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a bit of that manual hacking for tor-design.html too
svn:r10169
This commit is contained in:
Roger Dingledine
parent
7218188157
commit
6c7ae20ca8
@ -49,8 +49,8 @@ more than 30 nodes. We close with a list of open problems in anonymous communica
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<div class="p"><!----></div>
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<h2><a name="tth_sEc1">
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1</a> Overview</h2>
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<a name="sec:intro">
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1</a> Overview</h2>
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</a>
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<div class="p"><!----></div>
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@ -92,7 +92,7 @@ extending to a new node.
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<div class="p"><!----></div>
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<b>Separation of "protocol cleaning" from anonymity:</b>
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Onion Routing originally required a separate "application
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proxy" for each supported application protocol-most of which were
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proxy" for each supported application protocol — most of which were
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never written, so many applications were never supported. Tor uses the
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standard and near-ubiquitous SOCKS [<a href="#socks4" name="CITEsocks4">32</a>] proxy interface, allowing
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us to support most TCP-based programs without modification. Tor now
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@ -130,7 +130,7 @@ streams along each circuit to improve efficiency and anonymity.
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<b>Leaky-pipe circuit topology:</b> Through in-band signaling
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within the circuit, Tor initiators can direct traffic to nodes partway
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down the circuit. This novel approach
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allows traffic to exit the circuit from the middle-possibly
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allows traffic to exit the circuit from the middle — possibly
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frustrating traffic shape and volume attacks based on observing the end
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of the circuit. (It also allows for long-range padding if
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future research shows this to be worthwhile.)
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@ -146,7 +146,7 @@ or flooding and send less data until the congestion subsides.
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<div class="p"><!----></div>
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<b>Directory servers:</b> The earlier Onion Routing design
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planned to flood state information through the network-an approach
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planned to flood state information through the network — an approach
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that can be unreliable and complex. Tor takes a simplified view toward distributing this
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information. Certain more trusted nodes act as <em>directory
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servers</em>: they provide signed directories describing known
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@ -164,7 +164,7 @@ from his node.
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<div class="p"><!----></div>
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<b>End-to-end integrity checking:</b> The original Onion Routing
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design did no integrity checking on data. Any node on the
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circuit could change the contents of data cells as they passed by-for
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circuit could change the contents of data cells as they passed by — for
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example, to alter a connection request so it would connect
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to a different webserver, or to `tag' encrypted traffic and look for
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corresponding corrupted traffic at the network edges [<a href="#minion-design" name="CITEminion-design">15</a>].
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@ -218,8 +218,8 @@ Routing project in Section <a href="#sec:conclusion">10</a>.
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<div class="p"><!----></div>
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<h2><a name="tth_sEc2">
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2</a> Related work</h2>
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<a name="sec:related-work">
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2</a> Related work</h2>
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</a>
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<div class="p"><!----></div>
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@ -377,8 +377,8 @@ Eternity and Free Haven.
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<div class="p"><!----></div>
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<h2><a name="tth_sEc3">
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3</a> Design goals and assumptions</h2>
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<a name="sec:assumptions">
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3</a> Design goals and assumptions</h2>
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</a>
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<div class="p"><!----></div>
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@ -404,7 +404,7 @@ this goal for non-anonymous users talking to hidden servers,
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however; see Section <a href="#sec:rendezvous">5</a>.)
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<div class="p"><!----></div>
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<b>Usability:</b> A hard-to-use system has fewer users-and because
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<b>Usability:</b> A hard-to-use system has fewer users — and because
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anonymity systems hide users among users, a system with fewer users
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provides less anonymity. Usability is thus not only a convenience:
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it is a security requirement [<a href="#econymics" name="CITEeconymics">1</a>,<a href="#back01" name="CITEback01">5</a>]. Tor should
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@ -474,8 +474,8 @@ to the network.
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<div class="p"><!----></div>
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<h3><a name="tth_sEc3.1">
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3.1</a> Threat Model</h3>
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<a name="subsec:threat-model">
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3.1</a> Threat Model</h3>
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</a>
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<div class="p"><!----></div>
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@ -504,7 +504,7 @@ which points in the network he should attack.
