
\end{tabular}
\end{center}

\noindent Let us have a look at the picture in Step 1: The
cave has a tunnel which forks at some point. Both forks are
connected in a loop. At the deep end of the loop is a magic
wand. The point of the magic wand is that Alice knows the
secret word for how to open it. She wants to keep the word
secret, but wants to convince Bob that she knows it.

One way of course would be to let Bob follow her, but then he
would also find out the secret. This does not work. So let us
first fix the rules: At the beginning Alice and Bob are
outside the cave. Alice goes in alone and takes either tunnel
labelled $A$ in the picture, or the other one labelled $B$.
She waits at the magic wand for the instructions from Bob, who
also goes into the gave and observes what happens at the fork.
He has no knowledge which tunnel Alice took and calls out
(Step 2) that she should emerge from tunnel $A$. If she knows
the problem, this will not be a problem for Alice. If she was 
already in the A-segment of the tunnel, then she just comes
back. If she was in the B-segment of the tunnel she will
say the magic work and goes through the want to emerge
from $A$ as requested by Bob.

Let us have a look at the case where Alice cheats, that is not
knows the secret. She would still go into the cave and use,
for example the $B$-segment of the tunnel. If now Bob says she
should emerge from $B$, she was lucky. But if he says she
should emerge from $A$ then Alice is in trouble and Bob will
find out she does not know the secret. So in order to fool Bob
she needs a protocol that anticipate his call, and already go
into this tunnel. 


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