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+<pre>
+  BIP: 156
+  Layer: Peer Services
+  Title: Dandelion - Privacy Enhancing Routing
+  Author: Brad Denby <bdenby@cmu.edu>
+          Andrew Miller <soc1024@illinois.edu>
+          Giulia Fanti <gfanti@andrew.cmu.edu>
+          Surya Bakshi <sbakshi3@illinois.edu>
+          Shaileshh Bojja Venkatakrishnan <shaileshh.bv@gmail.com>
+          Pramod Viswanath <pramodv@illinois.edu>
+  Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-0156
+  Status: Draft
+  Type: Standards Track
+  Created: 2017-06-09
+  License: CC0-1.0
+</pre>
+
+==Abstract==
+
+Bitcoin's transaction spreading protocol is vulnerable to deanonymization
+attacks. Dandelion is a transaction routing mechanism that provides formal
+anonymity guarantees against these attacks. When a node generates a transaction
+without Dandelion, it transmits that transaction to its peers with independent,
+exponential delays. This approach, known as diffusion in academia, allows
+network adversaries to link transactions to IP addresses.
+
+Dandelion mitigates this class of attacks by sending transactions over a
+randomly selected path before diffusion. Transactions travel along this path
+during the "stem phase" and are then diffused during the "fluff phase" (hence
+Dandelion). We have shown that this routing protocol provides near-optimal
+anonymity guarantees among schemes that do not introduce additional encryption
+mechanisms.
+
+==Motivation==
+
+Transaction diffusion in Bitcoin is vulnerable to deanonymization attacks.
+Because transactions are sent to peers with independent, exponential delays,
+messages spread through the network in a statistically symmetric manner. This
+pattern allows colluding spy nodes to infer the transaction source. Breaking
+this symmetry prevents the attack. However, we have shown that an adversary with
+knowledge of the network topology can launch a much more effective "fingerprint"
+attack if the symmetry breaking is not done properly.
+
+Consider a botnet-style adversary with access to the P2P graph. Botnets of size
+comparable to the Bitcoin P2P network are common and cheap, and these
+adversaries can learn the network structure with probe messages. We have shown
+that such an adversary can achieve total deanonymization of the entire network
+after observing less than ten transactions per node.
+
+Dandelion is a practical, lightweight privacy solution that provides the Bitcoin
+network formal anonymity guarantees. While other privacy solutions aim to
+protect individual users, Dandelion protects anonymity by limiting the
+capability of adversaries to deanonymize the entire network.
+
+==How Dandelion Works==
+
+Dandelion enhances user privacy by sending transactions through an anonymity
+phase before diffusing them throughout the network. At a high level, Dandelion
+enhances privacy by (i) breaking the symmetry of diffusion and (ii) mixing
+transactions by forwarding messages from different sources along the same path.
+
+Dandelion routing can be conceptualized in three phases. First, a privacy graph
+is constructed. In practice, this privacy graph is constructed in a fully
+decentralized manner and is a subgraph of the existing Bitcoin P2P network.
+Next, transactions are forwarded along this privacy graph during the "stem
+phase." Finally, messages are broadcast to the network during the "fluff phase"
+using the typical method of diffusion.
+
+[[File:bip-dandelion/1-dandelion.png|framed|center|alt=An illustration of Dandelion routing|Figure 1]]
+Figure 1
+
+In order to select the privacy graph in a decentralized manner, each node
+selects a subset of its outbound peers to be Dandelion destinations. Dandelion
+transactions (transactions in their stem phase) that arrive at this node via
+inbound connections are forwarded to these Dandelion destinations.
+
+In an ideal setting, we have found that a Hamiltonian circuit provides
+near-optimal privacy guarantees. However, constructing a Hamiltonian circuit
+through the Bitcoin P2P network in a decentralized, trustless manner is not
+feasible. Thus, we recommend that each node select two Dandelion destinations
+uniformly at random without replacement from its list of outbound peers. Our
+tests have shown that this method provides comparable privacy with increased
+robustness.
