187 lines
6.9 KiB
ReStructuredText
187 lines
6.9 KiB
ReStructuredText
=================================
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BitTorrent DHT security extension
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=================================
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:Author: Arvid Norberg, arvid@rasterbar.com
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:Version: 1.0.0
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.. contents:: Table of contents
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:depth: 2
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:backlinks: none
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BitTorrent DHT security extension
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---------------------------------
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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.
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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.
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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.
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considerations
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--------------
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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.
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In order to successfully launch an attack, you just need to find 8 IPs
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whose hash will be *closest* 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.
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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.
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``2^20 * 8 = 8388608``
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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.
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With IPv4, snooping would require a /8 IP block, giving access to 16.7 million
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Ips.
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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.
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Node ID restriction
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-------------------
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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.
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The expression to calculate a valid ID prefix (from an IPv4 address) is::
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sha1((ip & 0x01071f7f) .. r)
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And for an IPv6 address (``ip`` is the high 64 bits of the address)::
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sha1((ip & 0x000103070f1f3f7f) .. r)
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``r`` 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.
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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.
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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 (``r``) used to generate the hash.
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.. image:: ip_id_v4.png
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.. image:: ip_id_v6.png
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Example code code for calculating a valid node ID::
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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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test vectors:
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.. parsed-literal::
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IP rand example node ID
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============ ===== ==========================================
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124.31.75.21 1 **f766f9f5** 0c5d6a4ec8a88e4c6ab4c28b95eee4 **01**
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21.75.31.124 86 **7ee04779** 4e7a08645677bbd1cfe7d8f956d532 **56**
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65.23.51.170 22 **76a626ff** bc8f112a3d426c84764f8c2a1150e6 **16**
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84.124.73.14 65 **beb4e619** 1bb1fe518101ceef99462b947a01ff **41**
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43.213.53.83 90 **ace5613a** 5b7c4be0237986d5243b87aa6d5130 **5a**
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The bold parts of the node ID are the important parts. The rest are
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random numbers.
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bootstrapping
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-------------
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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, *all* 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 ``r`` dictionary) called ``ip``. 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.
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A DHT node which receives an ``ip`` 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.
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enforcement
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-----------
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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 ``announce_peer`` 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.
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Any peer on a local network address is exempt from this node ID verification.
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This includes the following IP blocks:
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10.0.0.0/8
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reserved for local networks
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172.16.0.0/12
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reserved for local networks
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192.168.0.0/16
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reserved for local networks
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169.254.0.0/16
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reserved for self-assigned IPs
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127.0.0.0/8
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reserved for loopback
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backwards compatibility and transition
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--------------------------------------
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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.
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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.
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forward compatibility
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---------------------
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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.
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