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-
-
-
-\begin{proposition}
-If there are no refresh operations, then any adversary who links
-coins can recognize blinding factors.
-\end{proposition}
-
-\begin{proof}
-In effect, coin withdrawal transcripts consist of numbers $b m^d \mod n$
-
-The blinding factor is created with a full domain hash
-\end{proof}
-
-
-We say a blind signature
-linkable if some probabilistic polynomial
-time (PPT) adversary has a non-negligible advantage indentifying
-the
-
-
-, given some withdrawal and refresh
-transcripts
-
-
-
-
-
-We say a coin $C_0$ is {\em linkable} to the withdrawal or refresh
-operation in which it was created if some probabilistic polynomial
-time (PPT) adversary has a non-negligible advantage in guessing
-which of $\{ C_0, C_1 \}$ were created in that operation,
- where $C_1$ is an unrelated third coin.
-
-% TODO: Compare this definition with some from the literature
-% TODO: Should this definition be broadened?
-
-.. reference literate about withdrawal ..
-
-\begin{proposition}
-In the random oracle model,
-if a coin created by refresh is linkable to the refresh operation
-that created it, then some PPT adversary has a non-negligible
-advantage in determining the shared secret of an eliptic curve
-Diffie-Hellman key exchange on curve25519.
-\end{proposition}
-
-% Intuitively this follows from \cite{Rudich88}[Theorem 4.1], but
-% we provide slightly more formality.
-
-\begin{proof}
-Assume a PPT adversary $A$ has a non-negligible advantage in solving
-the linking problem.
-
-We have two curve points $C = c G$ and $T = t G$ for which
-we wish to compute the shared secret $c t G$.
-
-We make $C$ into a coin by singing it with a denomination key
-invented for this purpose. We let $T^{(1)}$ denote $T$ and
-invent $\kappa-1$ linking keys $T^{(2)},\ldots,T^{(\kappa)}$.
-
-We shall extract the shared secret by constructing an algorithm
-that runs the refresh protocol and then runs $A$ using the natural
-simulation of a random oracle, namely answering new queries with
-random bits, yet recording the answers in a database so as to
-provide idendical answers to identical queries.
-
-We may take $\gamma=1$ by restarting the exchange with a clean
-database. As a result, the exchange never checks the commitment
-covering $T^{(1)}$, but this alone does not suffice to discount
-the any information contained in the commitment.
-
-Instead, we observe that our commitments consist of random oracle
-queries distinct from anything else in the protocol, so they contain
-no information of use to $A$, and can safely be omitted.
-
-We do not know $c t G$ so our simulation cannot run the KDF to
-derive the new coin that $A$ can link.
-
-
-... random oracle ..
-\end{proof}
-
-In principle, one might worry if coins created in the same withdrawal
-or refresh opeartion might be linkable to one another without being
-linkable to the operation, but addressing this concern would take us
-somewhat far afield and require similar methods.
-
-
-