Black Ops of TCP/IP 2011 (Black Hat USA 2011)

Black Ops Of TCP/IP 2011
Dan Kaminsky, Chief Scientist, DKH

Intro
I’m Dan Kaminsky
I write code
Not here to fix authentication
Working on that
Not here to make DNSSEC scale
Working on that too

What I’m here for
Return to form
As a community, we’ve sort of stopped looking at network security
Mapping networks
Evading firewalls
Subverting design assumptions
This is probably the right thing – looking at attacks:
Acquire Beachhead
SQLi the web front end
PDF the client backend
Lilypad
Use acquired credentials to break everything else
Netsec is only so relevant in such an environment

So?
We’re going to look into it anyway. Maybe we’ll find something interesting.

BitCoin
“BitCoin turns nerd forums into libertarian forums”
It’s infected everything else in nerddom, why not this talk?
What is it?
Attempt at making a digital currency with no central bank
A system with economic properties I don’t know anything about
An overlay network upon the Internet that people think has certain properties

BitCoin In A Nutshell
Built on doing three things
TRANSFER: “I Alice, give Bob 2.1 BTC”
Alice signs the declaration to Bob’s public key
GOSSIP: “Heh everyone! Did you hear that Alice gave Bob 2.1 BTC?”
Alice sends that declaration into a peer to peer network that gossips the change
APPEND: “Everyone, the official registry of transactions should now include Alice paying Bob, Charlie paying David, and so on.”
This is gossiped too
Requires solving a problem so hard, it takes the world 10 minutes for someone to do it
If it takes less than 10 minutes, it’s not hard enough
Crypto lets you make things hard enough
Solving the problem gives you 50 BTC (today) to Transfer

The Truth Of Bitcoin
…this is not my BitCoin talk 
Go to dankaminsky.com for a more detailed deck
BitCoin is actually really impressive
Entire classes of bugs are just missing
The first five times you think you understand it, you don’t
BitCoin has fixed almost all flaws that aren’t forced by the design

The Main Flaws (there are a few more)
Does not scale
Totally not anonymous

Scalability (from BitCoin’s Own Wiki)
Bandwidth
“Let's assume an average rate of 2000tps, so just VISA…. Shifting 60 gigabytes of data in, say, 60 seconds means an average rate of 1 gigabyte per second, or 8 gigabits per second.”
CPU
”A network node capable of keeping up with VISA would need roughly 50 cores + whatever is used for mining (done by separate machines/GPUs).”
Storage
“ A 3 terabyte hard disk costs less than $200 today and will be cheaper still in future, so you'd need one such disk for every 21 days of operation (at 1gb per block).”

OK, so you end up with supernodes and normal nodes
What are the characteristics of supernodes?
They’re banks
“Welcome to the new boss, who looks suspiciously like the old boss”
I’m not saying banks are bad or anything
The “peer to peer” model of BitCoin eventually goes away; as soon as the thing gets big, the entire thing switches to a banking model
With all the elements of banking people think BitCoin is immune to, without necessarily the properties people like
However, until then…

An Interesting Question
Travis Goodspeed: “Heh Dan, any chance BitCoin can be used as a samizdat service?”
Samizdat (Russian: самиздат; Russian pronunciation: [səmᵻˈzdat]) was a key form of dissidentactivity across the Soviet bloc in which individuals reproduced censored publications by hand and passed the documents from reader to reader.
An old challenge
The Internet is usually about sending data, ephemerally
Can we use it to store data, indefinitely?
Well, if BitCoin is eventually going to require a 3TB HD every 21 days…and is going to need to keep that data forever…

Len.
Our community recently lost one of its shining lights
If one executes:
strings --bytes=20 ~/.bitcoin/blk0001.dat
Strings extracts human readable text from any blob of data
Usually used to find hardcoded interesting stuff in executables, like default passwords
The block database of all transactions ever pushed into BitCoin, run through a filter that extracts all human readable text from the (presently) 450MB file…

BitLen

…and just because it would have made Len laugh

How This Works
In BitCoin, Alice gives money to Bob by issuing a sort of challenge
“Whoever can sign a message with the public key that hashes to the following bytes, may claim this money.”
Well, bytes are bytes
Instead of pushing the hash of a public key (20 bytes), we push 20 characters of a testimonial

Side Effect
This does cost BTC
About 1.0BTC in total
There’s minimums to transferring money
This does destroy the money
The network thinks somewhere, there must be a public key with a hash of “Len was our friend.”
I am OK with this.
It is the cyber equivalent of pouring one out for your homies.

