[taler-donau] branch master updated: [doc] Clarification of protocol definitions and examples, separate definitions file

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     new 49ef33e  [doc] Clarification of protocol definitions and examples, 
separate definitions file
49ef33e is described below

commit 49ef33e206538eb4ca6d59e2cb1d13d95357c049
Author: Pius Loosli <loosp2@bfh.ch>
AuthorDate: Tue Oct 31 00:45:32 2023 +0100

    [doc] Clarification of protocol definitions and examples, separate 
definitions file
---
 doc/flows/definitions.tex |  72 ++++++++++++++++++++++
 doc/flows/main.tex        | 153 ++++++++++++++++++++--------------------------
 2 files changed, 139 insertions(+), 86 deletions(-)

diff --git a/doc/flows/definitions.tex b/doc/flows/definitions.tex
new file mode 100644
index 0000000..61dfd46
--- /dev/null
+++ b/doc/flows/definitions.tex
@@ -0,0 +1,72 @@
+\section{Notation \& Definitions }
+\subsection{Notation}
+\begin{itemize}
+ \item $\langle a, b, ... \rangle$ : Pair/tuple
+\end{itemize}
+
+\subsection{Definitions}
+\begin{itemize}
+  \item \textbf{Cryptographic Hash Function} $H(m) = h$ where $m$ is a message 
and $h$ the resulting hash.
+  
+  \item \textbf{Blinding function} $blind(m, b)$ where $m$ is the message to 
blind and $b$ the blinding factor to apply. The blinding can be done with 
either the RSA blind signature scheme or the Blinded Clause-Schnorr signature 
scheme.
+  
+  \item \textbf{Keygen} TODO
+
+  \item \textbf{Signing}
+    \begin{itemize}
+      \item \textbf{Classic/lightweight signing (e.g. EdDSA):} 
+      \begin{align}
+        \fbox{$\sigma := sign(m,k)$}
+      \end{align}
+      where $m$ is a message and $k$ is the private key used to sign the 
message.\\
+      Applications:
+      \begin{itemize}
+        \item A charity signs a collection of \emph{BUDI-key-pair} before 
transfering them to the Donau to issue \emph{Donation Receipts}
+        \item The Donau computes the \emph{donation statement signature} for a 
donor for a specific year
+      \end{itemize}
+      
+      \item \textbf{Blind signing(e.g. RSA/CS):}
+      \begin{align}
+        \fbox{$\overline{\beta} := blind\_sign(\overline{m},k)$}
+      \end{align}
+      where $\overline{m}$ is a blinded message and $k$ is the private key 
used to blind sign the message.\\
+      Application:
+      \begin{itemize}
+        \item The Donau blind signs \emph{Blinded Unique Donor Identifier}s 
received from the charity with the private key matching the public key in the 
received \emph{BUDI-key-pair}
+      \end{itemize}
+    \end{itemize}
+
+  \item \textbf{Donation Unit} $ = (K_x^{pub}, K_x^{priv})$ where $x$ is the 
associated value (e.g. 2EUR):
+    Smallest structure representing a donation confirmation unit.
+    Consists of a Public key $K_x^{pub}$ and Private key $K_x^{priv}$. 
Equivalent in Taler is "denomination".
+
+  \item \textbf{Donor Identifier} $i = H(\texttt{taxid}, \texttt{salt})$
+    
+  \item \textbf{Unique Donor Identifier} $u := \langle i, \texttt{nonce} 
\rangle$
+  where \texttt{nonce} ...%TODO
+
+  \item \textbf{Blinded Unique Donor Identifier} 
+  \begin{align}
+    \fbox{$\overline{u} := blind(u)$}
+  \end{align}
+  blinded to protect the privacy of the donor
+  
+  \begin{itemize}
+   \item \textbf{Blinded Unique Donor Identifier-key-pair} or 
\textbf{BUDI-key-pair}
+  \begin{align}
+      \fbox{$\mu := \langle \overline{u}, H(K_x^{pub}) \rangle$}
+  \end{align}
+where $H(K_x^{pub})$ indicates which donation unit key should be used by the 
Donau to sign the resulting donation receipt. Thus, this hash carries the 
information about the exact value the final donation receipt should carry.
+  \end{itemize}
+
+
+  \item \textbf{Donation Receipt} $r = \langle u, \beta, H(K_x^{pub}) \rangle$ 
where $\beta$ is the unblinded signature: Sent to the Donau to get the donation 
Statement.
+
+  \item \textbf{Donation Statement signature}
+  Signature to attest the amount donated in a particular year by a specific 
donor.
+  \begin{align}
+\fbox{$\sigma_s = sign(\langle i, \texttt{amount}_{total}, \texttt{year} 
\rangle, D^{priv})$}
+\end{align}
+where $D^{priv}$ is the private key from the Donau.
+
+\end{itemize}
diff --git a/doc/flows/main.tex b/doc/flows/main.tex
index fa06efb..224d330 100644
--- a/doc/flows/main.tex
+++ b/doc/flows/main.tex
@@ -20,101 +20,77 @@
 \maketitle
 
