\documentclass[a4paper,10pt]{article}
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\usepackage{listings}
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  {morekeywords={if, then, else, end, let, and, for, in, while}}
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basicstyle=\sffamily\small, keywordstyle=\sffamily\bfseries}


\title{Pacemaker}

\begin{document}

\maketitle

\section{Base protocol}

\subsection{Principle}

\begin{itemize}
  \item{\bf Phase 1:} The server generates a unique seed and initiates
  a seed pulse which is diffused through the network. Upon receiving the seed,
  each peer signs it with its private key and hashes it. This hash is called the
  \emph{token} of the peer.
  \item{\bf Phase 2:} Each peer starts collecting the tokens of its
  neighbors and add its own hash to the collected hashes, obtaining a
  peer-token map. The hash of this map yields a new token which is sent back to
  the neighbors. This progressively builds a hash tree until the root
  server itselfs hashes the tokens of its children.
  \item{\bf Phase 3:} The server initiates a new pulse with the map it has
  collected from its childrens. Upon receiving the pulse, each peer adds to the
  pulse the map associated with the token that has been collected during 
  Phase 2. Thus, each peer receives a list of hash maps
  corresponding to a branch from the root to itself in the hash tree computed during phase 2. 
\end{itemize}

\subsection{Messages}

It is assumed that each peer $p$ has a pair of public/private key
\textsf{(K$^p_{pub}$, K$^p_{priv}$)} and has access to the 3
following cryptographic primitives:
\begin{itemize}
 \item{\sf sign(data, K$_{priv}$):} Returns a signature for \textsf{data} 
 using the private key \textsf{K$_{priv}$}.
 \item{\sf verify(S, data, K$_{pub}$):} Verifies that \textsf{S} is a
 signature for \textsf{data} that was created using the private key
 \textsf{K$_{priv}$} associated with \textsf{K$_{pub}$}.

 \item{\sf hash(data):} Returns the hash of \textsf{data}.
\end{itemize}

The key pair of the server will be denoted by \textsf{(KS$_{pub}$,
KS$_{priv}$)}. \textsf{KS$_{pub}$} is assumed to be known by every peer. 

The following messages are used in the protocol :
\begin{itemize}
  \item{\sf Seed($i$, seed$^i$, S$^i_{seed}$):} used to diffuse the seed
  pulse during phase 1, where:
  \begin{itemize}
  	\item $i$ is the round number.
  	\item \textsf{seed$^i$} is the seed of round $i$.
  	\item \textsf{S$^i_{pulse}$} is \textsf{sign(<$i$, seed$^i$>, KS$_{priv}$)}.
  \end{itemize}
  
  \item{\sf SeedReply($i$, H$_p$):} used to build the hash tree during Phase 2,
  where \textsf{H$_p$} is the token received from peer $p$, that is, the hash of
  the collected map of the tokens received by peer $p$.
  
  \item{\sf Pulse($i$, seed$^i$, H$^i$, level, branch$^i$, S$^i_{pulse}$):} used
  during Phase 3. Where:
  \begin{itemize}
    \item \textsf{H$^i$} is the hash of the map built by the server during
    phase 2.
    \item \textsf{level} is the current level in the branch built from the root
    server to the current peer.
    \item \textsf{branch$^i$} is a branch of hash maps going from the server
    to the current peer. This branch is a part of the hash tree built during
    phase 2.
    \item \textsf{S$^i_{pulse}$} is \textsf{sign(<i, seed$^i$, H$^i$>, KS$_{priv}$)}
  \end{itemize}
\end{itemize}
 
The protocol also provides the following request/reply logic to challenge the
availability of a peer:

\begin{itemize}
  \item{\sf Availability($i$, bits):} used by a peer to send its avaibility
  history up to round $i$. \textsf{bits} is a bit array where a $0$ (resp. 1)
  bit at index $j$ means that the peer was absent (resp. available) during round
  $j$.
  \item{\sf Challenge($i$):} used to ask a peer the proof of its presence
  during round $i$.
  
  \item{\sf Proof($i$, seed$^i$, branch$^i$, S$^i_{pulse}$, S$^i_p$):} to answer
  a challenge, where \textsf{S$^i_p$} is \textsf{sign(<i, seed$^i$>,
  K$^p_{priv}$)}.
\end{itemize}

