Continue writing

chap-proofs.tex

@@ -102,10 +102,34 @@ In cryptology, it is also important to consider the success probability of algor
 an attack is successful if the probability that it succeed is noticeable.
 
 \index{Negligible function}
-\textsc{Notation.} Let $f : \NN \to [0,1]$ be a function. The function $f$ is called \emph{negligible} if $f(n) = n^{-\omega(1)}$, and this is written $f(n) = \negl[n]$. Non-negligible functions are called \emph{noticeable} functions. And if $f = 1- \negl[n]$, $f$ is called \emph{overwhelming}.
+\scbf{Notation.} Let $f : \NN \to [0,1]$ be a function. The function $f$ is called \emph{negligible} if $f(n) = n^{-\omega(1)}$, and this is written $f(n) = \negl[n]$. Non-negligible functions are called \emph{noticeable} functions. And if $f = 1- \negl[n]$, $f$ is called \emph{overwhelming}.
 
 Once that we define the notions related to the core of the proof, we have to define the objects on what we work on.
-Namely, defining what we want to prove, and the hypotheses on which we rely.
+Namely, defining what we want to prove, and the hypotheses on which we rely, also called ``hardness assumption''.
+
+The details of the hardness assumptions we use are given in Chapter~\ref{chap:structures}. Nevertheless, some notions are common to these and are evoked here.
+
+The amount of confidence one can put in a hardness assumption is given by many criteria.
+
+First of all, a weaker assumption is preferred to a stronger one if it is possible.
+To illustrate this, let us consider the two following assumptions:
+
+\begin{definition}[Discrete logarithm] \label{de:DLP}
+  \index{Discrete Logarithm!Assumption}
+  \index{Discrete Logarithm!Problem}
+  The \emph{discrete algorithm problem} is defined as follows. Let $(\GG, \cdot)$ be a cyclic group of order $p$.
+  Given $g,h \in \GG$, the goal is to find an integer $a \in \Zp$ such that: $g^a = h$.
+
+  The \textit{discrete logarithm assumption} is the intractability of this problem.
+\end{definition}
+
+\begin{definition}[Decisional Diffie Hellman] \label{de:DDH} \index{Discrete Logarithm!Decisional Diffie-Hellman}
+  Let $\GG$ be a cyclic group of order $p$. The \emph{decisional Diffie-Hellman} ($\DDH$) problem is the following.
+  Given $(g, g_1, g_2, g_3) = (g, g^a, g^b, g^c) \in \GG^4$, the goal is to decide if $c = ab$ or if $c$ is sampled uniformly in $\GG$.
+
+  The \textit{\DDH assumption} is the intractability of the problem for any $\ppt$ algorithm.
+\end{definition}
+The discrete logarithm assumption is implied by the decisional Diffie-Hellman assumption for instance. Indeed, if we can solve the discrete logarithm problem, then it suffices to compute the discrete logarithm of $g_1$, let say $a$, and then check whether $g_2^a = g_3$. Thus it is preferable to work with the discrete logarithm problem if it is possible. 
 
 \section{Random-Oracle Model, Standard Model and Half-Simulatability}