 \newpage
-\section{Notation \& Definitions }
-\subsection{Notation}
-\begin{itemize}
- \item $\langle a, b, ... \rangle$ : Pair/tuple
-\end{itemize}
-
-\subsection{Definitions}
-\begin{itemize}
-  \item \textbf{Cryptographic Hash Function} $H(m) = h$ where $m$ is a message 
and $h$ the resulting hash.
-  
-  \item \textbf{Blinding function} $blind(m, b)$ where $m$ is the message to 
blind and $b$ the blinding factor to apply. The blinding can be done with 
either the RSA blind signature scheme or the Blinded Clause-Schnorr signature 
scheme.
-  
-  \item \textbf{Keygen} TODO
-
-  \item \textbf{Signing}
-    \begin{itemize}
-      \item \textbf{Classic/lightweight signing (e.g. EdDSA):} 
-      \begin{align}
-        \fbox{$\sigma := sign(m,k)$}
-      \end{align}
-      where $m$ is a message and $k$ is the private key used to sign the 
message.\\
-      Applications:
-      \begin{itemize}
-        \item A charity signs a collection of \emph{BUDI-key-pair} before 
transfering them to the Donau to issue \emph{Donation Receipts}
-        \item The Donau computes the \emph{donation statement signature} for a 
donor for a specific year
-      \end{itemize}
-      
-      \item \textbf{Blind signing(e.g. RSA/CS):}
-      \begin{align}
-        \fbox{$\overline{\beta} := blind\_sign(\overline{m},k)$}
-      \end{align}
-      where $\overline{m}$ is a blinded message and $k$ is the private key 
used to blind sign the message.\\
-      Application:
-      \begin{itemize}
-        \item The Donau blind signs \emph{Blinded Unique Donor Identifier}s 
received from the charity with the private key matching the public key in the 
received \emph{BUDI-key-pair}
-      \end{itemize}
-    \end{itemize}
-
-  \item \textbf{Donation Unit} $ = (K_x^{pub}, K_x^{priv})$ where $x$ is the 
associated value (e.g. 2EUR):
-    Smallest structure representing a donation confirmation unit.
-    Consists of a Public key $K_x^{pub}$ and Private key $K_x^{priv}$. 
Equivalent in Taler is "denomination".
-
-  \item \textbf{Donor Identifier} $i = H(\texttt{taxid}, \texttt{salt})$
-    
-  \item \textbf{Unique Donor Identifier} $u := \langle i, \texttt{nonce} 
\rangle$
-  where \texttt{nonce} ...%TODO
-
-  \item \textbf{Blinded Unique Donor Identifier} $\overline{u} := blind(u)$, 
blinded to protect the privacy of the donor
-  
-  \begin{itemize}
-   \item \textbf{BUDI-key-pair} $\mu := \langle \overline{u}, H(K_x^{pub}) 
\rangle$ where $H(K_x^{pub})$ indicates which donation unit key should be used 
by the Donau to sign the resulting donation receipt. Thus, this hash carries 
the information about the exact value the final donation receipt should carry.
-  \end{itemize}
-
-
-  \item \textbf{Donation Receipt} $r = \langle u, \beta, H(K_x^{pub}) \rangle$ 
where $\beta$ is the unblinded signature: Sent to the Donau to get the donation 
Statement.
 
-  \item \textbf{Donation Statement} $\sigma_s = sign(\langle i, 
\texttt{amount}_{total}, \texttt{year} \rangle, D^{priv})$ where $D^{priv}$ is 
the private key from the Donau: Signature to attest the amount donated in a 
particular year by a specific donor.
+\input{definitions.tex}
 
-\end{itemize}
 
 \section{Overview}
 \includegraphics[width=\textwidth]{overview}
 
-\subsection{Step 0: Key generation and Initial setup}
+\newpage
+\section{Protocol Detail}
+
+\subsection{Key generation and initial setup}
+\subsubsection{Initial Donau setup}
 \begin{enumerate}
   \item The Donau generates a public key $D^{pub}$ and private key $D^{priv}$.
 