+
+During stem phase routing, there is a question of how to route messages in order
+to protect privacy. For example, if two Dandelion transactions arrive at a node
+from different inbound peers, to which Dandelion destination(s) should these
+transactions be sent? We have found that some choices are much better than
+others.
+
+Consider the case in which each Dandelion transaction is forwarded to a
+Dandelion destination selected uniformly at random. This approach results in a
+fingerprint attack allowing network-level botnet adversaries to achieve total
+deanonymization of the P2P network after observing less than ten transactions
+per node.
+
+[[File:bip-dandelion/2-attack.png|framed|center|alt=An illustration of a fingerprint attack|Figure 2]]
+Figure 2
+
+During a fingerprint attack, a botnet-style adversary with knowledge of the
+graph structure first simulates transaction propagation. This offline step lets
+the adversary generate fingerprints for each network node. During the online
+attack, the adversary collects transactions at its spy nodes and matches these
+observations to the simulated fingerprints. Our simulations have shown that this
+attack results in devastating, network-wide deanonymization.
+
+[[File:bip-dandelion/3-attack-plot.png|framed|center|alt=A plot illustrating total deanonymization|Figure 3]]
+Figure 3
+
+To avoid this issue, we suggest "per-inbound-edge" routing. Each inbound peer is
+assigned a particular Dandelion destination. Each Dandelion transaction that
+arrives via this peer is forwarded to the same Dandelion destination. 
+Per-inbound-edge routing breaks the described attack by blocking an adversary's
+ability to construct useful fingerprints. Fingerprints arise when routing
+decisions are made independently per transaction at each node. In this case, two
+transactions from the same node generally take different paths through the
+network. Crucially, this results in multiple, unique data points that are
+aggregated to match with a fingerprint.
+
+Dandelion ensures that two transactions from the same node take the same network
+path, limiting adversaries to the far-left of the graph in Figure 3. In other
+words, adversary knowledge is limited to the case of one observed message rather
+than a rich profile of multiple transaction paths. Dandelion also breaks the
+symmetry of diffusion, making the source of the transaction difficult to infer.
+
+[[File:bip-dandelion/4-dandelion-plot.png|framed|center|alt=A plot illustrating limited deanonymization|Figure 4]]
+Figure 4
+
+After a transaction has traveled along a Dandelion stem for a random number of
+hops, it transitions into the fluff phase of routing. The transaction is shared
+with the network through the existing process of diffusion. In practice, this
+fluff mechanism is enforced by a weighted coin flip at each node. If the random
+value is below some threshold, the Dandelion transaction is transformed into a
+typical transaction. In our testing, we have chosen a probability of ten percent
+that a given Dandelion transaction enters fluff phase when leaving a given node.
+This value strikes a good balance between stem path length and transaction
+spreading latency.
+
+Note that Dandelion's expected precision guarantees are a population-level
+metric, whereas the expected recall guarantees can be interpreted as an
+individual-level metric. Expected recall is equivalent to the probability that
+an adversary associates a single transaction with a given source. These
+guarantees are probabilistic. They do not address scenarios in which a node has
+been eclipsed by other nodes, or when a node is specifically targeted by an
+ISP-like adversary. Individuals who are concerned about targeted deanonymization
+should still use Tor.
+
+At a high level, Dandelion is like an "anonymity inoculation" for the public at
+large - including users who are not aware of Bitcoin's privacy issues. Higher
+adoption leads to greater benefits, even for users who do not use Tor. Early
+adopters of Dandelion still receive privacy benefits. In the worst case when no
+neighbors support Dandelion, transactions make at least one hop before
+diffusing. Note that any solution based only on routing cannot be perfectly
+anonymous due to the fundamental lower bounds on precision and recall shown in
+the original Dandelion paper. Dandelion provides near-optimal anonymity
+guarantees among such solutions.
+
+==Specification==
+
+Dandelion can be specified with a handful of features: Dandelion transaction
+support, Dandelion routing data and logic, periodic Dandelion route shuffling,
+memory pool logic, the fluff mechanism, transaction embargoes, and Dandelion
+transaction logic. Specification details are summarized below.
