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<pre>
1) <a href="#tunnel.overview">Tunnel overview</a>
2) <a href="#tunnel.operation">Tunnel operation</a>
2.1) <a href="#tunnel.preprocessing">Message preprocessing</a>
2.2) <a href="#tunnel.gateway">Gateway processing</a>
2.3) <a href="#tunnel.participant">Participant processing</a>
2.4) <a href="#tunnel.endpoint">Endpoint processing</a>
2.5) <a href="#tunnel.padding">Padding</a>
2.6) <a href="#tunnel.fragmentation">Tunnel fragmentation</a>
2.7) <a href="#tunnel.alternatives">Alternatives</a>
2.7.1) <a href="#tunnel.nochecksum">Don't use a checksum block</a>
2.7.2) <a href="#tunnel.reroute">Adjust tunnel processing midstream</a>
2.7.3) <a href="#tunnel.bidirectional">Use bidirectional tunnels</a>
2.7.4) <a href="#tunnel.smallerhashes">Use smaller hashes</a>
3) <a href="#tunnel.building">Tunnel building</a>
3.1) <a href="#tunnel.peerselection">Peer selection</a>
3.2) <a href="#tunnel.request">Request delivery</a>
3.3) <a href="#tunnel.pooling">Pooling</a>
4) <a href="#tunnel.throttling">Tunnel throttling</a>
5) <a href="#tunnel.mixing">Mixing/batching</a>
</pre>
<h2>1) <a name="tunnel.overview">Tunnel overview</a></h2>
<p>Within I2P, messages are passed in one direction through a virtual
tunnel of peers, using whatever means are available to pass the
message on to the next hop. Messages arrive at the tunnel's
gateway, get bundled up for the path, and are forwarded on to the
next hop in the tunnel, which processes and verifies the validity
of the message and sends it on to the next hop, and so on, until
it reaches the tunnel endpoint. That endpoint takes the messages
bundled up by the gateway and forwards them as instructed - either
to another router, to another tunnel on another router, or locally.</p>
<p>Tunnels all work the same, but can be segmented into two different
groups - inbound tunnels and outbound tunnels. The inbound tunnels
have an untrusted gateway which passes messages down towards the
tunnel creator, which serves as the tunnel endpoint. For outbound
tunnels, the tunnel creator serves as the gateway, passing messages
out to the remote endpoint.</p>
<p>The tunnel's creator selects exactly which peers will participate
in the tunnel, and provides each with the necessary confiruration
data. They may vary in length from 0 hops (where the gateway
is also the endpoint) to 9 hops (where there are 7 peers after
the gateway and before the endpoint). It is the intent to make
it hard for either participants or third parties to determine
the length of a tunnel, or even for colluding participants to
determine whether they are a part of the same tunnel at all
(barring the situation where colluding peers are next to each other
in the tunnel). Messages that have been corrupted are also dropped
as soon as possible, reducing network load.</p>
<p>Beyond their length, there are additional configurable parameters
for each tunnel that can be used, such as a throttle on the size or
frequency of messages delivered, how padding should be used, how
long a tunnel should be in operation, whether to inject chaff
messages, whether to use fragmentation, and what, if any, batching
strategies should be employed.</p>
<p>In practice, a series of tunnel pools are used for different
purposes - each local client destination has its own set of inbound
tunnels and outbound tunnels, configured to meet its anonymity and
performance needs. In addition, the router itself maintains a series
of pools for participating in the network database and for managing
the tunnels themselves.</p>
<p>I2P is an inherently packet switched network, even with these
tunnels, allowing it to take advantage of multiple tunnels running
in parallel, increasing resiliance and balancing load. Outside of
the core I2P layer, there is an optional end to end streaming library
available for client applications, exposing TCP-esque operation,
including message reordering, retransmission, congestion control, etc.</p>
<h2>2) <a name="tunnel.operation">Tunnel operation</a></h2>
<p>Tunnel operation has four distinct processes, taken on by various
peers in the tunnel. First, the tunnel gateway accumulates a number
of tunnel messages and preprocesses them into something for tunnel
delivery. Next, that gateway encrypts that preprocessed data, then
forwards it to the first hop. That peer, and subsequent tunnel
participants, unwrap a layer of the encryption, verifying the
integrity of the message, then forward it on to the next peer.