A peer can also choose to send its availability history up to round $i$

\subsection{Protocol}

\subsubsection{Proof building}
\begin{lstlisting}[title=Server at round $i$]
  let phase$^i$ = Seeding;
  let seed$^i$ = random_seed();
  let S$^i_{seed}$ = sign(<$i$,seed$^i$>, KS$_{priv}$);
  let M$^i_{seed}$ = Seed($i$, seed$^i$, S$^i_{seed}$);
  let map$^i$ = $\emptyset$
  $\forall q \in$ NS$_{server}$, send($q$, M$^i_{seed}$);

  wait $T$;

  phase$^i$ = Idle;
  H$^i$ = Hash(map$^i$);
  let S$^i_{pulse}$ = sign(<$i$,seed$^i$,H$^i$>, KS$_{priv}$);
  let M$^i_{pulse}$ = Pulse($i$, seed$^i$, H$^i$, 0, 
                      $\{$ 0 $\rightarrow$ map$^i$ $\}$, S$^i_{pulse}$); 
  $\forall q \in$ NS$_{server}$, send($q$, M$^i_{pulse}$);
\end{lstlisting}

\begin{lstlisting}[title={Server or peer receiving \textsf{SeedReply($i$,
X$_q$)} from peer $q$}]
  if phase$^i$ = Seeding then
    map$^i$ = $\{ q \rightarrow $X$_q \} \oplus$map$^i$;
  end if
\end{lstlisting}

\begin{lstlisting}[title={Every $\Delta << T$ seconds, on peer $p$}]
  if phase$^i$ = Seeding then
    let H$^i$ = H(map$^i$);
    let M$^i_{seedreply}$ = SeedReply($i$, H$^i$);
    nreplies$^i$ = nreplies$^i$ + 1;
    replies$^i$[nreplies$^i$] = (H$^i$, map$^i$);
    $\forall q \in$ NS$_p$, send($q$, M$^i_{seedreply}$);    
  end if
\end{lstlisting}

\begin{lstlisting}[title={Peer $p$ receiving M$^i_{seed}$ = \textsf{Seed($i$,
seed$^i$, S$^i_{seed}$)}}]
  if
    verify_sig(S$^i_{seed}$, <$i$,seed$^i$>, KS$_{pub}$)
  then
    $\forall q \in$ NS$_p$, send($q$, M$^i_{seed}$);    

    let S$^i_p$ = sign(<$i$,seed$^i$>, K$^p_{priv}$);
    let map$^i$ = $\{p \rightarrow H(S^i_p)\}$;
    nreplies$^i$ = 0;
    phase$^i$ = Seeding;
  end if
\end{lstlisting}

\begin{lstlisting}[title={Peer $p$ receiving \textsf{Pulse($i$, seed$^i$, H$^i$,
level, tree$^i$, S$^i_{pulse}$)}}]
  if
    verify_sig(S$^i_{pulse}$, <$i$,seed$^i$,H$^i$>, KS$_{pub}$) 
  then
     phase$^i$ = Idle;
     included$^i$ = $\bot$;
     if
       $\exists$ $\{$ $p$ $\rightarrow$ H$^i$ $\}$ $\in$ tree$^i$[level] and
       $\exists$ $n$ $|$ replies[$n$] = (H$^i$, oldmap$^i$) and
       included$^i$ $<$ $n$ and
       verify_tree(H$^i$, level, tree$^i$)
     then
       let tree'$^i$ = tree$^i$$\oplus$$\{$level+1 $\rightarrow$ oldmap$^i$$\}$;
       let M$^i_{pulse}$ = Pulse($i$, seed$^i$, H$^i$, level+1, tree'$^i$, S$^i_{pulse}$);
       History[$i$] = M$^i_{pulse}$;
       included$^i$ = $n$;
       $\forall q \in$ children$^p$, send($q$, M$^i_{pulse}$);
    end if
  end if
\end{lstlisting}

\subsubsection{Challenges}
\begin{lstlisting}[title=Peer $p$ sending to $q$ its availability at round $i$]
  let bits$^i$ = new bitfield[$N_t$];
  for $x$ in [1..$N_t$]
    if History[$i-N_t+x$] = $\emptyset$ then
      bits$^i$[$x$] := 0;
    else
      bits$^i$[$x$] := 1;
    end if
  end for  
  let M$^i_{avail}$ = Availability($i$, bits);
  send($q$, M$^i_{avail}$)
\end{lstlisting}

\begin{lstlisting}[title={Peer $p$ receiving \textsf{Challenge($i$)} from peer
$q$}]
  let M$^i_{pulse}$ = History[$i$];
  send($q$, Proof M$^i_{pulse}$)
\end{lstlisting}

\begin{lstlisting}[title={Peer $q$ receiving from peer $p$ \textsf{Proof($i$, nonce, K$^i_{pub}$, trees$^i$, S$^i_p$, S$^i_{pulse}$, S$_{proof}$)}}]
  if
     ($i$, nonce, $p$) $\in$ challenges and
     verify_sig(S$^i_{pulse}$, <$i$,seed$^i$,$K^i_{pub}$,trees$^i$[0]>,KS$_{pub}$) and
     verify_sig(S$^i_p$, <$i$, seed$^i$ >, K$^p_{pub}$) and
     S$^i_p$ $\in$ trees$^i$ and
     $\forall n>0$, H(trees$^i$[n]) $\in$ trees$^i$[n-1] and
     verify_sig(S$_{proof}$, < nonce, H$^p$, H$^q$ >, K$^i_{pub}$)
  then
     good_reply($p$)
  else
     bad_reply($p$)
  end if
\end{lstlisting}

\end{document}