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nistws.tex
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\begin{document}
\title[Diversity and Transparency for ECC]{Diversity and Transparency for ECC}
\author[J.-P. Flori]{Jean-Pierre Flori, Jérôme Plût, Jean-René
Reinhard, and Martin Ekerå}
\institute[ANSSI]{ANSSI and NCSA/SW}
\date{June 11, 2015}
\begin{frame}<handout:0> \titlepage
\end{frame}
\sectionframe{Standardization}
\begin{frame}\frametitle{Need for standardization?}
\begin{block}{}
\strong{In general,} the group of rational points of an elliptic curve
behaves as a ``generic group'':
the DLOG problem has \strong{exponential} complexity, provided:
\end{block}
\begin{itemize}
\item The curve cardinality includes a \imp{large prime factor} $q$.
\begin{itemize}
\item Solution: use curves with (almost) prime cardinality.
\end{itemize}
\item The DLOG problem can not be transferred into \imp{weaker} groups.
\begin{itemize}
\item Solution: avoid weak curves.
\end{itemize}
\end{itemize}
\begin{block}{}
Applying these solutions is \strong{computationally expensive}:
curves can not be generated on demand.
\end{block}
\end{frame}
\begin{frame}\frametitle{Standardized curves}
\begin{center}\begin{tableau}{lcll}
\entete Year & & Curves & Sizes \\
2000 & \includegraphics[height=1em]{nist} & NIST & 192, 224, 256, 384, 521\\
2005 & \includegraphics[height=2em]{brainpool} & Brainpool & 160, 192, 224, 256, 320, 384, 512\\
2010 & \includegraphics[height=2em]{oscca} & OSCCA & 256 \\
2011 & \includegraphics[height=2em]{anssi} & ANSSI & 256 \\
\end{tableau}\end{center}
\begin{itemize}
\item Plus a few academic propositions (Curve25519/41417, NUMS, Ed448-Goldilocks, \ldots).
\end{itemize}
\end{frame}
\begin{frame}\frametitle{Need for a second round?}
The first curves were standardized in years 2000
when:
\begin{itemize}
\item it was possible to find curves with prime cardinality
(SEA algorithm);
\item weak classes of curves were identified.
\end{itemize}
\begin{block}{}
We think that these curves are still secure\ldots
\end{block}
\ldots but new concerns emerged since then:
\begin{itemize}
\item what about the generation process?
(is there some hidden secret vulnerability?)
\item what about side-channel attacks?
\item what about scientific progess in related domains (e.g. DLOG in finite fields)?
\end{itemize}
\begin{block}{}
It is a good time to standardize new curves.
\end{block}
\end{frame}
\sectionframe{Security}
\begin{frame}\frametitle{Five classes of criteria}
\begin{enumerate}
\item The \strong{DLOG} problem should be hard.
\item Implementations should be \strong{safe} (e.g. resist \imp{side-channel attacks}).
\item The curve should exhibit no \strong{particularities}.
\item Implementations can be \strong{optimized}.
\item (The curve exhibits \strong{interesting} properties.)
\end{enumerate}
\begin{block}{Tradeoffs}
Some conditions are \strong{incompatible}:
this is a good reason to standardize \imp{different} (families of) curves.
\end{block}
\begin{block}{Base field}
We only deal with \imp{prime base fields} as we think that \imp{extension fields}
introduce more vulnerabilities without valuable properties.
\end{block}
\end{frame}
\subsection{DLOG problem difficulty}
\begin{frame}\frametitle{DLOG problem difficulty}
\begin{itemize}
\item \strong{Large prime subgroup}:
Attacks with complexity~$O(√q)$ exist where $q$ is the largest prime factor of $N$.
\begin{block}{}
It is mandatory that:
\begin{itemize}
\item \imp{$q ≈ N$} ($\prob ≈ \frac{1}{\log p}$, costly).
\item At best \imp{$q = N$} (\emph{no complete addition law!}).
\end{itemize}
\end{block}
\bigskip
\item \strong{Weak curves}:
For some curves the DLOG problem can be transferred into a weaker finite field.
\begin{block}{}
It is mandatory that:
\begin{itemize}
\item \imp{$Δ ≠ 0$} ($\prob ≈ 1$, free);
\item \imp{$N ≠ p$} ($\prob ≈ 1$, free);
\item the \imp{embedding degree} must be large ($\prob ≈ 1$, costly).