+
+===Dandelion transaction support===
+
+During the stem phase, transactions are "Dandelion transactions." When a
+Dandelion transaction enters fluff phase, it becomes a typical Bitcoin
+transaction. Dandelion transactions and typical transactions differ only in
+their <code>NetMsgType</code>.
+
+Dandelion (stem phase) transactions MUST be differentiable from typical Bitcoin
+transactions.
+
+===Dandelion routing data and logic===
+
+Dandelion routing during the stem phase requires notions of inbound peers,
+outbound peers, Dandelion destinations, and Dandelion routes. Inbound peers
+consist of all currently connected peers that initiated the peer connection.
+Outbound peers consist of all currently connected peers that were connected to
+by this node. Dandelion destinations are a subset of outbound peers. The number
+of Dandelion destinations is limited by the
+<code>DANDELION_MAX_DESTINATIONS</code> parameter. In the reference
+implementation, this parameter is set to two. Our tests have shown that this
+value provides both privacy and robustness (see the reference paper for more
+details on the parameter tradeoffs). Dandelion routes are a map of inbound peers
+to Dandelion destinations. Every inbound peer is mapped to a Dandelion
+destination.
+
+Note that a Dandelion node may choose a different
+<code>DANDELION_MAX_DESTINATIONS</code> parameter without splitting from the
+privacy graph. When mapping inbound connections to outbound connections for
+Dandelion routes, we implement the following routing logic. First, select a set
+of Dandelion destinations from the set of outbound peers. This set of Dandelion
+destinations is of size less than or equal to
+<code>DANDELION_MAX_DESTINATIONS</code>. For each inbound connection, first
+identify the subset of Dandelion destinations with the least number of routes.
+For example, some subset of Dandelion destinations may be affiliated with zero
+routes while all other Dandelion destinations are affiliated with one or more
+routes. From this subset, select one Dandelion destination uniformly at random.
+Establish a Dandelion route from the inbound connection to this Dandelion
+destination.
+
+For a given Dandelion routing epoch, two distinct Dandelion destinations SHOULD
+be selected uniformly at random from the set of outbound connections. All
+Dandelion transactions that arrive via a given inbound connection MUST be
+transmitted to the same Dandelion destination. When choosing a Dandelion
+destination for a given inbound connection, the destination MUST be selected
+uniformly at random from the set of Dandelion destinations with the least number
+of inbound connections mapped to them.
+
+===Periodic Dandelion route shuffling===
+
+The map of Dandelion routes is cleared and reconstructed every ten minutes on
+average. We have chosen the value of ten minutes heuristically in order to make
+privacy graph learning difficult for adversaries. Note that a Dandelion node may
+choose a different average shuffle time without splitting from the privacy
+graph.
+
+Dandelion routes MUST be cleared and reconstructed at random intervals.
+Dandelion routes SHOULD be cleared and reconstructed every ten minutes on
+average.
+
+===Memory pool logic===
+
+Dandelion transactions are segregated from typical transactions. The
+<code>mempool</code> remains unchanged. Another instance of the
+<code>CTxMemPool</code> class, called the <code>stempool</code>, is used for
+Dandelion transactions. Information flows from <code>mempool</code> to
+<code>stempool</code> in order to ensure proper transaction propagation.
+Information does not flow from <code>stempool</code> to <code>mempool</code>,
+except when a Dandelion transaction fluffs into a typical transaction.
+
+When a Dandelion transaction arrives, the transaction MUST be added to the
+stempool and MUST NOT be added to the mempool. When a typical Bitcoin
+transaction arrives, the transaction MUST be added to the mempool and MUST be
+added to the stempool. When a Dandelion transaction fluffs, the transaction MUST
+be added to the mempool.
+
+===The fluff mechanism===
+
+When relaying a Dandelion transaction along a Dandelion route, there is a 10%
+chance that the Dandelion transaction becomes a typical Bitcoin transaction and
+is therefore relayed via diffusion. In our testing, this value strikes a good
+balance between stem path length and transaction spreading latency. Note that a
+Dandelion node may choose a different chance of fluffing without splitting from
+the privacy graph.