Eventually, the message arrives at the endpoint where the messages
bundled by the gateway are split out again and forwarded on as
requested.</p>
<h3>2.1) <a name="tunnel.preprocessing">Message preprocessing</a></h3>
<p>When the gateway wants to deliver data through the tunnel, it first
gathers zero or more I2NP messages (no more than 32KB worth),
selects how much padding will be used, and decides how each I2NP
message should be handled by the tunnel endpoint, encoding that
data into the raw tunnel payload:</p>
<ul>
<li>2 byte unsigned integer specifying the # of padding bytes</li>
<li>that many random bytes</li>
<li>a series of zero or more { instructions, message } pairs</li>
</ul>
<p>The instructions are encoded as follows:</p>
<ul>
<li>1 byte value:<pre>
bits 0-1: delivery type
(0x0 = LOCAL, 0x01 = TUNNEL, 0x02 = ROUTER)
bit 2: delay included? (1 = true, 0 = false)
bit 3: fragmented? (1 = true, 0 = false)
bit 4: extended options? (1 = true, 0 = false)
bits 5-7: reserved</pre></li>
<li>if the delivery type was TUNNEL, a 4 byte tunnel ID</li>
<li>if the delivery type was TUNNEL or ROUTER, a 32 byte router hash</li>
<li>if the delay included flag is true, a 1 byte value:<pre>
bit 0: type (0 = strict, 1 = randomized)
bits 1-7: delay exponent (2^value minutes)</pre></li>
<li>if the fragmented flag is true, a 4 byte message ID, and a 1 byte value:<pre>
bits 0-6: fragment number
bit 7: is last? (1 = true, 0 = false)</pre></li>
<li>if the extended options flag is true:<pre>
= a 1 byte option size (in bytes)
= that many bytes</pre></li>
<li>2 byte size of the I2NP message</li>
</ul>
<p>The I2NP message is encoded in its standard form, and the
preprocessed payload must be padded to a multiple of 16 bytes.</p>
<h3>2.2) <a name="tunnel.gateway">Gateway processing</a></h3>
<p>After the preprocessing of messages into a padded payload, the gateway
encrypts the payload with the eight keys, building a checksum block so
that each peer can verify the integrity of the payload at any time, as
well as an end to end verification block for the tunnel endpoint to
verify the integrity of the checksum block. The specific details follow.</p>
<p>The encryption used is such that decryption
merely requires running over the data with AES in CTR mode, calculating the
SHA256 of a certain fixed portion of the message (bytes 16 through $size-288),
and searching for that hash in the checksum block. There is a fixed number
of hops defined (8 peers after the gateway) so that we can verify the message
without either leaking the position in the tunnel or having the message
continually "shrink" as layers are peeled off. For tunnels shorter than 9
hops, the tunnel creator will take the place of the excess hops, decrypting
with their keys (for outbound tunnels, this is done at the beginning, and for
inbound tunnels, the end).</p>
<p>The hard part in the encryption is building that entangled checksum block,
which requires essentially finding out what the hash of the payload will look
like at each step, randomly ordering those hashes, then building a matrix of
what each of those randomly ordered hashes will look like at each step.
To visualize this a bit:</p>
<table border="1">
<tr><td colspan="2"></td>
<td><b>IV</b></td><td><b>Payload</b></td>
<td><b>eH[0]</b></td><td><b>eH[1]</b></td>
<td><b>eH[2]</b></td><td><b>eH[3]</b></td>
<td><b>eH[4]</b></td><td><b>eH[5]</b></td>
<td><b>eH[6]</b></td><td><b>eH[7]</b></td>
<td><b>V</b></td>
</tr>
<tr><td rowspan="2"><b>peer0</b><br /><font size="-2">key=K[0]</font></td><td><b>recv</b></td>
<td>IV[0]</td><td>P[0]</td>
<td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>
<td>V[0]</td>
</tr>
<tr><td><b>send</b></td>
<td rowspan="2">IV[1]</td><td rowspan="2">P[1]</td>
<td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2">H(P[1])</td>
<td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td>
<td rowspan="2">V[1]</td>
</tr>
<tr><td rowspan="2"><b>peer1</b><br /><font size="-2">key=K[1]</font></td><td><b>recv</b></td>
</tr>
<tr><td><b>send</b></td>
<td rowspan="2">IV[2]</td><td rowspan="2">P[2]</td>
<td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td>
<td rowspan="2"></td><td rowspan="2"></td><td rowspan="2">H(P[2])</td><td rowspan="2"></td>
<td rowspan="2">V[2]</td>
</tr>
<tr><td rowspan="2"><b>peer2</b><br /><font size="-2">key=K[2]</font></td><td><b>recv</b></td>
</tr>
<tr><td><b>send</b></td>
<td rowspan="2">IV[3]</td><td rowspan="2">P[3]</td>
<td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td>
<td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2">H(P[3])</td>
<td rowspan="2">V[3]</td>
</tr>
<tr><td rowspan="2"><b>peer3</b><br /><font size="-2">key=K[3]</font></td><td><b>recv</b></td>
</tr>
<tr><td><b>send</b></td>
<td rowspan="2">IV[4]</td><td rowspan="2">P[4]</td>
<td rowspan="2">H(P[4])</td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td>
<td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td>
<td rowspan="2">V[4]</td>
</tr>
<tr><td rowspan="2"><b>peer4</b><br /><font size="-2">key=K[4]</font></td><td><b>recv</b></td>
</tr>
<tr><td><b>send</b></td>
<td rowspan="2">IV[5]</td><td rowspan="2">P[5]</td>
<td rowspan="2"></td><td rowspan="2"></td><td rowspan="2">H(P[5])</td><td rowspan="2"></td>
<td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td>