\end{itemize}
\end{block}
\end{itemize}
\end{frame}
\subsection{Safe implementation}
\begin{frame}\frametitle{Safe implementation}
\begin{block}{}
Even though the DLOG problem is \imp{hard} on the curve,
implementations might \strong{leak} information.
\end{block}
Example: scalar multiplication using naive ``double-and-add'' algorithm.
\begin{center}
\includegraphics[width=24em,height=8em]{spa.pdf}
\hskip 1.2em
\begin{tabular}{|p{1.45em}|p{1.45em}|p{1.45em}|p{1.45em}|p{1.45em}|p{1.45em}|p{1.45em}|p{1.45em}|}
\hline
D&A&D&D& D&A&D&A\\
\hline
\multicolumn{2}{|c|}{1} &
\vrule width 0pt height 2ex
0 & 0 & \multicolumn{2}{|c|}{1} & \multicolumn{2}{|c|}{1} \\
\hline
\end{tabular}
\end{center}
\end{frame}
\begin{frame}\frametitle{Classical countermeasures}
\begin{itemize}
\item Against \strong{simple} attacks:
avoid branching depending on secret elements.
\begin{itemize}
\item ``double-and-add'' always;
\item Montgomery ladder.
\end{itemize}
\item Against \strong{differential} attacks:
avoid using secrets elements repeatedly.
\begin{itemize}
\item secret \imp{masking};
\item curve \imp{masking};
\item point \imp{masking}.
\end{itemize}
\end{itemize}
\medskip
\begin{block}{}
This is not enough: information can still \strong{leak}!
\end{block}
\end{frame}
\begin{frame}\frametitle{Further countermeasures}
\begin{block}{Masking inefficiency}
Avoid base field with \imp{special prime} cardinality (\emph{no fast reduction!}).
\end{block}
\begin{block}{Exceptional cases}
Use a curve with a \imp{complete} addition law (\emph{no prime cardinality!}).
\end{block}
\begin{block}{Special points}
Ensure no points with a \imp{zero coordinate} exist (\emph{no complete addition law!}).
\end{block}
\end{frame}
\begin{frame}\frametitle{Misbehavior resistance}
\begin{block}{Subgroup attacks}
Ensure no \imp{small subgroups} exist ($\prob = 1$ if $N$ is prime, \emph{no complete addition law!}).
\end{block}
\medskip
\begin{block}{Twist attacks}
Use a \imp{twist} with prime cardinality (\strong{$\prob ≈ \frac{1}{\log p}$}, \emph{does not leverage all checks!}).
\end{block}
\end{frame}
\subsection{Genericity}
\begin{frame}\frametitle{Resist attacks to come?}
\begin{itemize}
\item What if we don't know all classes of \strong{weak} curves?
\item Avoid producing too ``\imp{special}'' curves!
\item Verify properties satisfied with~\strong{$\prob ≈ 1$} in the sense of the DLOG problem difficulty.
\item In particular, some \strong{numbers attached to the curve} should be ``\imp{large enough}''.
\end{itemize}
\begin{block}{}
The curve should look \strong{generic}.
\end{block}
\end{frame}
\begin{frame}\frametitle{Numbers attached to a curve}
\begin{block}{Discriminant (and class number) of the endomorphism ring}
The \imp{discriminant} $D_E$ is $≥ √p$ with~$\prob ≈ 1-O(1/√p)$ (\emph{no pairings, no fast endomorphism!}).
\end{block}
\begin{block}{Embedding degree}
The \imp{embedding degree} is~$≥ p^{1/4}$
with~$\prob ≥ 1-1/√p$ (\emph{no pairings!}).
\end{block}
\begin{block}{Class number smoothness}
In general, the \imp{class number} $h_E$ has at least a prime divisor~$≥ (\log p)^{O(1)}$.
\end{block}
\end{frame}
\begin{frame}\frametitle{Numbers attached to a curve (II)}
\begin{block}{Twist cardinality}
In general, the \imp{twist cardinality} $N'$ has at least
a prime divisor $≥ (\log p)^{O(1)}$.
\end{block}
\begin{block}{DLOG in the base field}
\begin{itemize}
\item The base field cardinality $p$ should be \strong{pseudo-random}
(\emph{no fast reduction!}).
\item In general, $p-1$ has a prime divisor $≥ (\log p)^{O(1)}$.