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Our adversary might try to link an initiator Alice with her
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communication partners, or try to build a profile of Alice's
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behavior. He might mount passive attacks by observing the network edges
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and correlating traffic entering and leaving the network-by
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and correlating traffic entering and leaving the network — by
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relationships in packet timing, volume, or externally visible
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user-selected
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options. The adversary can also mount active attacks by compromising
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@ -516,7 +516,7 @@ network stops; or by introducing patterns into traffic that can later be
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detected. The adversary might subvert the directory servers to give users
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differing views of network state. Additionally, he can try to decrease
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the network's reliability by attacking nodes or by performing antisocial
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activities from reliable nodes and trying to get them taken down-making
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activities from reliable nodes and trying to get them taken down — making
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the network unreliable flushes users to other less anonymous
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systems, where they may be easier to attack. We summarize
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in Section <a href="#sec:attacks">7</a> how well the Tor design defends against
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@ -526,8 +526,8 @@ each of these attacks.
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<div class="p"><!----></div>
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<h2><a name="tth_sEc4">
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4</a> The Tor Design</h2>
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<a name="sec:design">
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4</a> The Tor Design</h2>
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</a>
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<div class="p"><!----></div>
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@ -570,8 +570,8 @@ fairness issues.
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<div class="p"><!----></div>
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<h3><a name="tth_sEc4.1">
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4.1</a> Cells</h3>
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<a name="subsec:cells">
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4.1</a> Cells</h3>
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</a>
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<div class="p"><!----></div>
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@ -627,8 +627,8 @@ in more detail below.
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</center>
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<div class="p"><!----></div>
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<h3><a name="tth_sEc4.2">
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4.2</a> Circuits and streams</h3>
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<a name="subsec:circuits">
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4.2</a> Circuits and streams</h3>
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</a>
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<div class="p"><!----></div>
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@ -662,8 +662,9 @@ without harming user experience.<br />
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</a>
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</center>
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<div class="p"><!----></div>
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<font size="+1"><b>Constructing a circuit</b></font><a name="subsubsec:constructing-a-circuit">
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</a><br />
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<a name="subsubsec:constructing-a-circuit"></a>
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<font size="+1"><b>Constructing a circuit</b></font>
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<br />
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A user's OP constructs circuits incrementally, negotiating a
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symmetric key with each OR on the circuit, one hop at a time. To begin
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creating a new circuit, the OP (call her Alice) sends a
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@ -704,7 +705,7 @@ extend one hop further.
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<div class="p"><!----></div>
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This circuit-level handshake protocol achieves unilateral entity
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authentication (Alice knows she's handshaking with the OR, but
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the OR doesn't care who is opening the circuit-Alice uses no public key
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the OR doesn't care who is opening the circuit — Alice uses no public key
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and remains anonymous) and unilateral key authentication
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(Alice and the OR agree on a key, and Alice knows only the OR learns
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it). It also achieves forward
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@ -797,8 +798,8 @@ attack [<a href="#freedom21-security" name="CITEfreedom21-security">4</a>]
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<div class="p"><!----></div>
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<h3><a name="tth_sEc4.3">
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4.3</a> Opening and closing streams</h3>
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<a name="subsec:tcp">
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4.3</a> Opening and closing streams</h3>
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</a>
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<div class="p"><!----></div>
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@ -818,7 +819,7 @@ now accepts data from the application's TCP stream, packaging it into
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the chosen OR.
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<div class="p"><!----></div>
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There's a catch to using SOCKS, however-some applications pass the
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There's a catch to using SOCKS, however — some applications pass the
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alphanumeric hostname to the Tor client, while others resolve it into
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an IP address first and then pass the IP address to the Tor client. If
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the application does DNS resolution first, Alice thereby reveals her
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@ -855,8 +856,8 @@ connections.
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<div class="p"><!----></div>
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<h3><a name="tth_sEc4.4">
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4.4</a> Integrity checking on streams</h3>
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<a name="subsec:integrity-checking">
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4.4</a> Integrity checking on streams</h3>
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</a>
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<div class="p"><!----></div>
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@ -920,8 +921,8 @@ receive a bad hash.