Can we get higher bandwidth?
BitCoin lets you send money to a public key directly, rather than its hash
10x increase from 20 bytes to 200 bytes
This is not a bug
BitCoin allows for extra data in a signature

Signature Expansion
BitCoin works with small programs
The program from the receiver is: “Put this signature and public key on a stack”
The program from the sender is: “Take the signature and public key off the stack and make sure they’re good.”
The receiver can put extra stuff on the stack, and yes, it still works just fine
This is in fact a bug that is visible purely from being pedantic about the English Language

Illicit Signature Expansion
Signatures can’t cover themselves
Chicken and Egg
So signatures also don’t cover the presence or absence of additional data within themselves
Block appending does cover additional data
But there is time between when transactions are first created and emitted, and when they’re included in a block append
So it turns out anyone can add additional data to an otherwise valid transaction

Limited Usefulness
If you’re just some random relay, gossiping the information, you have to compete with the real version
Transaction fees limit you to about 1KB of embed per 0.01 BTC (14 cents)
This does not apply to you if you generate the signature with extra data, because then you can pay fees
This does not apply to you if you calculate the block – you can include as much as you want, up to present 2MB limit, and force everyone else to carry
Still better than 20 bytes per 0.01 BTC 
Yes, Travis, bitcoinfs is totally possible

What about Anonymity? Looking at blockexplorer.com
Transaction Sources: These are all the same ID
Transaction Dst’s:One of these IDs is (likely) all of the IDs on the left

Graphs (from Reid/Harrigan)

Problem: Linking pseudonyms isn’t enough
Reid/Harrigan get lucky
One BitCoin source publishes the IPs it gives money to
Another user posted to a forum seeking donations to a linked ID
They’re linking pseudonyms within BC, but they’re not linking to IP via out of band processes
The published audit trail is noisy and deniable
“Naturally, much of this analysis is circumstantial. We cannot say for certain whether or not these ﬂows imply a shared agency in both incidents. There is always the possibility of drawing false inferences.”
Is there another source of data?

P2P!
There are two sources of transaction information in BitCoin
The “blocks” that have been set in stone
The “loose transactions” waiting to be merged into blocks
These (effectively) always refer to a single identity
Both are “gossiped” around the network
Big relay race; Alice tells Bob and Charlie, Bob tells David and Eric, Charlie tells Frank and Gary

Subverting the Relay Race
An attacker can just connect to every node in the public cloud at once
“But that could take 50,000 connections!”
Yeah, we can do that in Python now. Kernels don’t suck anymore (well as much).
When you’re connected to every node, the first node to inform you of a transaction is the source of it
“Done relay it because done done it”
More or less true, and absolutely over time
(Bonus: You can accelerate your own transactions, by relaying them to everyone yourself)
BlitCoin – accelerated probing of BitCoin

Discovering Nodes
Just scan the Internet on 8333/TCP
Join the IRC channels!
#bitcoin, #bitcoin00 to #bitcoin99 on LFNET
BitBot
Recursively ask every node about every other node it knows about
“get_addr” message
Can start from hardcoded seeds

Statement From Gavin Andreson, lead dev on BitCoin
Bitcoin transactions are more private than credit card or PayPal transactions, but are less private than physical-world cash transactions. Unless you are very careful in the way you use Bitcoin (and you have the technical know-how to use it with other anonymizing technologies like Tor or i2p), you should assume that a persistent, motivated attacker will be able to associate your IP address with yourbitcoin transactions.

