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\section{Conclusion}
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\label{sec:Conclusion}
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Any formal system relies on a trusted base. In this section we describe our
chain of trust.
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\subheading{Trusted Code Base of the proof.}
Our proof relies on a trusted base, i.e. a foundation of definitions that must be
correct. One should not be able to prove a false statement in that system, \eg, by
proving an inconsistency.
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In our case we rely on:
\begin{itemize}
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  \item \textbf{Calculus of Inductive Constructions}. The intuitionistic logic
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  used by Coq must be consistent in order to trust the proofs. As an axiom,
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  we assume that the functional extensionality is also consistent with that logic.
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  $$\forall x, f(x) = g(x) \implies f = g$$
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\begin{lstlisting}[language=Coq]
Lemma f_ext: forall (A B:Type),
  forall (f g: A -> B),
  (forall x, f(x) = g(x)) -> f = g.
\end{lstlisting}

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  \item \textbf{Verifiable Software Toolchain}. This framework developed at
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  Princeton allows a user to prove that a Clight code matches pure Coq
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  specification.
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  \item \textbf{CompCert}. The formally proven compiler. We trust that the Clight
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  model captures correctly the C17 standard.
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  Our proof also assumes that the TweetNaCl code will behave as expected if
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  compiled under CompCert.
  % We do not provide guarantees for other C compilers such as Clang or GCC.
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  \item \textbf{\texttt{clightgen}}. The tool making the translation from \textbf{C} to
  \textbf{Clight}. It is the first step of the compilation.
  VST does not support the direct verification of \texttt{o[i] = a[i] + b[i]}.
  This required us to rewrite the lines into:
\begin{lstlisting}[language=C]
aux1 = a[i];
aux2 = b[i];
o[i] = aux1 + aux2;
\end{lstlisting}
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  The trust of the proof relies on the trust of a correct translation from the
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  initial version of \emph{TweetNaCl} to \emph{TweetNaclVerifiableC}.
  \texttt{clightgen} comes with \texttt{-normalize} flag which
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  factors out function calls and assignments from inside subexpressions.
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  The changes required for C code to make it verifiable are now minimal.
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  \item Last but not the least, we must trust the \textbf{Coq kernel} and its
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  associated libraries; the \textbf{Ocaml compiler} on which we compiled Coq;
  the \textbf{Ocaml Runtime} and the \textbf{CPU}. Those are common to all proofs
  done with this architecture \cite{2015-Appel,coq-faq}.
\end{itemize}
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\subheading{Corrections in TweetNaCl.}
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As a result of this verification, we removed superfluous code.
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Indeed indexes 17 to 79 of the \TNaCle{i64 x[80]} intermediate variable of
\TNaCle{crypto_scalarmult} were adding unnecessary complexity to the code,
we removed them.
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Peter Wu and Jason A. Donenfeld brought to our attention that the original
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WIP    
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\TNaCle{car25519} function carried a risk of undefined behavior if \texttt{c}
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is a negative number.
\begin{lstlisting}[language=Ctweetnacl]
c=o[i]>>16;
o[i]-=c<<16; // c < 0 = UB !
\end{lstlisting}
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We replaced this statement with a logical \texttt{and}, proved correctness,
and thus solved this problem.
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\begin{lstlisting}[language=Ctweetnacl]
o[i]&=0xffff;
\end{lstlisting}
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We believe that the type change of the loop index (\TNaCle{int} instead of \TNaCle{i64})
does not impact the trust of our proof.
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\subheading{A complete proof.}
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We provide a mechanized formal proof of the correctness of the X25519
implementation in TweetNaCl.
We first formalized X25519 from RFC~7748~\cite{rfc7748} in Coq. Then we proved
that TweetNaCl's implementation of X25519 matches our formalization.
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In a second step we extended the Coq library for elliptic curves \cite{BartziaS14}
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by Bartzia and Strub to support Montgomery curves. Using this extension we
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proved that the X25519 from the RFC and therefore its implementation in TweetNaCl matches
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the mathematical definitions as given in~\cite[Sec.~2]{Ber06}.