More more more

thesis/sections/security_of_eddsa/uf-nma_implies_suf-cma.tex

@@ -109,11 +109,11 @@ The proof starts by providing an algorithm which generates correctly distributed
 \begin{proof}
 	\item \paragraph{\underline{$G_0:$}} Let $G_0$ be defined in figure \ref{fig:uf-nma_implies_suf-cma_games} by excluding all boxes except the black one and let $G_0$ be $\text{\cma}$. By definition,
 	
-	\[ \advantage{\text{EdDSA},\adversary{A}}{\cma}(\secparamter) = \Pr[\text{\cma}^{\adversary{A}} \Rightarrow 1] = \Pr[G_0^{\adversary{A}} \Rightarrow 1] \].
+	\[ \advantage{\text{EdDSA},\adversary{A}}{\cma}(\secparamter) = \Pr[\text{\cma}^{\adversary{A}} \Rightarrow 1] = \Pr[G_0^{\adversary{A}} \Rightarrow 1]. \]
 	
 	\item \paragraph{\underline{$G_1:$}} $G_1$ is now defined by replacing the black box with the blue one. This change inlines the call to the hash function and introduces a bad flag, which is set in the case that the hash value is already set. This change is only conceptual, since it does not alter the behavior of the oracle. Hence,
 	
-	\[ \Pr[G_0^{\adversary{A}} \Rightarrow 1] = \Pr[G_1^{\adversary{A}} \Rightarrow 1] \].
+	\[ \Pr[G_0^{\adversary{A}} \Rightarrow 1] = \Pr[G_1^{\adversary{A}} \Rightarrow 1]. \]
 	
 	\item \paragraph{\underline{$G_2:$}} $G_2$ also includes the abort instruction in the red box. The abort condition is triggered if the $bad$ flag is set. Without loss of generality it is assumed that the adversary queries the \sign oracle only once with each message since the signature generated is deterministic and an adversary would not gain more information by multiple queries with the same message. For each individual sign query the probability for the $bad$ flag to be set is at most $\frac{\hashqueries}{2^{-\log_2(\lceil \frac{2^{2b} - 1}{L} \rceil 2^{-2b})}}$. The only parameter, which is unknown to the adversary prior to calling the \sign oracle is the commitment $R$. For an adversary to trigger the abort condition he has to guess the commitment $\groupelement{R}$ used during on of the \sign queries. $-\log_2(\lceil \frac{2^{2b} - 1}{L} \rceil 2^{-2b})$ is the min entropy of $\groupelement{R}$. $r'$ is chosen uniformly at random from $\{0,1\}^{2b}$ and then reduced modulo $L$ when multiplied with the generator $\groupelement{B}$. At first there are $2^{2b}$ possible values for $r'$. After the reduction module $L$ there are $min\{2^{2b}, L\}$ possible values left for $r'$. In the case that the values $L$ is smaller than $2^{2b}$ (this is the case in most instantiations of EdDSA) then the $r'$'s are not uniformly distributed in $\field{L}$. Since an adversary could use this information the min entropy of $\groupelement{R}$ has to be considered, which takes this into account. By the Union bound over all oracle queries $\oraclequeries$ we obtain $\Pr[bad] \leq \frac{\oraclequeries \hashqueries}{2^{-\log_2(\lceil \frac{2^{2b} - 1}{L} \rceil 2^{-2b})}}$. Since $G_1$ and $G_2$ are identical-until-bad games, we have