   \item The Donau generates the \emph{donation units} consisting of 
$K_x^{pub}$ and $K_x^{priv}$ where $x$ is the associated value.
+\end{enumerate}
 
-  \item The charity generates the key pair $(C^{pub}, C^{priv})$ and downloads 
the \emph{donation units} from the donau.
-
-  \item The Donau administrator registers the public key $C_{pub}$ and sets 
the yearly donation limit for the charities.
+\subsubsection{Charity setup (Charity side and Donau side)}
+\begin{enumerate}
+  \item The \textbf{charity} generates the key pair $(C^{pub}, C^{priv})$ and 
downloads the \emph{donation units} from the donau.
+  
+  \item The \textbf{charity} transmits $C^{pub}$ to the party responsible for 
Donau administration using a \textbf{secure channel}.
+  
+  \item The party in charge of \textbf{Donau administration} ensures that the 
applying party is authentic and if it is publicly recognized as charity 
organisation. Furthermore, it ensures that all eventual other checks required 
by law are done. If everything is clear, it registers the public key $C^{pub}$ 
and sets the yearly donation limit for the charitiy.
 \end{enumerate}
 
-\subsection{Step 1: Attest donation}
-\subsubsection{Donor donates to charity}
+
+\subsection{Continuously during tax period: get donation receipts}
+\subsubsection{Overview}
+
+
+\subsubsection{Donor donates to charity and transmits unique donor ids (future 
donation receipts)}
 \begin{enumerate}
-  \item The donor downloads the $DU$'s public keys $K_x^{pub}$ for the 
corresponding year from the Donau.
+  \item The donor downloads the \emph{donation unit} public keys $K_x^{pub}$ 
for the corresponding year from the Donau. (if not already done)
+  
+  \item The donor splits the donation amount into a sum of \emph{donation 
units} offered by the Donau. \\
+  \emph{Example: With donation units \{1,2,4\} available, and a donation with 
a total value of 7, the donation amount is split into the sum 4+2+1.}
 
-  \item The donor generates a unique donor identifier $UDI$ for every $DU$. 
For every $DU$ the donor generates:
+  \item The donor generates as many \emph{unique donor identifiers} as there 
are terms in the calculated sum.
+  \emph{Example: In our example, there will be 3 unique donor identifiers: one 
per donation unit, so one for the value 4, one for the value 2, one for the 
value 1.\\
+%   TODO make footnote out of this
+  (if one donation unit is present more than once in the sum, then there is 
more than one unique donor identifier required for said donation unit. This 
depnds upon the offered donation units.)}
     \begin{align}
-      UDI_1 &= \langle H(\texttt{TAXID}, \texttt{SALT}), \texttt{NONCE}_1 
\rangle \\
-      ... \\
-      UDI_i &= \langle H(\texttt{TAXID}, \texttt{SALT}), \texttt{NONCE}_i 
\rangle
+      i &= H(\texttt{taxid, salt})\\ 
+      u_1 &= \langle i, \texttt{nonce}_1 \rangle \\
+      u_2 &= \langle i, \texttt{nonce}_2 \rangle \\
+      u_3 &= \langle i, \texttt{nonce}_3 \rangle \\
     \end{align}
 
-  \item The donor blinds the $UDI$'s using a \textbf{different} blinding 
factor $b$ for every $UDI$.
-
+  \item The donor blinds the \emph{unique donor identifiers} using a 
\textbf{different} blinding factor $b$ for every \emph{unique donor 
identifier}.\\
+  \emph{Example:}
   \begin{align}
-    BKP_1 &= \langle Blind(BUDI_1, K_x^{pub}, b_1), H(K_x^{pub}) \rangle \\
-    ... \\
-    BKP_i &= \langle Blind(BUDI_i, K_x^{pub}, b_i), H(K_x^{pub}) \rangle
+    \overline u_1 &= blind (u_1, b_1) \\
+    \overline u_2 &= blind (u_2, b_2) \\
+    \overline u_3 &= blind (u_3, b_3) \\
+  \end{align}
+  
+  \item So far, the \emph{unique donor identifiers} do not carry information 
about their value. The \textbf{intended effective value is now indicated} by 
grouping each \emph{unique donor identifier} with the according (hash of the) 
\emph{donation unit} public key $P^{pub}_x$. \\
+  We call these pairs \emph{blinded unique donor identifier-key-pair}, or 
\emph{budi-key-pair} for short\\
+  \\
+  It is only the \textbf{intended effective} value because the value will only 
be attributed later on with the signature of the Donau.
+  
+  \emph{Example: Note: The public key is not in relation with the sequential 
index of the budi-key-pair, it only relates to the value of the pair!}
+  \begin{align}
+       \overline \mu_1 &= \langle \overline u_1, 
\color{red}{P^{pub}_1}\color{black}{} \rangle \\
+       \overline \mu_2 &= \langle \overline u_2, 
\color{red}{P^{pub}_2}\color{black}{} \rangle \\
+       \overline \mu_3 &= \langle \overline u_3, 
\color{red}{P^{pub}_4}\color{black}{} \rangle \\
   \end{align}
 