<td rowspan="2">V[5]</td>
</tr>
<tr><td rowspan="2"><b>peer5</b><br /><font size="-2">key=K[5]</font></td><td><b>recv</b></td>
</tr>
<tr><td><b>send</b></td>
<td rowspan="2">IV[6]</td><td rowspan="2">P[6]</td>
<td rowspan="2"></td><td rowspan="2">H(P[6])</td><td rowspan="2"></td><td rowspan="2"></td>
<td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td>
<td rowspan="2">V[6]</td>
</tr>
<tr><td rowspan="2"><b>peer6</b><br /><font size="-2">key=K[6]</font></td><td><b>recv</b></td>
</tr>
<tr><td><b>send</b></td>
<td rowspan="2">IV[7]</td><td rowspan="2">P[7]</td>
<td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td>
<td rowspan="2"></td><td rowspan="2">H(P[7])</td><td rowspan="2"></td><td rowspan="2"></td>
<td rowspan="2">V[7]</td>
</tr>
<tr><td rowspan="2"><b>peer7</b><br /><font size="-2">key=K[7]</font></td><td><b>recv</b></td>
</tr>
<tr><td><b>send</b></td>
<td>IV[8]</td><td>P[8]</td>
<td></td><td></td><td></td><td></td><td>H(P[8])</td><td></td><td></td><td></td>
<td>V[8]</td>
</tr>
</table>
<p>In the above, P[8] is the same as the original data being passed through the
tunnel (the preprocessed messages), and V[8] is the SHA256 of eH[0-7] as seen on
peer7 after decryption. For
cells in the matrix "higher up" than the hash, their value is derived by encrypting
the cell below it with the key for the peer below it, using the end of the column
to the left of it as the IV. For cells in the matrix "lower down" than the hash,
they're equal to the cell above them, decrypted by the current peer's key, using
the end of the previous encrypted block on that row.</p>
<p>With this randomized matrix of checksum blocks, each peer will be able to find
the hash of the payload, or if it is not there, know that the message is corrupt.
The entanglement by using CTR mode increases the difficulty in tagging the
checksum blocks themselves, but it is still possible for that tagging to go
briefly undetected if the columns after the tagged data have already been used
to check the payload at a peer. In any case, the tunnel endpoint (peer 7) knows
for certain whether any of the checksum blocks have been tagged, as that would
corrupt the verification block (V[8]).</p>
<p>The IV[0] is a random 16 byte value, and IV[i] is the first 16 bytes of
H(D(IV[i-1], K[i-1])). We don't use the same IV along the path, as that would
allow trivial collusion, and we use the hash of the decrypted value to propogate
the IV so as to hamper key leakage.</p>
<h3>2.3) <a name="tunnel.participant">Participant processing</a></h3>
<p>When a participant in a tunnel receives a message, they decrypt a layer with their
tunnel key using AES256 in CTR mode with the first 16 bytes as the IV. They then
calculate the hash of what they see as the payload (bytes 16 through $size-288) and
search for that hash within the decrypted checksum block. If no match is found, the
message is discarded. Otherwise, the IV is updated by decrypting it and replacing it
with the first 16 bytes of its hash. The resulting message is then forwarded on to
the next peer for processing.</p>
<h3>2.4) <a name="tunnel.endpoint">Endpoint processing</a></h3>
<p>When a message reaches the tunnel endpoint, they decrypts and verifies it like
a normal participant. If the checksum block has a valid match, the endpoint then
computes the hash of the checksum block itself (as seen after decryption) and compares
that to the decrypted verification hash (the last 32 bytes). If that verification
hash does not match, the endpoint takes note of the tagging attempt by one of the
tunnel participants and perhaps discards the message.</p>
<p>At this point, the tunnel endpoint has the preprocessed data sent by the gateway,
which it may then parse out into the included I2NP messages and forwards them as
requested in their delivery instructions.</p>
<h3>2.5) <a name="tunnel.padding">Padding</a></h3>
<p>Several tunnel padding strategies are possible, each with their own merits:</p>
<ul>
<li>No padding</li>
<li>Padding to a random size</li>
<li>Padding to a fixed size</li>
<li>Padding to the closest KB</li>
<li>Padding to the closest exponential size (2^n bytes)</li>
</ul>
<p><i>Which to use? no padding is most efficient, random padding is what
we have now, fixed size would either be an extreme waste or force us to
implement fragmentation. Padding to the closest exponential size (ala freenet)
seems promising. Perhaps we should gather some stats on the net as to what size
messages are, then see what costs and benefits would arise from different
strategies?</i></p>
<h3>2.6) <a name="tunnel.fragmentation">Tunnel fragmentation</a></h3>
<p>For various padding and mixing schemes, it may be useful from an anonymity
perspective to fragment a single I2NP message into multiple parts, each delivered
seperately through different tunnel messages. The endpoint may or may not
support that fragmentation (discarding or hanging on to fragments as needed),
and handling fragmentation will not immediately be implemented.</p>
<h3>2.7) <a name="tunnel.alternatives">Alternatives</a></h3>
<h4>2.7.1) <a name="tunnel.nochecksum">Don't use a checksum block</a></h4>
<p>One alternative to the above process is to remove the checksum block
completely and replace the verification hash with a plain hash of the payload.
