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first draft of secure semireliable UDP protocol

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<code>$Id$</code>
<h1>Secure Semireliable UDP (SSU)</h1>
<b>DRAFT</b>
<p>
The goal of this protocol is to provide secure and authenticated
semireliable, undordered message delivery, exposing only a minimal
amount of data easily discernable to third parties. It should
support high degree communication as well as TCP-friendly congestion
control, and may include PMTU detection. It should be capable of
efficiently moving bulk data at rates sufficient for home users.
In addition, it should support techniques for addressing network
obstacles, like most NATs or firewalls.</p>
<h2><a name="addressing">Addressing and introduction</a></h2>
<p>To contact an ESU peer, one of two sets of information is necessary:
a direct address, for when the peer is publicly reachable, or an
indirect address, for using a third party to introduce the peer.
There is no restriction on the number of addresses a peer may have.</p>
<pre>
Direct: udp://host:port/introKey
Indirect: udp://tag@relayhost:port/relayIntroKey/targetIntroKey
</pre>
<p>These introduction keys are delivered through an external channel
and must be used when establishing a session key. For the indirect
address, the peer must first contact the relayhost and ask them for
an introduction to the peer known at that relayhost under the given
tag. If possible, the relayhost sends a message to the addressed
peer telling them to contact the requesting peer, and also gives
the requesting peer the IP and port on which the addressed peer is
located. In addition, the peer establishing the connection must
already know the public keys of the peer they are connecting to (but
not necessary to any intermediary relay peer).</p>
<h2><a name="header">Header</a></h2>
<p>All UDP datagrams begin with a MAC and an IV, followed by a variable
size payload encrypted with the appropriate key. The MAC used is
HMAC-SHA256, truncated to 16 bytes, while the key is a full AES256
key. The specific construct of the MAC is the first 16 bytes from:</p>
<pre>
HMAC-SHA256(payload, HMAC-SHA256(IV || payloadLength, key))
</pre>
<p>The payload itself is AES256/CBC encrypted with the IV and the key,
with replay prevention addressed within its body, explained below.</p>
<h2><a name="payload">Payload</a></h2>
<p>Within the AES encrypted payload, there is a minimal common structure
to the various messages - a one byte flag and a four byte sending
timestamp (*seconds* since the unix epoch). The flag byte contains
the following bitfields:</p>
<pre>
bits 0-3: payload type
bit 4: rekey?
bit 5: extended options included
bits 6-7: reserved
</pre>
<p>If the rekey flag is set, 32 bytes of keying material follow the
timestamp. If the extended options flag is set, a one byte option
size value is appended to, followed by that many extended option
bytes, which are currently uninterpreted.</p>
<p>When rekeying, the keying material is fed into a SHA256 to produce
the new key, though that key is not immediately used. The other
side should also reply with the rekey flag set and that same keying
material. Once both sides have sent and received those values, the
new key should be used and the previous key discarded. It may be
useful to keep the old key around briefly, to address packet loss
and reordering.</p>
<pre>
Header: 37+ bytes
+----+----+----+----+----+----+----+----+
| MAC |
| |
+----+----+----+----+----+----+----+----+
| IV |
| |
+----+----+----+----+----+----+----+----+
|flag| time | (optionally |
+----+----+----+----+----+ |
| this may have 32 byte keying material |
| and/or a one+N byte extended options) |
+---------------------------------------|
</pre>
<h2><a name="messages">Messages</a></h2>
<h3><a name="sessionRequest">SessionRequest (type 0)</a></h3>
<table border="1">
<tr><td align="right" valign="top"><b>Peer:</b></td>
<td>Alice to Bob</td></tr>
<tr><td align="right" valign="top"><b>Data:</b></td>
<td><ul>
<li>256 byte X, to begin the DH agreement</li>
<li>1 byte IP address size</li>
<li>that many byte representation of Bob's IP address</li>
<li>N bytes, currently uninterpreted (later, for challenges)</li>
</ul></td></tr>
<tr><td align="right" valign="top"><b>Key used:</b></td>
<td>introKey</td></tr>
</table>
<pre>
+----+----+----+----+----+----+----+----+
| X, as calculated from DH |
| |
. . .
| |
+----+----+----+----+----+----+----+----+
|size| that many byte IP address (4-16) |
+----+----+----+----+----+----+----+----+
| arbitrary amount |
| of uninterpreted data |
. . .