\end{itemize}
\end{block}
\end{frame}
\begin{frame}\frametitle{Summary}
\begin{center}\begin{tableau}{*{5}{l}}
\entete & NIST & Brainpool & ANSSI & OSCCA\\
$N$ prime & ✓ & ✓ & ✓ & ✓ \\
$p$ ordinary & & ✓ & ✓ & ✓ \\
Complete law & & & & \\
Twist secure & & & & \\
Generic & &✓&✓&✓ \\
\entete & NUMS & Curve25519/41417 & Ed448-Goldilocks &\\
$N$ prime & & & & \\
$p$ ordinary & & & & \\
Complete law &✓ &✓ &✓ & \\
Twist secure &✓ &✓ &✓ & \\
Generic & & & & \\
\end{tableau}\end{center}
\end{frame}
\subsection{Optimized implementation}
\begin{frame}\frametitle{Optimized implementation}
\begin{itemize}
\item Curves with \imp{$N < p$} points (half of them).
\item Fast computation of \imp{square roots} ($p \neq 3 \pmod{4}$).
\item Fast modular \imp{reduction} (special primes, \emph{inefficient masking!}).
\item \imp{Small coefficients} for the curve equation (\emph{no genericity!}).
\item Weierstrass equations with $a = -3$ (fixed fraction of them).
\item More \imp{specific forms} of curves (fixed fraction of them, some entail \emph{no prime cardinality!}).
\end{itemize}
\end{frame}
\subsection{Diversity}
\begin{frame}\frametitle{Different criteria for different uses}
\begin{itemize}
\item The aforementioned criteria are \strong{conflicting}.
\item In particular, \imp{tradeoffs} to be made between genericity/speed\ldots
\item \ldots but also between optimization/side-channel security.
\bigskip
\item Only the first class of criteria is mandatory to ensure
the \imp{DLOG problem difficulty}.
\item The other classes of criteria mostly affect speed and
ease of implementation.
\end{itemize}
\bigskip
\begin{block}{}
Use (and standardize) \strong{different} (families of) curves!
\end{block}
\end{frame}
\begin{frame}\frametitle{Real zoo}
\begin{center}
\begin{tabular}{cc}
\includegraphics[width=.4\hsize]{weierstrass} & \includegraphics[width=.4\hsize]{edwards} \\
Weierstrass & Edwards \\
\end{tabular}
\end{center}
\end{frame}
\begin{frame}\frametitle{Real zoo (II)}
\begin{center}
\begin{tabular}{cc}
\includegraphics[width=.4\hsize]{jacobi} & \includegraphics[width=.4\hsize]{hessian} \\
Jacobi & Hess \\
\end{tabular}
\end{center}
\end{frame}
\begin{frame}\frametitle{Finite field zoo}
\begin{center}
\begin{tabular}{cc}
\includegraphics[width=.45\hsize]{frog} & \includegraphics[width=.45\hsize]{cockroach} \\
Frog & Cockroach \\
\end{tabular}
\end{center}
\end{frame}
\begin{frame}\frametitle{Finite field zoo (II)}
\begin{center}
\begin{tabular}{cc}
\includegraphics[width=.45\hsize]{walrus} & \includegraphics[width=.45\hsize]{bunny} \\
Walrus & Bunny \\
\end{tabular}
\end{center}
\end{frame}
\sectionframe{Transparency}
\subsection{Certificates for elliptic curves}
\begin{frame}\frametitle{Architecture}
\begin{itemize}
\item Provide curves fulfilling a selection of \strong{criteria}\ldots
\item \ldots together with a \strong{certificate} for faster
verification of:
\begin{itemize}
\item the number of points,
\item the discriminant and class number properties,
\item the embedding degree.
\end{itemize}
\bigskip
\item A \strong{deterministic} algorithm to sample curves\ldots
\item \ldots and producing a \strong{certificate}:
\begin{itemize}
\item Completely \imp{reproducible} generation process.
\item Either pseudo-random (for genericity) or
by enumeration of increasing values (for efficiency).
\item Certify every step,
including \imp{rejected} curves.
\end{itemize}
\end{itemize}
\end{frame}
\begin{frame}\frametitle{Cardinality of curves}
\begin{block}{Prime order}
\begin{itemize}
\item \strong{Certificate}: $(G, q, Π)$
where $G ≠ 0$ is s.t. $q · G = 0$ with $q ≥ p-2√p+1$,
and $Π$~a primality proof for~$q$.
