i2p.itoopie/router/doc/tunnel-alt-creation.html

<code>$Id: tunnel-alt.html,v 1.9 2005/07/27 14:04:07 jrandom Exp $</code>
<pre>
1) <a href="#tunnelCreate.overview">Tunnel creation</a>
1.1) <a href="#tunnelCreate.requestRecord">Tunnel creation request record</a>
1.2) <a href="#tunnelCreate.hopProcessing">Hop processing</a>
1.3) <a href="#tunnelCreate.replyRecord">Tunnel creation reply record</a>
1.4) <a href="#tunnelCreate.requestPreparation">Request preparation</a>
1.5) <a href="#tunnelCreate.requestDelivery">Request delivery</a>
1.6) <a href="#tunnelCreate.endpointHandling">Endpoint handling</a>
1.7) <a href="#tunnelCreate.replyProcessing">Reply processing</a>
2) <a href="#tunnelCreate.notes">Notes</a>
</pre>

<h2 id="tunnelCreate.overview">1) Tunnel creation encryption:</h2>

<p>The tunnel creation is accomplished by a single message passed along
the path of peers in the tunnel, rewritten in place, and transmitted
back to the tunnel creator.  This single tunnel message is made up
of a fixed number of records (8) - one for each potential peer in
the tunnel.   Individual records are asymmetrically encrypted to be
read only by a specific peer along the path, while an additional
symmetric layer of encryption is added at each hop so as to expose
the asymmetrically encrypted record only at the appropriate time.</p>

<h3 id="tunnelCreate.requestRecord">1.1) Tunnel creation request record</h3>

<p>Cleartext of the record, visible only to the hop being asked:</p><pre>
  bytes     0-3: tunnel ID to receive messages as
  bytes    4-35: local router identity hash
  bytes   36-39: next tunnel ID
  bytes   40-71: next router identity hash
  bytes  72-103: AES-256 tunnel layer key
  bytes 104-135: AES-256 tunnel IV key
  bytes 136-167: AES-256 reply key
  bytes 168-183: reply IV
  byte      184: flags
  bytes 185-188: request time (in hours since the epoch)
  bytes 189-222: uninterpreted / random padding</pre>

<p>The next tunnel ID and next router identity hash fields are used to
specify the next hop in the tunnel, though for an outbound tunnel
endpoint, they specify where the rewritten tunnel creation reply
message should be sent.</p>

<p>The flags field currently has two bits defined:</p><pre>
 bit 0: if set, allow messages from anyone
 bit 1: if set, allow messages to anyone, and send the reply to the
        specified next hop in a tunnel message</pre>

<p>That cleartext record is ElGamal 2048 encrypted with the hop's
public encryption key and formatted into a 528 byte record:</p><pre>
  bytes   0-15: SHA-256-128 of the current hop's router identity
  bytes 16-527: ElGamal-2048 encrypted request record</pre>

<p>Since the cleartext uses the full field, there is no need for
additional padding beyond <code>SHA256(cleartext) + cleartext</code>.</p>

<h3 id="tunnelCreate.hopProcessing">1.2) Hop processing</h3>

<p>When a hop receives a TunnelBuildMessage, it looks through the 8
records contained within it for one starting with their own identity
hash (trimmed to 8 bytes).  It then decryptes the ElGamal block from
that record and retrieves the protected cleartext.  At that point,
they make sure the tunnel request is not a duplicate by feeding the 
AES-256 reply key into a bloom filter and making sure the request
time is within an hour of current.  Duplicates or invalid requests
are dropped.</p>

<p>After deciding whether they will agree to participate in the tunnel
or not, they replace the record that had contained the request with
an encrypted reply block.  All other records are AES-256/CBC
encrypted with the included reply key and IV (though each is
encrypted separately, rather than chained across records).</p>

