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<h1><span>Rasterbar Software</span></h1>
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<h2><span>Software developement and consulting</span></h2>
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<div id="main">
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<h1 class="title">BitTorrent DHT security extension</h1>
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<col class="docinfo-name" />
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<col class="docinfo-content" />
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<tbody valign="top">
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<tr><th class="docinfo-name">Author:</th>
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<td>Arvid Norberg, <a class="last reference external" href="mailto:arvid@rasterbar.com">arvid@rasterbar.com</a></td></tr>
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<tr><th class="docinfo-name">Version:</th>
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<td>1.0.0</td></tr>
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<div class="contents topic" id="table-of-contents">
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<p class="topic-title first">Table of contents</p>
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<ul class="simple">
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<li><a class="reference internal" href="#id1" id="id2">BitTorrent DHT security extension</a></li>
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<li><a class="reference internal" href="#considerations" id="id3">considerations</a></li>
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<li><a class="reference internal" href="#node-id-restriction" id="id4">Node ID restriction</a></li>
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<li><a class="reference internal" href="#bootstrapping" id="id5">bootstrapping</a></li>
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<li><a class="reference internal" href="#enforcement" id="id6">enforcement</a></li>
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<li><a class="reference internal" href="#backwards-compatibility-and-transition" id="id7">backwards compatibility and transition</a></li>
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<li><a class="reference internal" href="#forward-compatibility" id="id8">forward compatibility</a></li>
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</ul>
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</div>
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<div class="section" id="id1">
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<h1>BitTorrent DHT security extension</h1>
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<p>The purpose of this extension is to make it harder to launch a few
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specific attacks against the BitTorrent DHT and also to make it harder
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to snoop the network.</p>
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<p>Specifically the attack this extension intends to make harder is launching
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8 or more DHT nodes which node-IDs selected close to a specific target
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info-hash, in order to become the main nodes hosting peers for it. Currently
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this is very easy to do and lets the attacker not only see all the traffic
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related to this specific info-hash but also block access to it by other
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peers.</p>
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<p>The proposed guard against this is to enforce restrictions on which node-ID
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a node can choose, based on its external IP address.</p>
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</div>
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<div class="section" id="considerations">
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<h1>considerations</h1>
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<p>One straight forward scheme to tie the node ID to an IP would be to hash
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the IP and force the node ID to share the prefix of that hash. One main
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draw back of this approach is that an entities control over the DHT key
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space grows linearly with its control over the IP address space.</p>
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<p>In order to successfully launch an attack, you just need to find 8 IPs
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whose hash will be <em>closest</em> to the target info-hash. Given the current
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size of the DHT, that is quite likely to be possible by anyone in control
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of a /8 IP block.</p>
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<p>The size of the DHT is approximately 8.4 million nodes. This is estmiated
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by observing that a typical routing table typically has about 20 of its
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top routing table buckets full. That means the key space is dense enough
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to contain 8 nodes for every combination of the 20 top bits of node IDs.</p>
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<blockquote>
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<tt class="docutils literal">2^20 * 8 = 8388608</tt></blockquote>
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<p>By controlling that many IP addresses, an attacker could snoop any info-hash.
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By controlling 8 times that many IP addresses, an attacker could actually
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take over any info-hash.</p>
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<p>With IPv4, snooping would require a /8 IP block, giving access to 16.7 million
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Ips.</p>
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<p>Another problem with hashing the IP is that multiple users behind a NAT are
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forced to run their DHT nodes on the same node ID.</p>
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</div>
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<div class="section" id="node-id-restriction">
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<h1>Node ID restriction</h1>
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<p>In order to avoid the number node IDs controlled to grow linearly by the number
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of IPs, as well as allowing more than one node ID per external IP, the node
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ID can be restricted at each class level of the IP.</p>
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<p>The expression to calculate a valid ID prefix (from an IPv4 address) is:</p>
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<pre class="literal-block">
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sha1((ip & 0x01071f7f) .. r)
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</pre>
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<p>And for an IPv6 address (<tt class="docutils literal">ip</tt> is the high 64 bits of the address):</p>
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<pre class="literal-block">
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sha1((ip & 0x000103070f1f3f7f) .. r)
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</pre>
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<p><tt class="docutils literal">r</tt> is a random number in the range [0, 7]. The resulting integer,
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representing the masked IP address is supposed to be big-endian before
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hashed. The ".." means concatenation.</p>
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<p>The details of implementing this is to evaluate the expression, store the
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result in a big endian 64 bit integer and hash those 8 bytes with SHA-1.</p>
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<p>The first 4 bytes of the node ID used in the DHT MUST match the first 4
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bytes in the resulting hash. The last byte of the hash MUST match the
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random number (<tt class="docutils literal">r</tt>) used to generate the hash.</p>
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<img alt="ip_id_v4.png" src="ip_id_v4.png" />
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<img alt="ip_id_v6.png" src="ip_id_v6.png" />