\item \imp{Size} and \imp{verification} in~$O(\log² p)$, generally only generated once.
\end{itemize}
\end{block}
\medskip
\begin{block}{Composite order}
\begin{itemize}
\item \strong{Certificate}: $(P, n, c)$,
where $P ≠ 0$ is s.t. $n · P = 0$ with $n < 2 (√p-1)²$,
and $c$~a composition witness for~$n$.
\item \imp{Size} in $O(\log p)$, \imp{generation} and \imp{verification} in~$O(\log² p)$.
\item More efficient \imp{verification} using early-abort SEA information about
small torsion points.
\end{itemize}
\end{block}
\end{frame}
\subsection{Generation process}
\begin{frame}\frametitle{Example}
\begin{itemize}
\item \imp{Sampling function} from the \strong{seed}~$s$:
\begin{itemize}
\item $p$ = smallest prime $≥ s$;
\item $g$ = smallest generator of $\F_p^{×}$;
\item equations of the form $y^2 = x^3 - 3x + b$, $b = g, g^2, …$.
\end{itemize}
\item \imp{Conditions}:
\begin{itemize}
\item $N$ and $N'$ prime;
\item $Δ ≠ 0$, $N, N' ≠ p, p + 1$;
\item embedding degrees of~$E$, $E'$ at least~$p^{1/4}$;
\item class number $≥ p^{1/4}$.
\end{itemize}
\end{itemize}
\end{frame}
\begin{frame}\frametitle{Certificate}
From the seed $s = 2015$: $p = 2017$, $g = 5$,
\begin{block}{\small\tt Curve}\small\tt
(2017, -3, 625)\\
order = 2063, point = (0, 25)\\
twist\_order = 1973\\
disc\_factors = \{6043\}\\
class\_number = 9, form = (17,3,89)\\
embedding\_degree = 1031, factors = \{2, 1031\}\\
twist\_embedding\_degree = 493, factors = \{2, 17, 29\}
\end{block}
\vskip -1.7em
\begin{block}{\small\tt Rejected curves}\small\tt
((2017, -3, 5), composite, 2065, witness, 1679,
point, (1,258))\\
((2017, -3, 25), torsion\_point, 3, point, (448, 288))\\
((2017, -3, 125), torsion\_point, 2, point, (982, 0))
\end{block}
\end{frame}
\begin{frame}\frametitle{Non-manipulability}
\begin{itemize}
\item Such a process produces \strong{deterministically}
a curve from:
\begin{itemize}
\item a set of \imp{conditions} (including \strong{numerical bounds}),
\item a \imp{sampling function} (including potential \strong{seed}).
\end{itemize}
\begin{block}{}
No \imp{rigidity} but still \strong{transparency}.
\end{block}
\item Only a few \imp{conditions} will actually affect the process:
\begin{itemize}
\item twist security,
\item smoothness bounds.
\end{itemize}
\item When a \strong{seed} is needed, suspicion can be avoided:
\begin{itemize}
\item using a \imp{share-commitment} scheme;
\item using \imp{unpredictable} and \imp{unmanipulable} values
(sports results, stock values, lottery results, sunspot observations, \ldots).
\end{itemize}
\end{itemize}
\end{frame}
\begin{frame}\frametitle{Seed generation}
\begin{center}
%\includegraphics[width=0.7\hsize]{keyboard_cat}
\animategraphics[loop,autoplay,width=0.7\hsize]{12}{animated_cat/keyboard_cat-}{0}{93}
\end{center}
\end{frame}
\sectionframe{Conclusion}
\begin{frame}\frametitle{Diversity and Transparency for ECC}
\begin{block}{\strong{Diversity}}
International standards should:
\begin{itemize}
\item not restrict to a single curve or family of related elliptic curves;
\item include a ``generic'' elliptic curve.
\end{itemize}
\end{block}
\bigskip
\begin{block}{\strong{Transparency}}
All details about the generation process should be:
\begin{itemize}
\item public and ``transparent'';
\item annonced before the actual generation.
\end{itemize}
\end{block}
\end{frame}
\begin{frame}\frametitle{Questions?}
\begin{center}
\includegraphics[width=.45\hsize]{rockwell_speech}
\end{center}
\end{frame}
\end{document}