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<div class="p"><!----></div>
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<h3><a name="tth_sEc4.5">
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4.5</a> Rate limiting and fairness</h3>
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<a name="subsec:rate-limit">
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4.5</a> Rate limiting and fairness</h3>
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</a>
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<div class="p"><!----></div>
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@ -955,8 +956,8 @@ attacks.
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<div class="p"><!----></div>
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<h3><a name="tth_sEc4.6">
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4.6</a> Congestion control</h3>
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<a name="subsec:congestion">
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4.6</a> Congestion control</h3>
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</a>
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<div class="p"><!----></div>
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@ -1018,8 +1019,8 @@ and delay; see Section <a href="#sec:in-the-wild">8</a>.
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<div class="p"><!----></div>
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<h2><a name="tth_sEc5">
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5</a> Rendezvous Points and hidden services</h2>
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<a name="sec:rendezvous">
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5</a> Rendezvous Points and hidden services</h2>
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</a>
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<div class="p"><!----></div>
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@ -1144,7 +1145,7 @@ those users can switch to accessing Bob's service via
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the Tor rendezvous system.
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<div class="p"><!----></div>
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Bob's introduction points are themselves subject to DoS-he must
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Bob's introduction points are themselves subject to DoS — he must
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open many introduction points or risk such an attack.
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He can provide selected users with a current list or future schedule of
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unadvertised introduction points;
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@ -1170,7 +1171,7 @@ by the hash of his public key. Bob's webserver is unmodified,
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and doesn't even know that it's hidden behind the Tor network.
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<div class="p"><!----></div>
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Alice's applications also work unchanged-her client interface
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Alice's applications also work unchanged — her client interface
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remains a SOCKS proxy. We encode all of the necessary information
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into the fully qualified domain name (FQDN) Alice uses when establishing her
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connection. Location-hidden services use a virtual top level domain
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@ -1205,20 +1206,20 @@ service, to encourage volunteers to offer introduction and rendezvous
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services. Tor's introduction points do not output any bytes to the
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clients; the rendezvous points don't know the client or the server,
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and can't read the data being transmitted. The indirection scheme is
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also designed to include authentication/authorization-if Alice doesn't
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also designed to include authentication/authorization — if Alice doesn't
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include the right cookie with her request for service, Bob need not even
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acknowledge his existence.
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<div class="p"><!----></div>
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<h2><a name="tth_sEc6">
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6</a> Other design decisions</h2>
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<a name="sec:other-design">
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6</a> Other design decisions</h2>
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</a>
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<div class="p"><!----></div>
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<h3><a name="tth_sEc6.1">
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6.1</a> Denial of service</h3>
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<a name="subsec:dos">
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6.1</a> Denial of service</h3>
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</a>
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<div class="p"><!----></div>
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@ -1270,8 +1271,8 @@ extra complexity still require investigation.
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<div class="p"><!----></div>
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<h3><a name="tth_sEc6.2">
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6.2</a> Exit policies and abuse</h3>
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<a name="subsec:exitpolicies">
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6.2</a> Exit policies and abuse</h3>
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</a>
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<div class="p"><!----></div>
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@ -1304,7 +1305,7 @@ nodes that will connect anywhere. On the other end are <em>middleman</em>
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nodes that only relay traffic to other Tor nodes, and <em>private exit</em>
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nodes that only connect to a local host or network. A private
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exit can allow a client to connect to a given host or
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network more securely-an external adversary cannot eavesdrop traffic
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network more securely — an external adversary cannot eavesdrop traffic
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between the private exit and the final destination, and so is less sure of
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Alice's destination and activities. Most onion routers in the current
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network function as
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@ -1348,7 +1349,7 @@ an adversary needs to monitor for traffic analysis, and places a
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greater burden on the exit nodes. This tension can be seen in the
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Java Anon Proxy
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cascade model, wherein only one node in each cascade needs to handle
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abuse complaints-but an adversary only needs to observe the entry
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abuse complaints — but an adversary only needs to observe the entry
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and exit of a cascade to perform traffic analysis on all that
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cascade's users. The hydra model (many entries, few exits) presents a
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different compromise: only a few exit nodes are needed, but an
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@ -1367,8 +1368,8 @@ project [<a href="#darkside" name="CITEdarkside">37</a>] give us a glimpse
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<div class="p"><!----></div>
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<h3><a name="tth_sEc6.3">
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6.3</a> Directory Servers</h3>
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<a name="subsec:dirservers">
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6.3</a> Directory Servers</h3>
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</a>
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<div class="p"><!----></div>
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@ -1403,7 +1404,7 @@ to bootstrap each client's view of the network.
