diff --git a/network-privacy-assessment/network-privacy.pdf b/network-privacy-assessment/network-privacy.pdf index 51549a7..f18d8f2 100644 Binary files a/network-privacy-assessment/network-privacy.pdf and b/network-privacy-assessment/network-privacy.pdf differ diff --git a/network-privacy-assessment/network-privacy.tex b/network-privacy-assessment/network-privacy.tex index afad456..bb0705a 100644 --- a/network-privacy-assessment/network-privacy.tex +++ b/network-privacy-assessment/network-privacy.tex @@ -467,7 +467,7 @@ general-purpose Loopix-based mixnets~\cite{Piotrowska:2017:LAS}. Further, we review private information retrieval (PIR) as a method which can be used in conjunction with the above systems. -\textbf{Dandelion.} +\subsection{Dandelion.} Dandelion~\cite{BojjaVenkatakrishnan:2017:DRB} and Dandelion++~\cite{Fanti:2018:DLC} is a lightweight gossip protocol aimed at adding additional network privacy for @@ -494,7 +494,9 @@ a super node to link a specific transaction to the node which originally published that transaction. -\textbf{Tor.} +\subsection{Tor.} +\label{tor-intro} + Tor~\cite{tor-specification} is an anonymity network that today has over 2.5 million users and a network size of over 6,500 nodes. Tor supports applications that require low latency, @@ -513,8 +515,16 @@ participants, Tor avoids epistemic or routing attacks unlike completely decentralized networks which cannot guarantee a user or relay's view of the network is authentic or that all users fall within a global anonymity set. +In Zcash, Tor integration can be implemented at several points: between senders +and the Zcash network, between Zcash peers, or between receivers and the Zcash +network. Because our focus is on preventing \emph{end-to-end} correlation +attacks, we focus our attention on integrating Tor to submit and receive +transactions only. While integrating Tor between Zcash peers could have some +utility, as demonstrated by our analysis, this protection can easily be +circumvented by malicious peers handling entry/exit traffic to the network. -\textbf{Loopix-based Mixnets.} + +\subsection{Loopix-based Mixnets.} While a range of mix network designs have been introduced in the literature, in this assessment we consider only those which instantiate the Loopix~\cite{Piotrowska:2017:LAS} design, which provides improved latency @@ -531,7 +541,7 @@ the network. However, alternative network distribution mechanisms can be used, but similarly may be subject to epistemic and path-routing attacks similar to other distributed networks. -\textbf{Private Information Retrieval (PIR).} +\subsection{Private Information Retrieval (PIR).} Even though the above network anonymity systems disassociate a sender or receiver's identity from the transaction, nodes can still observe which blocks are being fetched and perform fingerprinting attacks using this information. @@ -758,7 +768,8 @@ following steps to provide stronger security and privacy guarantees. In looking at Table~\ref{network-zcash-assessment}, the strongest protections are offered by the combination of PIR along with a network anonymity tool such -as Tor or a mixnet. Because use of an anonymity network protects against +as Tor or a mixnet between senders and receivers and the edges of the Zcash +network. Because use of an anonymity network protects against directly leaking the network identity of a user, and PIR protects against disclosing the contents of a user's query, the combination of these methods leaves little room for an attacker to gain advantage. @@ -774,12 +785,19 @@ itself. Again, this observation requires that the receiver can hide the contents of their query. -As such, our immediate-term plan to proceed with Tor integration, and will -assess PIR as a second step. +As such, our immediate-term plan to proceed with Tor integration to facilitate +sending and receiving transactions over Tor, and will +assess PIR to improve receiver query privacy as a second step. We will continue to observe the development of production-ready mixnets as a possible option in the future, with an eye towards large-scale adoption in order to ensure sufficiently large anonymity sets for users. +As discussed in Section~\ref{tor-intro}, sending Peer-to-Peer traffic between +Zcash nodes may frustrate some attackers' ability to perform end-to-end +correlation attacks, but has no effect on other adversaries such +as malicious light wallets. As such, we deem integrating Tor in the P2P gossip +protocol for Zcash as low priority. + \section{Conclusion} \label{conclusion}