-\item The donor sends the $BKP$'s as well as the corresponding payment to the 
charity.
+  \item The donor sends the $BKP$'s as well as the corresponding payment to 
the charity.
 \end{enumerate}
 
 \subsubsection{Charity sends signed $BKP$'s to Donau}
@@ -165,36 +141,41 @@
   \end{align}
 \end{enumerate}
 
-\subsection{Step 2: Donor sends the $DR$'s to the Donau to get the 
\textbf{Donation Statement} $DS$.}
+\subsection{After effective tax period: get tax statement for period from 
Donau}
+
+\subsubsection{Donor sends the \emph{Donation receipts} to the Donau to get 
the \emph{Donation Statement}.}
 \begin{enumerate}
-  \item The donor sends the collection of all $DR$'s, to the Donau. The $DR$'s 
are sent manually once a year.
-  \item For each $DR$ the Donau:
+  \item The donor sends the collection of all \emph{donation receipts r} to 
the Donau. This happens manually once per period.\\\
+  It is not done continuously to obtain \emph{unlinkability} between the 
\textbf{issuance} of the donation receipts (which happens upon donation) and 
their \textbf{submission} for the \emph{donation statement}.
+  \item For each \emph{donation receipt} the Donau:
   \begin{itemize}
     \item checks that $K_x^{pub}$ is known.
 
     \item verifies that the signature $\beta$ is correct using the 
corresponding public key $K_x^{pub}$.
 
-    \item verifies that the hash of the $\texttt{TAXID}$ and the 
$\texttt{SALT}$ is the same as in other $DR$'s (With multiple wallets each 
wallet must simply obtain a separate $DS$!).
+    \item verifies that the \emph{donor identifier} is the same as in other 
\emph{donation receipts} 
+%     TODO: put this as footnote
+    (With multiple wallets each wallet must simply obtain a separate 
\emph{donation statement}!).
 
-    \item verifies that the $\texttt{NONCE}$ is unique and was not used before 
by the donor for the corresponding year.
+    \item verifies that the $\texttt{nonce}$ is unique and was not used before 
by the donor for the corresponding year.
   \end{itemize}
 
-  \item The Donau signs over the total amount,
-      year and $H(\texttt{TAXID}, \texttt{SALT})$ and sends the signature and 
the total amount so far back to the donor. This results in a final signature 
called the \textbf{Donation Statement} $DS$.
+  \item The Donau signs over the total \texttt{amount},
+      \texttt{year} and \emph{donor identifier} and sends the signature and 
the total amount so far back to the donor. This results in a final signature 
called the \textbf{\emph{Donation Statement signature}}.
 
     \begin{align}
-      DS = \Sigma(\langle \texttt{AMOUNT}_{Total}, \texttt{YEAR}, 
H(\texttt{TAXID}, \texttt{SALT}) \rangle, D^{priv})
+      \sigma_s = sign(\langle i, \texttt{amount}_{Total}, \texttt{year}) 
\rangle, D^{priv})
     \end{align}
 \end{enumerate}
 
-\subsection{Step 3: Donor sends the QR Code to a validator (tax office)}
+\subsubsection{Donor sends the QR Code to a validator (tax office)}
 \begin{enumerate}
-  \item The donor generates a QR code:
+  \item The donor generates a QR code which contains the following:
     \begin{align}
-      \texttt{QR} = \langle \texttt{TAXID}, \texttt{SALT}, \texttt{DS}, 
\texttt{YEAR}, \texttt{AMOUNT} \rangle
+      \texttt{QR} = \langle \texttt{taxid}, \texttt{salt}, \texttt{year}, 
\texttt{amount}, \text{$\sigma_s$} \rangle
     \end{align}
 
-  \item The validator scans the QR code and verifies the signature $DS$.
+  \item The validator scans the QR code and verifies the signature $\sigma_s$.
 \end{enumerate}
 
 \end{document}

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