What about Tor?
Tor indeed obfuscates IPs derived from outbound connections
It does nothing if you’re still listening on 8333/tcp and somebody sweeps the net for you
Bug filed in BitCoin to shut off listener when operating through Tor

What about unreachable nodes?
Most are behind NAT, and only connect via outbound links
The active inbound set is only 3000-8000 nodes
So, you just create 3000-8000 nodes and you’re half the gossip network
Probably only need a few hundred, since each node will collect ~7 peers and you only need one

Just how unreachable are they?
Many users are behind wireless routers
Routers implement NAT – outbound is easy, inbound is hard
“Poor Man’s Firewall”
Don’t mock it, it was more effective than real firewalling when it came out
Most home routers implement UPNP – Universal Plug And Play
UPNP allows nodes inside your network to ask the router to open up ports from the Internet
BitCoin now supports doing this by default
…but even if it didn’t…

How UPNP is supposed to work
Internal hosts send a multicast message out via SSDP (Simple Service Discovery Protocol)
1900/UDP M-SEARCH Multicast
Internal UPNP nodes – media players, routers, etc – respond w/ endpoints that can be twiddled via web services requests
1900/UDP NOTIFY Unicast
Responses are sometimes just flooded out, in the absence of M-SEARCH
SSDP NOTIFY messages are supposed to contain a randomized URL for UPNP messages to go to

Question
UPNP is supposed to only work on internal interfaces
“Hello Router, please let the outside world in.”
It would be tragic if routers listened to UPNP on external interface as well..
“Hello Router, please let the outside world (read: me) in”

Stats (and yes, Scanrand’s coming back)

More Stats
Not all listeners on 2869 are fully open
Would require fixed UPNP endpoints, instead of the randomized ones Microsoft uses
Many verified listeners though
Hundreds of thousands to millions
Entire countries have standardized NATs that are vulnerable

Your Princess Is In Another Castle
Turns out there’s a speaker talking at DEFCON about just this very subject!
I’m a little more careful about independent rediscoveries now 
Daniel Garcia found that there were open UPNP endpoints on the net last year
Track 3, Friday, 17:00
ArmijnHemelalso did some great work
upnp-hacks.org
Also noted that sometimes UPNP was exposed to outside world, back in ~2007
Still true, unfixed

What about outside the consumer space?
Corporate environments
Less about BitCoin and UPNP
More about web services and ACLs
Are there ways past corporate ACLs?
Access Control Lists
“Access to this IP is constrained to the following range”

Ye Olde Trick
IP Spoofing
Just pretend to be a source IP vaguely near the target, and you’ll probably pass ACLs
“But BCP’s!”
Real world, IP spoofing is not hard, as long as you’re not virtualized
IP spoofing – the one thing the cloud isn’t very good for

Is IP Spoofing Still Effective?
Sure! Let me just pull this DNS trick out of the archive…
Generate a query for “$RANDOM.attacker-domain.com”
Send query to all IPs on a network, from various IPs that network might trust
x.1.1.1 -> x.1.100.8
x.1.100.1 -> x.1.100.8
Response will go back to IP you don’t control – but first, the server will try to resolve $RANDOM.attacker-domain.com – from you!
(Yes, this was another way to exploit that bug.)
Granted, this only works for an obscure application like DNS and UDP…certainly nothing built on TCP

Understanding The Limits Of IP Spoofing
Most modern protocols run over TCP, a reliable communication protocol
1) Alice sends Bob a SYN, containing a random sequence number
2) Bob replies with a SYN|ACK, containing both Alice’s sequence number, and his own sequence number
3) Alice replies to Bob with an ACK, containing both sequence numbers
Data can be sent now
Sequence numbers become a sort of “password” for all future traffic
If Alice spoofs her IP, she doesn’t see Bob’s sequence number, so she can’t complete step 3

Sequence Numbers Didn’t Used To Be Random
Obviously if you can guess a sequence number, you can blindly inject into sessions
So, make them random?
Problem: Connections are identified by source port, dest port, source IP, and dest IP
24.1.2.3:50000 -> 4.2.2.1:53
Sometimes, ports are recycled from one connection to the next
What if a packet arrives from an old connection? It could look like it belongs in the new one!
Fix this by having random sequence numbers, unless id is the same, then we go sequential in time to maximize distance

