OTCP - Obfuscated TCP (11 Mar 2008)

Like open SMTP relays, TCP was developed in a kinder, gentler time. With Comcast forging RST packets to disrupt connections and UK ISPs looking to trawl the clickstreams of a nation and sell them (not to mention AT&T copying their backbone to the NSA) it's time that TCP got a little more paranoid.

The 'correct' solutions are something along the lines of IPSec, but there's no reason to suspect that anyone is going to start using that in droves any time soon. Application level crypto (TLS, SSH etc) is the correct solution for protecting the contents of packets (which would stop the clickstream harvesting style of attacks), but cannot protect the TCP layer (and HTTPS is still not the default for websites).

An opportunistic obfuscation layer, on by default, would start to address this. By making it transparent to use, it stands a chance of getting some small fraction of traffic to use it. If it were included in Linux distribution kernels we might hope to see it in the wild after a year or so. In certain sectors (BitTorrent users and trackers) we might see it much sooner.

Our attacker has a couple of weaknesses:

With that in mind I'm going to suggest the following:

SYN packets from OTCP hosts include an empty TCP option advertising their support. OTCP servers, upon seeing the offer in the RST packet, generate a random 64-bit number (n), less than a globally known prime and return 2^n mod p in another TCP option in the SYN,ACK. The client performs the end of a DH handshake and includes its random number in a third option in the next packet to the server.

The two hosts now have a shared key which they can use to MAC and encrypt each packet in the subsequent connection (the MAC will be carried in a TCP option). The MAC function includes the TCP header and payload, except the source and destination port numbers. The encryption only covers the TCP payload, not the IP nor TCP packet.

The hash function and cipher need to very fast and just strong enough; the key is only 64-bits. MD4 for the hash function and AES128 for the cipher, say. (benchmarks for different functions from the Crypto++ library). I suspect that the cipher needs to be a block cipher because packets get retransmitted and reordered. A block cipher in CTR mode based on the sequence number seems to be the best way to deal with this.

A getsockopt interface would allow userland to find out if a given connection is OTCP, and to get the shared key.

Q: Can't this be broken by man-in-the-middle attacks?

Yes. However, note that this would require interception of traffic which is much more costly than sniffers in parallel and legally more troublesome for the attacker. Additionally, userland crypto protocols could be extended to include the shared secret in their certified handshakes, thus giving them MITM-proof security which includes the TCP layer.

Q: Isn't the key size very small?

Yes. However, even if the key could be brute forced in 10 seconds; that's still far too much work for a device which is monitoring hundreds or thousands of connections per second.

Q: Doesn't this break NATs

NATs rewrite the IP addresses and port numbers in the packets, which we don't include in our MAC protection, so everything should work. If the NAT happens to rebuild the whole packet, the OTCP offer in the SYN packet will be removed. In this case we loose OTCP but, most importantly, we don't break any users.

NATs which monitor the application level and try to rewrite IP address in there will be broken by this. However, the number of protocols which do this is small and clients may be configured by default not to offer OTCP when the destination port number matches one of these protocols (IRC and FTP spring to mind). This is a hack, but the downside to users of OTCP must be as small as possible.

Q: So can't I break this by filtering the offer from the SYN packet?

Yes. Application level protocols could be extended to sense this downgrade attack and stop working, but mostly see the points above: it's much more expensive to do this since it needs to be done in the router and it's legally more troublesome for the attacker.

Q: Won't this take too much time?

It's additional CPU load, certainly. The Crypto++ and OpenSSL benchmarks suggest that a full core should be able to handle this at 1 Gbps. Most servers don't see anything like that traffic. Maybe more concerning is the DDoS possibility of using OTCP to force a server to do a 64-bit modexp with a single, unauthenticated packet. A very quick knock-up using the OpenSSL BN library suggests that a single Core2@2.33GHz can do about 50000 random generations and modexps per second. Since the keys are so small, I expect that a tuned implementation (using registers, not bigints) would be about 10x faster. You probably run out of bandwidth from all the SYNs before 500,000 SYNs per second second maxes a single core (it's about 37MB/s). So SYN floods shouldn't be any more of a problem.

Q: What about my high-performance network?

I suggest that offering OTCP be disabled by default for private address ranges. Also, distributions probably won't turn it on for their "server" releases. If all else fails, it'll be a sysctl.

Q: But then I'm wasting CPU time and packet space whenever I'm running SSH or HTTPS

Right. Userland can turn off OTCP using a sockopt if it wishes, or it could just not enable itself for the default destination ports which these protocols use. (Again, that would be an ugly intrusion of default port numbers into the kernel, but this idea wasn't that beautiful to begin with.)

Q: So, what's the plan?