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<div class="p"><!----></div>
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When a directory server receives a signed statement for an OR, it
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checks whether the OR's identity key is recognized. Directory
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servers do not advertise unrecognized ORs-if they did,
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servers do not advertise unrecognized ORs — if they did,
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an adversary could take over the network by creating many
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servers [<a href="#sybil" name="CITEsybil">22</a>]. Instead, new nodes must be approved by the
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directory
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@ -1414,7 +1415,7 @@ in Section <a href="#sec:maintaining-anonymity">9</a>.
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<div class="p"><!----></div>
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Of course, a variety of attacks remain. An adversary who controls
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a directory server can track clients by providing them different
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information-perhaps by listing only nodes under its control, or by
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information — perhaps by listing only nodes under its control, or by
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informing only certain clients about a given node. Even an external
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adversary can exploit differences in client knowledge: clients who use
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a node listed on one directory server but not the others are vulnerable.
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@ -1460,8 +1461,8 @@ central point.
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<div class="p"><!----></div>
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<h2><a name="tth_sEc7">
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7</a> Attacks and Defenses</h2>
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<a name="sec:attacks">
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7</a> Attacks and Defenses</h2>
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</a>
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<div class="p"><!----></div>
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@ -1558,7 +1559,7 @@ be completed. (Thanks to the perfect forward secrecy of session
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keys, the attacker cannot force nodes to decrypt recorded
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traffic once the circuits have been closed.) Additionally, building
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circuits that cross jurisdictions can make legal coercion
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harder-this phenomenon is commonly called "jurisdictional
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harder — this phenomenon is commonly called "jurisdictional
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arbitrage." The Java Anon Proxy project recently experienced the
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need for this approach, when
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a German court forced them to add a backdoor to
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@ -1580,7 +1581,7 @@ to solve this latter problem.
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<em>Run an onion proxy.</em> It is expected that end users will
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nearly always run their own local onion proxy. However, in some
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settings, it may be necessary for the proxy to run
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remotely-typically, in institutions that want
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remotely — typically, in institutions that want
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to monitor the activity of those connecting to the proxy.
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Compromising an onion proxy compromises all future connections
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through it.
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@ -1603,7 +1604,7 @@ that those ORs are trustworthy and independent, then occasionally
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some user will choose one of those ORs for the start and another
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as the end of a circuit. If an adversary
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controls m > 1 of N nodes, he can correlate at most
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([m/N])<sup>2</sup> of the traffic-although an
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([m/N])<sup>2</sup> of the traffic — although an
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adversary
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could still attract a disproportionately large amount of traffic
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by running an OR with a permissive exit policy, or by
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@ -1644,7 +1645,7 @@ some political heat.
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<div class="p"><!----></div>
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<em>Distribute hostile code.</em> An attacker could trick users
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into running subverted Tor software that did not, in fact, anonymize
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their connections-or worse, could trick ORs into running weakened
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their connections — or worse, could trick ORs into running weakened
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software that provided users with less anonymity. We address this
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problem (but do not solve it completely) by signing all Tor releases
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with an official public key, and including an entry in the directory
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@ -1741,8 +1742,8 @@ with a session key shared by Alice and Bob.