Example
RFC1948
F(srcip, dstip, srcport, dstport,secret)+time
F==MD5
F(1.2.3.4,2.3.4.5,50000,80,”abcd”)==12341234time==1111seq==12341234+1111==12342345
F(1.2.3.4,2.3.4.6,50000,80,”abcd”)==89991121time==2000seq==89991121+2000==89992121
F(1.2.3.4,2.3.4.5,50000,80,”abcd”)==12341234time==5000seq==12341234+5000==12346234

A Problem: Memory
What if somebody just floods us with connection attempts?
They don’t have to remember all of our “passwords”
They don’t even need to use their own IP addresses
We need to remember all of theirs
This is a SYN flood, and it’s old as dirt

Solution: SYN Cookies
Specified (if not invented) by Dan Bernstein in 1999
Finally on by default in Linux in 2008
The “password” turns into a challenge
“If you can send this back to me, I’ll accept your data”
Uses 3/4ths of the sequence number (24 bits) to store the hash of a secret and the four tuple, 5 bits for time, three bits for connection metadata
5 bits is exposed to everyone publicly, 3 bits don’t matter, so there’s 24 bits of security

Alas
Average of 8 million packets to bypass SYN cookies
May be less, due to fudge factors
Of course DJB knew this 
“No matter what function is used, the attacker will succeed in a connection forgery after millions of random ACK packets.”
But it’s a different reality than 1999
Sending 8M packets is easy now, we has the bandwidth
Forged connections have arbitrary sources
They get through your ACLs
They can contain arbitrary Web Services payloads
Definitely REST, maybe SOAP

Are you safe if you disable SYN cookies?
Well, not on Linux
Linux is RFC 1948 compliant for the lower 24 bits
Uses MD4, but still
Upper 8 bits?
Counter, starting at 0, increments every five minutes
Sequential
Shared between inbound and outbound connections
So, you send a query from your actual IP once or twice to find the offset, and blindly spoof a SYN and a payload-containing ACK
After 8M tries, you win

Impact on RST attacks
Tony Watson, “Slipping In The Window”
Noticed that only one 32 bit “password” was required for Resets (RSTs)
Noticed that the “password” only had to be in the “window” of valid data that could sequentially be sent
Window describes how many bytes a sender is allowed to transmit without a receiver acknowledging
Noticed that the “window” wasn’t even limited to 16 bits; was being expanded 5-8 bits more from “Window Scaling”
32-16-8 = 8 bits = 128 packets to kill a session on average
New possibility: 32 - 16 – 8 – 8 = 0 bits = 1 packet will always work (assuming full sized window)

Beyond RST: Injection?
RST handlers (usually) only check SEQ# (32 bits)
ACK handlers however check both SEQ# and ACK# (64 bits)
64 – (16 bits from Alice window) – (16 bits from server window) = 32 bits
2B packets for 50%
32 bits – (5 bits from Alice window scaling) – (5 bits from Bob window scaling) = 22 bits
1M packets for 50%
Uh oh
22 bits – (8 bits from Alice predictable high bits) – (8 bits from Bob predictable high bits) = 6 bits
16 packets for 50% 

Difficulty: Ports
Linux randomizes the source port of a new connection by default
You don’t worry about this when you’re doing an ACL bypass, because you control the source port and the dest port
You do have to worry about this when injecting into other sessions though
6 bits (from large windows and high bit disclosure) + 13 bits (port leakage) = 19 bits
250K packets for 50% injection even with port randomization
Note that sometimes a TCP client sets its source port (DNS, BGP)

Status
This is very old code in Linux
Predates the check in history of LinusTorvalds
They’re figuring out the right fix that won’t cause even more problems
There are many potential wrong fixes that are even worse

