113fbc1df3
* Merged in the i2p_0_6_1_10_PRE branch to the trunk, so CVS HEAD is no
longer backwards compatible (and should not be used until 0.6.1.1 is
out)
164 lines
8.2 KiB
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164 lines
8.2 KiB
HTML
<code>$Id: tunnel-alt-creation.html,v 1.1.2.1 2006/02/01 20:28:34 jrandom Exp $</code>
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<pre>
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1) <a href="#tunnelCreate.overview">Tunnel creation</a>
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1.1) <a href="#tunnelCreate.requestRecord">Tunnel creation request record</a>
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1.2) <a href="#tunnelCreate.hopProcessing">Hop processing</a>
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1.3) <a href="#tunnelCreate.replyRecord">Tunnel creation reply record</a>
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1.4) <a href="#tunnelCreate.requestPreparation">Request preparation</a>
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1.5) <a href="#tunnelCreate.requestDelivery">Request delivery</a>
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1.6) <a href="#tunnelCreate.endpointHandling">Endpoint handling</a>
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1.7) <a href="#tunnelCreate.replyProcessing">Reply processing</a>
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2) <a href="#tunnelCreate.notes">Notes</a>
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</pre>
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<h2 id="tunnelCreate.overview">1) Tunnel creation encryption:</h2>
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<p>The tunnel creation is accomplished by a single message passed along
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the path of peers in the tunnel, rewritten in place, and transmitted
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back to the tunnel creator. This single tunnel message is made up
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of a fixed number of records (8) - one for each potential peer in
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the tunnel. Individual records are asymmetrically encrypted to be
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read only by a specific peer along the path, while an additional
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symmetric layer of encryption is added at each hop so as to expose
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the asymmetrically encrypted record only at the appropriate time.</p>
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<h3 id="tunnelCreate.requestRecord">1.1) Tunnel creation request record</h3>
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<p>Cleartext of the record, visible only to the hop being asked:</p><pre>
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bytes 0-3: tunnel ID to receive messages as
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bytes 4-35: local router identity hash
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bytes 36-39: next tunnel ID
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bytes 40-71: next router identity hash
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bytes 72-103: AES-256 tunnel layer key
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bytes 104-135: AES-256 tunnel IV key
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bytes 136-167: AES-256 reply key
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bytes 168-183: reply IV
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byte 184: flags
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bytes 185-188: request time (in hours since the epoch)
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bytes 189-192: next message ID
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bytes 193-222: uninterpreted / random padding</pre>
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<p>The next tunnel ID and next router identity hash fields are used to
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specify the next hop in the tunnel, though for an outbound tunnel
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endpoint, they specify where the rewritten tunnel creation reply
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message should be sent. In addition, the next message ID specifies the
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message ID that the message (or reply) should use.</p>
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<p>The flags field currently has two bits defined:</p><pre>
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bit 0: if set, allow messages from anyone
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bit 1: if set, allow messages to anyone, and send the reply to the
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specified next hop in a tunnel message</pre>
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<p>That cleartext record is ElGamal 2048 encrypted with the hop's
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public encryption key and formatted into a 528 byte record:</p><pre>
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bytes 0-15: SHA-256-128 of the current hop's router identity
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bytes 16-527: ElGamal-2048 encrypted request record</pre>
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<p>Since the cleartext uses the full field, there is no need for
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additional padding beyond <code>SHA256(cleartext) + cleartext</code>.</p>
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<h3 id="tunnelCreate.hopProcessing">1.2) Hop processing</h3>
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<p>When a hop receives a TunnelBuildMessage, it looks through the 8
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records contained within it for one starting with their own identity
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hash (trimmed to 8 bytes). It then decryptes the ElGamal block from
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that record and retrieves the protected cleartext. At that point,
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they make sure the tunnel request is not a duplicate by feeding the
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AES-256 reply key into a bloom filter and making sure the request
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time is within an hour of current. Duplicates or invalid requests
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are dropped.</p>
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<p>After deciding whether they will agree to participate in the tunnel
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or not, they replace the record that had contained the request with
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an encrypted reply block. All other records are AES-256/CBC
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encrypted with the included reply key and IV (though each is
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encrypted separately, rather than chained across records).</p>
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<h3 id="tunnelCreate.replyRecord">1.3) Tunnel creation reply record</h3>
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<p>After the current hop reads their record, they replace it with a
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reply record stating whether or not they agree to participate in the
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tunnel, and if they do not, they classify their reason for
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rejection. This is simply a 1 byte value, with 0x0 meaning they