<h3 id="tunnelCreate.replyRecord">1.3) Tunnel creation reply record</h3>

<p>After the current hop reads their record, they replace it with a
reply record stating whether or not they agree to participate in the
tunnel, and if they do not, they classify their reason for
rejection.  This is simply a 1 byte value, with 0x0 meaning they
agree to participate in the tunnel, and higher values meaning higher
levels of rejection.  The reply is encrypted with the AES session
key delivered to it in the encrypted block, padded with random data
until it reaches the full record size:</p><pre>
  AES-256-CBC(SHA-256(padding+status) + padding + status, key, IV)</pre>

<h3 id="tunnelCreate.requestPreparation">1.4) Request preparation</h3>

<p>When building a new request, all of the records must first be 
built and asymmetrically encrypted.  Each record should then be
decrypted with the reply keys and IVs of the hops earlier in the
path.  That decryption should be run in reverse order so that the
asymmetrically encrypted data will show up in the clear at the
right hop after their predecessor encrypts it.</p>

<p>The excess records not needed for individual requests are simply
filled with random data by the creator.</p>

<h3 id="tunnelCreate.requestDelivery">1.5) Request delivery</h3>

<p>For outbound tunnels, the delivery is done directly from the tunnel
creator to the first hop, packaging up the TunnelBuildMessage as if
the creator was just another hop in the tunnel.  For inbound
tunnels, the delivery is done through an existing outbound tunnel
(and during startup, when no outbound tunnel exists yet, a fake 0
hop outbound tunnel is used).</p>

<h3 id="tunnelCreate.endpointHandling">1.6) Endpoint handling</h3>

<p>When the request reaches an outbound endpoint (as determined by the
'allow messages to anyone' flag), the hop is processed as usual,
encrypting a reply in place of the record and encrypting all of the
other records, but since there is no 'next hop' to forward the
TunnelBuildMessage on to, it instead places the encrypted reply
records into a TunnelBuildReplyMessage and delivers it to the
reply tunnel specified within the request record.  That reply tunnel
forwards the reply records down to the tunnel creator for
processing, as below.</p>

<p>When the request reaches the inbound endpoint (also known as the
tunnel creator), the router processes each of the replies, as below.</p>

<h3 id="tunnelCreate.replyProcessing">1.7) Reply processing</h3>

<p>To process the reply records, the creator simply has to AES decrypt
each record individually, using the reply key and IV of each hop in
the tunnel after the peer (in reverse order).  This then exposes the
reply specifying whether they agree to participate in the tunnel or
why they refuse.  If they all agree, the tunnel is considered
created and may be used immediately, but if anyone refuses, the
tunnel is discarded.</p>

<h2 id="tunnelCreate.notes">2) Notes</h2>
<ul>
<li>This does not prevent two hostile peers within a tunnel from
tagging one or more request or reply records to detect that they are
within the same tunnel, but doing so can be detected by the tunnel
creator when reading the reply, causing the tunnel to be marked as 
invalid.</li>
<li>This does not include a proof of work on the asymmetrically
encrypted section, though the 16 byte identity hash could be cut in
half with the later replaced by a hashcash function of up to 2^64
cost.  This will not immediately be pursued, however.</li>
<li>This alone does not prevent two hostile peers within a tunnel from
using timing information to determine whether they are in the same
tunnel.  The use of batched and synchronized request delivery
could help (batching up requests and sending them off on the
(ntp-synchronized) minute).  However, doing so lets peers 'tag' the
requests by delaying them and detecting the delay later in the
tunnel, though perhaps dropping requests not delivered in a small
window would work (though doing that would require a high degree of
clock synchronization).  Alternately, perhaps individual hops could
inject a random delay before forwarding on the request?</li>
<li>Are there any nonfatal methods of tagging the request?</li>
<li>This strategy came about during a discussion on the I2P mailing list
    between Michael Rogers, Matthew Toseland (toad), and jrandom regarding
    the predecessor attack.  See: <ul>
    <li><a href="http://dev.i2p.net/pipermail/i2p/2005-October/001073.html">Summary</a></li>
    <li><a href="http://dev.i2p.net/pipermail/i2p/2005-October/001064.html">Reasoning</a></li>
    </ul></li>
</ul>