A Digression
RFC1948 is an interesting construction
Sequential and ordered with the key
Random and unpredictable without
Can participate with either:
Aprivate component (the secret, mixed in with the 4-tuple), able to generate all possible sequence numbers
A public component (a sample sequence number), transmitted over the network, successfully received and retransmitted
Public/private cryptography with nothing but a password?
Clearly this is impossible
Only possible here because of intersection of network security and crypto

To be clear
Passwords are a bad idea
They’re constantly being lost and forgotten and stolen
They are responsible for 50% of compromises
They increasingly look like l33tspeak, and this is not helping
But, supposing we ignore all that…and assuming that we’re stuck with them…

An Old Challenge [0]
How do we use a password to log into a system without that system learning our password?
“We hash it!”
You’re still giving the server your plaintext password, it just isn’t storing it
If salt (random but public prefix) is omitted, attacker can precalculate hash->password database, notice when two users use the same one

An Old Challenge [1]
“We challenge you to hash against it properly”
“Send me the password hashed against $RANDOM”
Digest/NTLM are more advanced versions
Requires server to store plaintext password or password equivalent
“We require knowledge of password to go from keypair to shared session secret”
SPEKE/SRP
Requires both client and server to run fairly obscure code – good luck getting either deployed

So…
Is it possible (NOT ADVISABLE, OBVIOUSLY THIS IS A BAD IDEA) to build a system where the client only remembers a password, but the server:
Stores nothing but a normal public key
Deploys nothing but a standard challenge to make sure the client has the matching private key, derived unilaterally from a password?
In other words…
Can we construct a keypair out of a password?

A Foreboding Question
What vulnerability impacted all asymmetric cryptosystems, be they RSA, DSA, or ECC?

…ok…
Debian
Specifically, a change to the way Debian calculated random numbers in OpenSSL
It always calculated the same numbers 
All asymmetric cryptosystems use entropy as follows:
Collect: Grab random bits
Permute: Alter those bits until they meet certain requirements. Then emit a public/private keypair
Predictable entropy == Predictable keypairs, no matter the algorithm

Uh Oh
What if we turned the Debian bug…into a feature?
Cryptography is all about constructions
We have hash functions, stream ciphers, block ciphers, all of which can be constructed from eachother
Note too this is often a bad idea
We know how to take a password and construct an everlasting stream of psuedorandom numbers from it
“Predictable Entropy”
We can even do so in a way that is Hard, in both CPU time and Memory
scrypt

A TRULY TERRIFYING AND UGLY AND BAD AND AWESOME IDEA
What if you make the output of a password-seeded PRNG, the input to an asymmetric key generator?
You’d end up with 2048 bit RSA keypairs, with a “trapdoor” in the form of a password
This isn’t theoretical

Normal ssh-keygen
# ssh-keygen -f $RANDOM -N "" | grep -i root70:94:3d:4f:c8:c1:1a:a3:88:9a:77:d7:cf:9e:44:2a root
# ssh-keygen -f $RANDOM -N "" | grep -i root69:3e:11:4e:a5:5f:09:12:ac:e2:94:21:c4:3b:40:09 root
# ssh-keygen -f $RANDOM -N "" | grep -i rootd6:7c:3a:c2:d5:ec:84:88:9d:da:81:2b:6f:9a:c3:9b root

ssh-keygen using Phidelius
# LD_PRELOAD=./phidelius.so PH_PASS="hi grandma" ssh-keygen -f $RANDOM -N ""| grep -i rootad:0d:52:2a:72:be:77:e4:b5:ca:83:bb:4f:49:ce:d2 root
# LD_PRELOAD=./phidelius.so PH_PASS="hi grandma" ssh-keygen -f $RANDOM -N "" | grep -i rootad:0d:52:2a:72:be:77:e4:b5:ca:83:bb:4f:49:ce:d2 root

Enter Phidelius
Harry Potter, properly understood, is a story about the epic consequences of losing one’s password.
Fidelius is how passwords fail in the HP universe, so…
Phidelius hooks /dev/random, /dev/urandom, OpenSSL’s Random functions, and a few other tidbits to provide predictable entropy where it isn’t expected
Uses a modified version of scrypt to require ~1 second processing time, and about 256MB of RAM, per crack attempt
No GPU fun for you
Can be seeded with a file as well

What Phidelius Gives You
Generic, multi-application support for predictably generating keypairs from passwords
ssh-keygen for SSH keys
openssl for certificates
Phreebird for DNSSEC keys
Allows message signing, message encryption, client certificate authentication, etc. with nothing but a password
Solves the “log in with a password, without the system learning your password” problem thoroughly, without you having to store anything anywhere
With BitCoin, you could literally give money to the bearer of a word, or a photo.