+
+When a node prepares to transmit a Dandelion transaction, the node MUST flip a
+biased coin. If the outcome is "Dandelion transaction," then the node MUST
+transmit the transaction to the appropriate Dandelion destination. Otherwise,
+the node MUST convert the Dandelion transaction into a typical Bitcoin
+transaction. A Dandelion transaction SHOULD fluff into a typical Bitcoin
+transaction with a 10% probability.
+
+===Transaction embargoes===
+
+During the stem phase, transactions are relayed along a single path. If any node
+in this path were to receive the Dandelion transaction and go offline, then the
+transaction would cease to propagate. To increase robustness, every node that
+forwards a Dandelion transaction initializes a timer at the time of reception.
+If the Dandelion transaction does not appear in the memory pool by the time the
+timer expires, then the transaction enters fluff phase and is forwarded via
+diffusion.
+
+When a Dandelion transaction arrives, the node MUST set an embargo timer for a
+random time in the future. If the Dandelion transaction arrives as a typical
+Bitcoin transaction, the node MUST cancel the timer. If the timer expires before
+the Dandelion transaction is observed as a typical Bitcoin transaction, then the
+node MUST fluff the Dandelion transaction.
+
+===Dandelion transaction logic===
+
+The following cases define a node's behavior when receiving network packets
+referencing Dandelion transactions.
+* Receive INV for Dandelion TX: If the peer is inbound and the Dandelion transaction has not been received from this peer, then reply with GETDATA.
+* Receive GETDATA for Dandelion TX: If the peer is not inbound and the Dandelion transaction has been advertised to this peer, then reply with the Dandelion transaction.
+* Receive Dandelion TX: If the peer is inbound, then relay the Dandelion TX to the appropriate Dandelion destination.
+
+==Implementation==
+
+A reference implementation is available at the following URL:
+https://github.com/mablem8/bitcoin/tree/dandelion-feature-commits
+
+All features have been compressed into a single commit at the following URL:
+https://github.com/mablem8/bitcoin/tree/dandelion
+
+==Compatibility==
+
+Dandelion does not conflict with existing versions of Bitcoin. A Bitcoin node
+that supports Dandelion appears no differently to Bitcoin nodes running older
+software versions. Bitcoin nodes that support Dandelion can identify feature
+support through a probe message. Obviously, older nodes are not capable of
+Dandelion routing. If a Bitcoin node supporting Dandelion has no peers that also
+support Dandelion, then its behavior naturally decays to that of a Bitcoin node
+without Dandelion support due to the Dandelion transaction embargoes.
+
+==Acknowledgements==
+
+We would like to thank the Bitcoin Core developers and Gregory Maxwell in
+particular for their insightful comments, which helped to inform this
+implementation and some of the follow-up work we conducted. We would also like
+to thank the Mimblewimble development community for coining the term "stempool,"
+which we happily adopted for this implementation.
+
+==References==
+
+# An Analysis of Anonymity in Bitcoin Using P2P Network Traffic http://fc14.ifca.ai/papers/fc14_submission_71.pdf
+# Deanonymisation of clients in Bitcoin P2P network https://arxiv.org/abs/1405.7418
+# Discovering Bitcoin’s Public Topology and Influential Nodes https://cs.umd.edu/projects/coinscope/coinscope.pdf
+# (Sigmetrics 2017) Dandelion: Redesigning the Bitcoin Network for Anonymity https://arxiv.org/abs/1701.04439
+# (Sigmetrics 2018) Dandelion++: Lightweight Cryptocurrency Networking with Formal Anonymity Guarantees https://arxiv.org/pdf/1805.11060.pdf
+
+==Copyright==
+
+To the extent possible under law, the author(s) have dedicated all copyright and
+related and neighboring rights to this work to the public domain worldwide. This
+work is distributed without any warranty.
+
+You should have received a copy of the CC0 Public Domain Dedication with this
+work. If not, see https://creativecommons.org/publicdomain/zero/1.0/ .
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