| |
+----+----+----+----+----+----+----+----+
</pre>
<h3><a name="sessionCreated">SessionCreated (type 1)</a></h3>
<table border="1">
<tr><td align="right" valign="top"><b>Peer:</b></td>
<td>Bob to Alice</td></tr>
<tr><td align="right" valign="top"><b>Data:</b></td>
<td><ul>
<li>256 byte Y, to complete the DH agreement</li>
<li>1 byte IP address size</li>
<li>that many byte representation of Alice's IP address</li>
<li>2 byte port number (unsigned, big endian 2s complement)</li>
<li>0-15 pad bytes to reach the 16 byte boundary</li>
<li>4 byte relay tag which Alice can publish (else 0x0)</li>
<li>40 byte DSA signature of the critical exchanged data</li>
<li>N bytes, currently uninterpreted (later, for challenges)</li>
</ul></td></tr>
<tr><td align="right" valign="top"><b>Key used:</b></td>
<td>introKey for the data through the pad bytes, and the
sessionKey for the DSA signature</td></tr>
</table>
<pre>
+----+----+----+----+----+----+----+----+
| Y, as calculated from DH |
| |
. . .
| |
+----+----+----+----+----+----+----+----+
|size| that many byte IP address (4-16) |
+----+----+----+----+----+----+----+----+
| Port (A)| (pad to 16 byte boundary) |
+----+----+----+----+----+----+----+----+
| public relay tag | DSA signature |
+----+----+----+----+ |
| |
| |
| |
| |
+ +----+----+----+----+
| | arbitrary amount |
+----+----+----+----+ |
| of uninterpreted data |
+----+----+----+----+----+----+----+----+
</pre>
<h3><a name="sessionConfirmed">SessionConfirmed (type 2)</a></h3>
<table border="1">
<tr><td align="right" valign="top"><b>Peer:</b></td>
<td>Bob to Alice</td></tr>
<tr><td align="right" valign="top"><b>Data:</b></td>
<td><ul>
<li>1 byte identity fragment info:<pre>
bits 0-3: current identity fragment #
bits 4-7: total identity fragments</pre></li>
<li>N byte fragment of Alice's identity, sent over a number
of messages.</li>
<li>on the last identity fragment, the last 40 bytes contain
the DSA signature of the critical exchanged data</li>
</ul></td></tr>
<tr><td align="right" valign="top"><b>Key used:</b></td>
<td>sessionKey</td></tr>
</table>
<pre>
<b>Fragment 1 through N-1</b>
+----+----+----+----+----+----+----+----+
|info| fragment of Alice's full |
+----+ |
| identity keys |
. . .
| |
+----+----+----+----+----+----+----+----+
<b>Fragment N:</b>
+----+----+----+----+----+----+----+----+
|info| fragment of Alice's full |
+----+ |
| identity keys |
. . .
| |
+----+----+----+----+----+----+----+----+
| arbitrary amount of uninterpreted |
| data, up from the end of the |
| identity key to 40 bytes prior to |
| end of the current packet |
+----+----+----+----+----+----+----+----+
| DSA signature |
| |
| |
| |
| |
+----+----+----+----+----+----+----+----+
</pre>
<h3><a name="relayRequest">RelayRequest (type 3)</a></h3>
<table border="1">
<tr><td align="right" valign="top"><b>Peer:</b></td>
<td>Alice to Bob</td></tr>
<tr><td align="right" valign="top"><b>Data:</b></td>
<td><ul>
<li>4 byte relay tag</li>
<li>1 byte IP address size</li>
<li>that many byte representation of Bob's IP address</li>
<li>1 byte IP address size</li>
<li>that many byte representation of Alice's IP address</li>
<li>2 byte port number (of Alice)</li>
<li>1 byte challenge size</li>
<li>that many bytes to be relayed to Charlie in the intro</li>
<li>N bytes, currently uninterpreted</li>
</ul></td></tr>
<tr><td align="right" valign="top"><b>Key used:</b></td>
<td>introKey (or sessionKey, if Alice/Bob is established)</td></tr>
</table>
<pre>
+----+----+----+----+----+----+----+----+
| relay tag |size| that many |
+----+----+----+----+----+ +----|
| bytes making up Bob's IP address |size|
+----+----+----+----+----+----+----+----+
| that many bytes making up Alice's IP |
+----+----+----+----+----+----+----+----+
| Port (A)|size| that many challenge |