No pain server side
All time/memory hard requirements are limited to the client – the server just implements completely standard crypto

Primary Issues With Phidelius
The obvious ones
It uses passwords
Passwords tend to be low entropy
The not obvious ones
It’s fragile
An explicit scheme to use a password to seed an RSA key, for instance, fixes parameters like “How sure do we need to be that this number is prime?”
As an implicit scheme, it depends on assumptions that happen to be encoded into a particular version of a particular key generator
It’s hard to salt
All users of the common password “password” have the same public/private keypair!

Salting with Phidelius
Basic idea is that the private key is computed not just from the password, but from the public key as well
The public key is then the carrier of the salt
Works for protocols like SSL, fails for protocols like PGP
Also a good channel of parameters, like “scrypt doesn’t need to use 256MB of RAM”
Can be implemented with no magic code on server, but client needs magic code to embed metadata in public key, and to extract said magic during computation of private

But, to get back to TCP/IP…
Lets talk about one last thing we can do with networks.
We can find biased network policy, no matter how subtle
If biased networks are affecting you, this gives you proof.
If you are biasing your network, this is how proof will come.

The Topology
Link 1
Google.com
Link 2
ISP
Client
Home Router
Microsoft.com
Yahoo.com
Link 3

Understanding the Target
1) “Magic box” is deployed within ISP network, in front of all links
2) Box matches packets to policies, and applies different rules to different packets
Can be stateless – “Do I like this packet?”
Can be stateful – “This packet is part of a flow. Do I like this flow?”
3) Policies can be anything and can do anything
Limit maximum bandwidth
Increase minimum latency
Alter content

The Problem With Subtlety
Say bing.com is 50ms slower than google.com
Is this because of the ISP?
Or is this because google.com has better hosting?
There are many reasons why bing.com might be slower than google.com, granting plausible deniability

Requirement: Normalization
Whether the tester is accessing bing.com or google.com, the network path should be identical (or at least uncorrelated)
We call this normalization
That way, any changes would be the result not of path, but of policy (presumably, and ultimately detectably, at the ISP)

Simple Normalization: HTTP
Policy: “All flows associated with a HTTP request w/ Host: www.bing.com should be delayed by 50ms”
Detector: Configure a single server to accept HTTP requests for www.bing.com, www.google.com, etc.
Then set the client to use it as a proxy server
If traffic from the proxy server is faster for some names, than it is for others, you’ve just detected a HTTP-biased policy!

The Problem
1) This is very protocol dependent
HTTP can be made to do this at low work
Other protocols require lots of work to implement/emulate
2) The policy can always be specific to IP addresses
Sniff DNS to learn which IPs to cover
Doesn’t matter how many hundreds of test servers you have, if policies are only applied to genuine bing.com or google.com servers

The Solution: N00ter
N00ter: The Network Normalization Engine
Start with a VPN
Traffic is pushed from the Client to a Broker
An IP associated with the Broker contacts Servers, who reply to the Broker
The Broker sends traffic back to the Client
Normally, the ISP sees nothing because traffic between Client and Broker is encrypted
Now, instead of encrypting traffic from Broker to Client, send it back to the client
Unencrypted
Spoofed, as if there was no Broker

SPOOFING ALL THE INTERNET
We want the ISP to see our return traffic
We’re trying to trigger the response, that would normally be reserved for Bing/Google, for our normalized test server
Policy engine can’t tell, because we’re impersonating the real entities
Traffic took the same path
Traffic came from the same source
Why else would we see different Quality of Service?