This would simplify processing at the tunnel gateway and save 256 bytes of
bandwidth at each hop. On the other hand, attackers within the tunnel could
trivially adjust the message size to one which is easily traceable by
colluding external observers in addition to later tunnel participants. The
corruption would also incur the waste of the entire bandwidth necessary to
pass on the message. Without the per-hop validation, it would also be possible
to consume excess network resources by building extremely long tunnels, or by
building loops into the tunnel.</p>
<h4>2.7.2) <a name="tunnel.reroute">Adjust tunnel processing midstream</a></h4>
<p>While the simple tunnel routing algorithm should be sufficient for most cases,
there are three alternatives that can be explored:</p>
<ul>
<li>Delay a message within a tunnel at an arbitrary hop for either a specified
amount of time or a randomized period. This could be achieved by replacing the
hash in the checksum block with e.g. the first 16 bytes of the hash, followed by
some delay instructions. Alternately, the instructions could tell the
participant to actually interpret the raw payload as it is, and either discard
the message or continue to forward it down the path (where it would be
interpreted by the endpoint as a chaff message). The later part of this would
require the gateway to adjust its encryption algorithm to produce the cleartext
payload on a different hop, but it shouldn't be much trouble.</li>
<li>Allow routers participating in a tunnel to remix the message before
forwarding it on - bouncing it through one of that peer's own outbound tunnels,
bearing instructions for delivery to the next hop. This could be used in either
a controlled manner (with en-route instructions like the delays above) or
probabalistically.</li>
<li>Implement code for the tunnel creator to redefine a peer's "next hop" in
the tunnel, allowing further dynamic redirection.</li>
</ul>
<h4>2.7.3) <a name="tunnel.bidirectional">Use bidirectional tunnels</a></h4>
<p>The current strategy of using two seperate tunnels for inbound and outbound
communication is not the only technique available, and it does have anonymity
implications. On the positive side, by using separate tunnels it lessens the
traffic data exposed for analysis to participants in a tunnel - for instance,
peers in an outbound tunnel from a web browser would only see the traffic of
an HTTP GET, while the peers in an inbound tunnel would see the payload
delivered along the tunnel. With bidirectional tunnels, all participants would
have access to the fact that e.g. 1KB was sent in one direction, then 100KB
in the other. On the negative side, using unidirectional tunnels means that
there are two sets of peers which need to be profiled and accounted for, and
additional care must be taken to address the increased speed of predecessor
attacks. The tunnel pooling and building process outlined below should
minimize the worries of the predecessor attack, though if it were desired,
it wouldn't be much trouble to build both the inbound and outbound tunnels
along the same peers.</p>
<h4>2.7.4) <a name="tunnel.smallerhashes">Use smaller hashes</a></h4>
<p>At the moment, the plan is to reuse the existing SHA256 code and build
all of the checksum and verification hashes as 32 byte SHA256 values. 20
byte SHA1 would likely be more than sufficient, and perhaps smaller.</p>
<h2>3) <a name="tunnel.building">Tunnel building</a></h2>
<h3>3.1) <a name="tunnel.peerselection">Peer selection</a></h3>
<h3>3.2) <a name="tunnel.request">Request delivery</a></h3>
<h3>3.3) <a name="tunnel.pooling">Pooling</a></h3>
<h2>4) <a name="tunnel.throttling">Tunnel throttling</a></h2>
<h2>5) <a name="tunnel.mixing">Mixing/batching</a></h2>