+----+----+----+ |
| bytes to be delivered to Charlie |
+----+----+----+----+----+----+----+----+
| arbitrary amount of uninterpreted data|
+----+----+----+----+----+----+----+----+
</pre>
<h3><a name="relayResponse">RelayResponse (type 4)</a></h3>
<table border="1">
<tr><td align="right" valign="top"><b>Peer:</b></td>
<td>Bob to Alice</td></tr>
<tr><td align="right" valign="top"><b>Data:</b></td>
<td><ul>
<li>1 byte IP address size</li>
<li>that many byte representation of Charlie's IP address</li>
<li>2 byte port number</li>
<li>1 byte IP address size</li>
<li>that many byte representation of Alice's IP address</li>
<li>2 byte port number</li>
<li>N bytes, currently uninterpreted</li>
</ul></td></tr>
<tr><td align="right" valign="top"><b>Key used:</b></td>
<td>introKey (or sessionKey, if Alice/Bob is established)</td></tr>
</table>
<pre>
+----+----+----+----+----+----+----+----+
|size| that many bytes making up |
+----+ +----+----+
| Charlie's IP address | Port (C)|
+----+----+----+----+----+----+----+----+
|size| that many bytes making up |
+----+ +----+----+
| Alice's IP address | Port (A)|
+----+----+----+----+----+----+----+----+
| arbitrary amount of uninterpreted data|
+----+----+----+----+----+----+----+----+
</pre>
<h3><a name="relayIntro">RelayIntro (type 5)</a></h3>
<table border="1">
<tr><td align="right" valign="top"><b>Peer:</b></td>
<td>Bob to Charlie</td></tr>
<tr><td align="right" valign="top"><b>Data:</b></td>
<td><ul>
<li>1 byte IP address size</li>
<li>that many byte representation of Alice's IP address</li>
<li>2 byte port number (of Alice)</li>
<li>1 byte challenge size</li>
<li>that many bytes relayed from Alice</li>
<li>N bytes, currently uninterpreted</li>
</ul></td></tr>
<tr><td align="right" valign="top"><b>Key used:</b></td>
<td>sessionKey</td></tr>
</table>
<pre>
+----+----+----+----+----+----+----+----+
|size| that many bytes making up |
+----+ +----+----+
| Charlie's IP address | Port (C)|
+----+----+----+----+----+----+----+----+
|size| that many bytes of challenge |
+----+ |
| data relayed from Alice |
+----+----+----+----+----+----+----+----+
| arbitrary amount of uninterpreted data|
+----+----+----+----+----+----+----+----+
</pre>
<h3><a name="data">Data (type 6)</a></h3>
<table border="1">
<tr><td align="right" valign="top"><b>Peer:</b></td>
<td>Any</td></tr>
<tr><td align="right" valign="top"><b>Data:</b></td>
<td><ul>
<li>1 byte flags:<pre>
bit 0: explicit ACKs included
bit 1: explicit NACKs included
bit 2: numACKs included
bits 3-4: reserved for congestion control
bit 5: want reply
bits 6-7: reserved</pre></li>
<li>if explicit ACKs are included:<ul>
<li>a 1 byte number of ACKs</li>
<li>that many 4 byte MessageIds being fully ACKed</li>
</ul></li>
<li>if explicit NACKs are included:<ul>
<li>a 1 byte number of NACKs</li>
<li>that many 4 byte MessageIds + 1 byte fragmentNum NACKs</li>
</ul></li>
<li>if numACKs included:<ul>
<li>a 1 byte number for how many messages were fully
received in the last minute.</li></ul></li>
<li>1 byte number of fragments</li>
<li>that many message fragments:<ul>
<li>4 byte messageId</li>
<li>1 byte fragment info:<pre>
bits 0-4: fragment #
bit 5: isLast (1 = true)
bits 6-7: unused</pre></li>
<li>2 byte fragment size</li>
<li>that many bytes</li>
<li>1 byte fragment size</li></ul>
<li>N bytes padding, uninterpreted</li>
</ul></td></tr>
<tr><td align="right" valign="top"><b>Key used:</b></td>
<td>sessionKey</td></tr>
</table>
<pre>
+----+----+----+----+----+----+----+----+
|flag| (additional headers, determined |
+----+ |
| by the flags, such as ACKs, NACKs, or |
| simple rate of full ACKs) |
+----+----+----+----+----+----+----+----+
|#frg| messageId |info|fragSize |
+----+----+----+----+----+----+----+----+
| that many bytes of fragment data |
. . .