What About Forward Flow?
The policy engine in this scenario doesn’t see traffic from Client to Server
That’s encrypted, VPN style
What if it just didn’t trigger the filtering policy if it didn’t see both sides of the conversation?

ENTER ROTO N00TER
Normal N00ter: Spoof the server to the client
RotoNooter: Spoof the client to the server
Sample A: Client talks directly to the real Google
ISP sees SYN
Sample B: Client talks to real Google by way of Broker, who spoofs the Client. Google replies directly.
ISP does not see SYN
Both samples have the same path!
If they have different performance characteristics, it must be because of the segment of the network that no longer sees client traffic – the ISP!

Catch-22
If ISP applies policy to half-flows, N00ter can differentiate the performance of the spoofed half flow of Google, versus the spoofed half flow from Bing
If ISP applies policy only to full flows, RotoNooter can differentiate the performance of the full flow to and from the real Google, versus the half flow from the real Google
Either way, N00ter Wins
This is the endgame. Biased policies might as well be transparent, because they’re not going to be deniable.

Retaining Full Flows
Suppose you really want the ISP to see bidirectional traffic
Advantage: Triggers all policies. Also, opens up listeners for NATs, that might be inconvenient to get around
Disadvantage: If the ISP sees Client->Server traffic, then the Server sees Client->Server traffic
It may reply, interfere, complain, etc.

Strategy 1: Bad TCP Checksum
Client can tunnel valid traffic to Broker, and push packets with invalid TCP checksums to Server
Advantage: Invalid TCP checksums are ignored. Server won’t interfere. NAT almost certainly won’t check sums; Policy engine might not
Disadvantage: Policy engine could. NAT might fix sums.
Catch-22 with checksums
If policy is disabled when checksums are bad, policy can be proven by having Broker provide steady stream of good sums while ISP sees the bad ones

Strategy 2: Low TTL
Client can send traffic to Server with TTL that causes packets to be dropped in the middle of the Internet
Advantage: Legitimate traffic.
Disadvantage: Policy could note low TTL. Router may drop sessions from ICMP Time Exceeded messages. Sort of a router DoS.
Another Catch-22: Can probably even figure out which hop the policy engine lives at, by when precisely the flow policy shifts

Strategy 3: The Silent Splice
When a TCP stack receives a message not associated with an active socket, it’s supposed to RST
But many servers have firewalls that silently ignore unassociated messages
For Security!
We can have the Client complete a three way handshake with a server, snipe the connection with a RST from the Broker, and then splice a connection between Broker and Server, with what Client (and ISP) think is a connection between Client and Server
Packets from Client to Server will be ignored by server
Packets from Server to Client are actually spoofed by Broker
Policy Engine sees client talking to server. Policy Engine sees server talking to client. You can’t explain that.
100% Perfect Bidirectional Flow

A Bit Of Warning
If you’re passively monitoring network traffic, be aware that these techniques do mean a malicious client can make it look like they’re having a conversation with anyone
Particularly if the server ignores unassociated traffic
Keep complete traffic logs!
Validate checksums
Check TTLs

Where N00ter Is Now
Emulates half flows at present
Very very fast (written to the old LibPaketto code!)
Supports anything that runs over IP
If you want to know whether a network prefers XBOX360 traffic to Playstation 3 traffic, this’ll tell you.
N00ter is extremely neutral – It Just Works
Again, it’s just a VPN that exposes Server->Client traffic in the hopes it’ll get filtered 

Summary
Networks are neat
BitCoin isn’t anonymous
UPNP sometimes exposes itself to the outside world
ACLs can be bypassed using some interesting sequence number tricks and large number of packets
Passwords can be used to seed asymmetric crypto, though they probably shouldn’t
Subtle net neutrality hacks are doomed. Transparency or bust.
Research hosting thanks to N2K of 3Crowd and Doxx of LyonLabs
Anyone want to do some release engineering for me? 

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Black Ops of TCP/IP 2011 (Black Hat USA 2011)

by dakami

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