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<p>Example code code for calculating a valid node ID:</p>
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<pre class="literal-block">
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uint8_t* ip; // our external IPv4 or IPv6 address (network byte order)
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int num_octets; // the number of octets to consider in ip (4 or 8)
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uint8_t node_id[20]; // resulting node ID
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uint8_t v4mask[] = { 0x01, 0x07, 0x1f, 0x7f };
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uint8_t v6mask[] = { 0x00, 0x01, 0x03, 0x07, 0x0f, 0x1f, 0x3f, 0x7f };
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uint8_t* mask = num_octets == 4 ? v4_mask : v8_mask;
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for (int i = 0; i < num_octets; ++i)
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ip[i] &= mask[i];
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SHA_CTX ctx;
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SHA1_Init(&ctx);
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SHA1_Update(&ctx, (unsigned char*)ip, num_octets);
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uint32_t rand = rand() & 0xff;
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uint8_t r = rand & 0x7;
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SHA1_Update(&ctx, (unsigned char*)&r, 1);
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SHA1_Final(&ctx, node_id);
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for (int i = 4; i < 19; ++i) node_id[i] = std::rand();
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node_id[19] = rand;
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</pre>
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<p>test vectors:</p>
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<pre class="literal-block">
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IP rand example node ID
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============ ===== ==========================================
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124.31.75.21 1 <strong>f766f9f5</strong> 0c5d6a4ec8a88e4c6ab4c28b95eee4 <strong>01</strong>
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21.75.31.124 86 <strong>7ee04779</strong> 4e7a08645677bbd1cfe7d8f956d532 <strong>56</strong>
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65.23.51.170 22 <strong>76a626ff</strong> bc8f112a3d426c84764f8c2a1150e6 <strong>16</strong>
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84.124.73.14 65 <strong>beb4e619</strong> 1bb1fe518101ceef99462b947a01ff <strong>41</strong>
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43.213.53.83 90 <strong>ace5613a</strong> 5b7c4be0237986d5243b87aa6d5130 <strong>5a</strong>
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</pre>
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<p>The bold parts of the node ID are the important parts. The rest are
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random numbers.</p>
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</div>
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<div class="section" id="bootstrapping">
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<h1>bootstrapping</h1>
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<p>In order to set ones initial node ID, the external IP needs to be known. This
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is not a trivial problem. With this extension, <em>all</em> DHT requests whose node
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ID does not match its IP address MUST be serviced and MUST also include one
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extra result value (inside the <tt class="docutils literal">r</tt> dictionary) called <tt class="docutils literal">ip</tt>. The IP field
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contains the raw (big endian) byte representation of the external IP address.
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This is the same byte sequence used to verify the node ID.</p>
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<p>A DHT node which receives an <tt class="docutils literal">ip</tt> result in a request SHOULD consider restarting
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its DHT node with a new node ID, taking this IP into account. Since a single node
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can not be trusted, there should be some mechanism of determining whether or
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not the node has a correct understanding of its external IP or not. This could
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be done by voting, or only restart the DHT once at least a certain number of
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nodes, from separate searches, tells you your node ID is incorrect.</p>
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</div>
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<div class="section" id="enforcement">
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<h1>enforcement</h1>
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<p>Once enforced, write tokens from peers whose node ID does not match its external
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IP should be considered dropped. In other words, a peer that uses a non-matching
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ID MUST never be used to store information on, regardless of which request. In the
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original DHT specification only <tt class="docutils literal">announce_peer</tt> stores data in the network,
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but any future extension which stores data in the network SHOULD use the same
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restriction.</p>
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<p>Any peer on a local network address is exempt from this node ID verification.
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This includes the following IP blocks:</p>
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<dl class="docutils">
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<dt>10.0.0.0/8</dt>
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<dd>reserved for local networks</dd>
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<dt>172.16.0.0/12</dt>
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<dd>reserved for local networks</dd>
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<dt>192.168.0.0/16</dt>
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<dd>reserved for local networks</dd>
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<dt>169.254.0.0/16</dt>
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<dd>reserved for self-assigned IPs</dd>
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<dt>127.0.0.0/8</dt>
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<dd>reserved for loopback</dd>
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</dl>
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</div>
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<div class="section" id="backwards-compatibility-and-transition">
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<h1>backwards compatibility and transition</h1>
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<p>During some transition period, this restriction should not be enforced, and
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peers whose node ID does not match this formula relative to their external IP
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should not be blocked.</p>
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<p>Requests from peers whose node ID does not match their external IP should
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always be serviced, even after the transition period. The attack this protects
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from is storing data on an attacker's node, not servicing an attackers request.</p>
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</div>
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<div class="section" id="forward-compatibility">
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<h1>forward compatibility</h1>
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<p>If the total size of the DHT grows to the point where the inherent size limit
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in this proposal is too small, the modulus constants can be updated in a new
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proposal, and another transition period where both sets of modulus constants
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are accepted.</p>
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</div>
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</div>
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<span>Copyright © 2005 Rasterbar Software.</span>
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