| |
+----+----+----+----+----+----+----+----+
| messageId |info|fragSize | |
+----+----+----+----+----+----+----+ |
| that many bytes of fragment data |
. . .
| |
+----+----+----+----+----+----+----+----+
| messageId |info|fragSize | |
+----+----+----+----+----+----+----+ |
| that many bytes of fragment data |
. . .
| |
+----+----+----+----+----+----+----+----+
| arbitrary amount of uninterpreted data|
+----+----+----+----+----+----+----+----+
</pre>
<h2><a name="keys">Keys</a></h2>
<p>All encryption used is AES256/CBC with 16 byte keys and 16 byte IVs.
When using the introKey, both the initial message and any subsequent
reply use the introKey of the responder (Bob) - the responder does
not need to know the introKey of the requestor (Alice). The DSA
signing key used by Bob should already be known to Alice when she
contacts him, though Alice's DSA key may not already be known by
Bob.</p>
<p>Upon receiving a message, the receiver checks the from IP address
with any established sessions - if there is one or more matches,
those session keys are tested sequentially in the HMAC. If none
of those verify or if there are no matching IP addresses, the
receiver tries their introKey in the MAC. If that does not verify,
the packet is dropped. If it does verify, it is interpreted
according to the message type, though if the receiver is overloaded,
it may be dropped anyway.</p>
<p>If Alice and Bob have an established session, but Alice loses the
key for some reason and she wants to contact Bob, she may at any
time simply establish a new session through the SessionRequest and
related messages. If Bob has lost the key but Alice does not know
that, she will first attempt to prod him to reply, by sending a
DataMessage with the wantReply flag set, and if Bob continually
fails to reply, she will assume the key is lost and reestablish a
new one.</p>
<p>For the DH key agreement,
<a href="http://www.faqs.org/rfcs/rfc3526.html">RFC3526</a> 2048bit
MODP group (#14) is used:</p>
<pre>
p = 2^2048 - 2^1984 - 1 + 2^64 * { [2^1918 pi] + 124476 }
g = 2
</pre>
<p>The DSA p, q, and g are shared according to the scope of the
identity which created them.</p>
<h2><a name="messageSequences">Message sequences</a></h2>
<h3><a name="establishDirect">Connection establishment (direct)</a></h3>
<pre>
Alice Bob
SessionRequest---------------------&gt;
&lt;---------------------SessionCreated
SessionConfirmed-------------------&gt;
SessionConfirmed-------------------&gt;
SessionConfirmed-------------------&gt;
SessionConfirmed-------------------&gt;
&lt;--------------------------Data
</pre>
<h3><a name="establishIndirect">Connection establishment (indirect)</a></h3>
<pre>
Alice Bob Charlie
RelayRequest ----------------------&gt;
&lt;--------------RelayResponse RelayIntro-----------&gt;
&lt;--------------------------------------------Data (ignored)
SessionRequest--------------------------------------------&gt;
&lt;--------------------------------------------SessionCreated
SessionConfirmed------------------------------------------&gt;
SessionConfirmed------------------------------------------&gt;
SessionConfirmed------------------------------------------&gt;
SessionConfirmed------------------------------------------&gt;
&lt;---------------------------------------------------Data
</pre>
<h2><a name="sampleDatagrams">Sample datagrams</a></h2>
<b>Minimal data message (no fragments, no ACKs, no NACKs, etc)</b><br />
<i>(Size: 39 bytes)</i>
<pre>
+----+----+----+----+----+----+----+----+
| MAC |
| |
+----+----+----+----+----+----+----+----+
| IV |
| |
+----+----+----+----+----+----+----+----+
|flag| time |flag|#frg| |
+----+----+----+----+----+----+----+ |
| padding to fit a full AES256 block |
+----+----+----+----+----+----+----+----+
</pre>
<b>Minimal data message with payload</b><br />
<i>(Size: 46+fragmentSize bytes)</i>
<pre>
+----+----+----+----+----+----+----+----+
| MAC |
| |
+----+----+----+----+----+----+----+----+
| IV |
| |
+----+----+----+----+----+----+----+----+
|flag| time |flag|#frg|
+----+----+----+----+----+----+----+----+
messageId |info| fragSize| |
+----+----+----+----+----+----+ |
| that many bytes of fragment data |
. . .
| |
+----+----+----+----+----+----+----+----+
</pre>
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