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<div class="p"><!----></div>
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<h2><a name="tth_sEc8">
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8</a> Early experiences: Tor in the Wild</h2>
|
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<a name="sec:in-the-wild">
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8</a> Early experiences: Tor in the Wild</h2>
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</a>
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<div class="p"><!----></div>
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@ -1751,7 +1752,7 @@ As of mid-May 2004, the Tor network consists of 32 nodes
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matures. (For comparison, the current remailer network
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has about 40 nodes.) Each node has at least a 768Kb/768Kb connection, and
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many have 10Mb. The number of users varies (and of course, it's hard to
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tell for sure), but we sometimes have several hundred users-administrators at
|
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tell for sure), but we sometimes have several hundred users — administrators at
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several companies have begun sending their entire departments' web
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traffic through Tor, to block other divisions of
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their company from reading their traffic. Tor users have reported using
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@ -1766,7 +1767,7 @@ cells (a bit under half a gigabyte) per week. On average, about 80%
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of each 498-byte payload is full for cells going back to the client,
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whereas about 40% is full for cells coming from the client. (The difference
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arises because most of the network's traffic is web browsing.) Interactive
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traffic like SSH brings down the average a lot-once we have more
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traffic like SSH brings down the average a lot — once we have more
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experience, and assuming we can resolve the anonymity issues, we may
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partition traffic into two relay cell sizes: one to handle
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bulk traffic and one for interactive traffic.
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@ -1779,7 +1780,7 @@ issues since the network was deployed in October
|
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resolve bugs, and get a feel for what users actually want from an
|
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anonymity system. Even though having more users would bolster our
|
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anonymity sets, we are not eager to attract the Kazaa or warez
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communities-we feel that we must build a reputation for privacy, human
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communities — we feel that we must build a reputation for privacy, human
|
||||
rights, research, and other socially laudable activities.
|
||||
|
||||
<div class="p"><!----></div>
|
||||
@ -1816,8 +1817,8 @@ topology will help us choose among alternatives when the time comes.
|
||||
|
||||
<div class="p"><!----></div>
|
||||
<h2><a name="tth_sEc9">
|
||||
9</a> Open Questions in Low-latency Anonymity</h2>
|
||||
<a name="sec:maintaining-anonymity">
|
||||
9</a> Open Questions in Low-latency Anonymity</h2>
|
||||
</a>
|
||||
|
||||
<div class="p"><!----></div>
|
||||
@ -1913,7 +1914,7 @@ Will users abandon the system because of this brittleness? How well
|
||||
does the method in Section <a href="#subsec:dos">6.1</a> allow streams to survive
|
||||
node failure? If affected users rebuild circuits immediately, how much
|
||||
anonymity is lost? It seems the problem is even worse in a peer-to-peer
|
||||
environment-such systems don't yet provide an incentive for peers to
|
||||
environment — such systems don't yet provide an incentive for peers to
|
||||
stay connected when they're done retrieving content, so we would expect
|
||||
a higher churn rate.
|
||||
|
||||
@ -1921,8 +1922,8 @@ a higher churn rate.
|
||||
|
||||
<div class="p"><!----></div>
|
||||
<h2><a name="tth_sEc10">
|
||||
10</a> Future Directions</h2>
|
||||
<a name="sec:conclusion">
|
||||
10</a> Future Directions</h2>
|
||||
</a>
|
||||
|
||||
<div class="p"><!----></div>
|
||||
@ -1955,7 +1956,7 @@ we need to explore more approaches to limiting abuse, and understand
|
||||
why most people don't bother using privacy systems.
|
||||
|
||||
<div class="p"><!----></div>
|
||||
<em>Cover traffic:</em> Currently Tor omits cover traffic-its costs
|
||||
<em>Cover traffic:</em> Currently Tor omits cover traffic — its costs
|
||||
in performance and bandwidth are clear but its security benefits are
|
||||
not well understood. We must pursue more research on link-level cover
|
||||
traffic and long-range cover traffic to determine whether some simple padding
|
||||
@ -2484,3 +2485,4 @@ by <a href="http://hutchinson.belmont.ma.us/tth/">
|
||||
T<sub><font size="-1">T</font></sub>H</a>,
|
||||
version 3.59.<br />On 18 May 2004, 10:45.</small>
|
||||
</body></html>
|
||||
|
||||
|
||||
Loading…
Reference in New Issue
Block a user