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agree to participate in the tunnel, and higher values meaning higher
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levels of rejection. The reply is encrypted with the AES session
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key delivered to it in the encrypted block, padded with random data
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until it reaches the full record size:</p><pre>
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AES-256-CBC(SHA-256(padding+status) + padding + status, key, IV)</pre>
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<h3 id="tunnelCreate.requestPreparation">1.4) Request preparation</h3>
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<p>When building a new request, all of the records must first be
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built and asymmetrically encrypted. Each record should then be
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decrypted with the reply keys and IVs of the hops earlier in the
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path. That decryption should be run in reverse order so that the
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asymmetrically encrypted data will show up in the clear at the
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right hop after their predecessor encrypts it.</p>
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<p>The excess records not needed for individual requests are simply
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filled with random data by the creator.</p>
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<h3 id="tunnelCreate.requestDelivery">1.5) Request delivery</h3>
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<p>For outbound tunnels, the delivery is done directly from the tunnel
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creator to the first hop, packaging up the TunnelBuildMessage as if
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the creator was just another hop in the tunnel. For inbound
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tunnels, the delivery is done through an existing outbound tunnel
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(and during startup, when no outbound tunnel exists yet, a fake 0
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hop outbound tunnel is used).</p>
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<h3 id="tunnelCreate.endpointHandling">1.6) Endpoint handling</h3>
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<p>When the request reaches an outbound endpoint (as determined by the
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'allow messages to anyone' flag), the hop is processed as usual,
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encrypting a reply in place of the record and encrypting all of the
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other records, but since there is no 'next hop' to forward the
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TunnelBuildMessage on to, it instead places the encrypted reply
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records into a TunnelBuildReplyMessage and delivers it to the
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reply tunnel specified within the request record. That reply tunnel
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forwards the reply records down to the tunnel creator for
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processing, as below.</p>
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<p>When the request reaches the inbound endpoint (also known as the
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tunnel creator), the router processes each of the replies, as below.</p>
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<h3 id="tunnelCreate.replyProcessing">1.7) Reply processing</h3>
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<p>To process the reply records, the creator simply has to AES decrypt
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each record individually, using the reply key and IV of each hop in
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the tunnel after the peer (in reverse order). This then exposes the
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reply specifying whether they agree to participate in the tunnel or
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why they refuse. If they all agree, the tunnel is considered
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created and may be used immediately, but if anyone refuses, the
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tunnel is discarded.</p>
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<h2 id="tunnelCreate.notes">2) Notes</h2>
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<ul>
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<li>This does not prevent two hostile peers within a tunnel from
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tagging one or more request or reply records to detect that they are
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within the same tunnel, but doing so can be detected by the tunnel
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creator when reading the reply, causing the tunnel to be marked as
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invalid.</li>
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<li>This does not include a proof of work on the asymmetrically
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encrypted section, though the 16 byte identity hash could be cut in
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half with the later replaced by a hashcash function of up to 2^64
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cost. This will not immediately be pursued, however.</li>
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<li>This alone does not prevent two hostile peers within a tunnel from
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using timing information to determine whether they are in the same
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tunnel. The use of batched and synchronized request delivery
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could help (batching up requests and sending them off on the
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(ntp-synchronized) minute). However, doing so lets peers 'tag' the
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requests by delaying them and detecting the delay later in the
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tunnel, though perhaps dropping requests not delivered in a small
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window would work (though doing that would require a high degree of
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clock synchronization). Alternately, perhaps individual hops could
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inject a random delay before forwarding on the request?</li>
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<li>Are there any nonfatal methods of tagging the request?</li>
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<li>This strategy came about during a discussion on the I2P mailing list
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between Michael Rogers, Matthew Toseland (toad), and jrandom regarding
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the predecessor attack. See: <ul>
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<li><a href="http://dev.i2p.net/pipermail/i2p/2005-October/001073.html">Summary</a></li>
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<li><a href="http://dev.i2p.net/pipermail/i2p/2005-October/001064.html">Reasoning</a></li>
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</ul></li>
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</ul>
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