Date: Wed, 8 Sep 2010 11:24:36 -0500
From: Andrew Auernheimer <gluttony@...il.com>
To: Full Disclosure <full-disclosure@...ts.grok.org.uk>
Subject: [GOATSE SECURITY] Clench: Goatse's way to say
 "screw you" to certificate authorities

A GOATSE SECURITY RELEASE
Application layer authentication-inherent validation of public key
integrity without the use of a trusted third party
Andrew Auernhemer and Jordan Borges.

More readable version w/ reference links available here:
http://security.goatse.fr/clench-our-way-of-saying-screw-you-to-ssl-pki-forever

This is the initial unveiling of the pre-alpha version of Clench,
Goatse Security’s new simple password-based authentication mechanism
that rids most organizations of a need to rely upon an untrustable
third party to ensure against man-in-the-middle attacks.

un-tl;dr abstract: SSL is broken. Certificate authorities only exist
to let the US, Chinese, Turkish, Brazilian etc etc government or
Russian mob spy on you (whichever is interested first). Well, I guess
they also exist to line the pockets of assholes who want $10-50 for
pushing a button. Luckily, we’ve remedied this! We’ve established a
way that a client, using only standard password authentication, can
validate a server’s public key and ensure that no third party is
listening (without the use of a trusted third party such as a
certificate authority or manual fingerprint verification). Read on for
a wonderfully simple hack and proof of concept code!

Biggest problem we solve: “Trusted” third parties can’t be trusted and
criminals or hostile governments are free to launch man in the middle
attacks. Extensive research in this area has been done by by
Marlinspike, Dan Kaminsky and Mike Zusman which you really should
read.

    * Exploiting web application and business logic vulnerabilities of
certificate authorities allow the generation of valid certificates for
arbitrary domains you do not own.
    * The number of entities allowed to issue certificates is now
massive and not being effectively tracked– criminal organizations
outright buy certificate authorities and print certificates for
arbitrary domains they do not own.
    * Do you trust the Chinese government not to snoop on your
traffic? What about the United States government? Your browser trusts
both– either of them can snoop on your “secure” HTTPS sessions.
    * EV SSL is a joke.
    * Who watches the watchers?

The whole PKI architecture is broken and cannot be safely relied upon.
Any system of authentication which relies on a “trusted” third party
that you have no dominion over is flawed. DNSSEC is only an
incremental improvement with the same underlying flaw– I may trust the
ICANN, ISC, NIST, NTIA, the Department of Homeland Security, or
VeriSign more than the combined ineptitude and maliciousness of every
current SSL CA, but I still don’t trust them. The whole idea of a
trust anchor is fallacious.

We set out to solve this problem in a way that can reconcile three
realities of security:

    * Users cannot effectively comprehend anything but password
authentication. They don’t understand key management, and the task of
getting hundreds of thousands or millions of users to install a client
certificate or generate a keypair (and not accidentally reveal the
private key) is a Herculean task that few IT departments want to try.
    * Users cannot be trusted to manually verify fingerprints.
Seriously, they just won’t. Even the ones that perceive themselves as
sophisticated and security-conscious.
    * The network is now many times more hostile and open to attack
than the server.

So we had to create an authentication mechanism in which a user inputs
only a username and password, but ends up with a session immune from
man-in-the-middle attacks without the use of a trusted third party.

We looked at various authentication schemes. Of note was
Livejournal’s. Being the most popular blogging site in Russia, they
got user auth details jacked so many times by shady spammers jacking
routes with “legitimate” certs that they stopped transmitting the hash
plaintext over the wire. They give a nonce to the client and the
client hashes locally and then sends the hash to the server. The other
source of inspiration was the Socialist Millionaire protocol, which
Off-the-Record Messaging utilizes with a shared secret to verify
cryptographic fingerprints.

In almost every scenario where security is mission critical, you
already have a shared secret from the outset. When an enterprise user
is given their new account, or when someone sets up online banking at
their bank branch they are given a temporary password. A temporary
password is a shared secret, and shared secrets may be leveraged to
verify fingerprint information.

HERE'S HOW CLENCH WORKS:

   1. Client connects to server and sends hello.
   2. Server sends hello back, along with its cert.
   3. Standard Diffie-Helman key exchange happens in SSL/TLS/SSH
fashion. Initial handshake is finished, cypher spec is changed, now
here comes the magic:
   4. Server sends client the nonce value [ Ticks since unix epoch +
16 bytes of random data ]
   5. Client sends userid/username to server.
   6. Client types in password, but password is not sent to server.
Both sides generate a hash.
      Client generates y, a hash of  [ client password + server's
public key, as client sees it + nonce from step 4 ]
      Server generates x, a hash of [ client password + server's own
public key + nonce from step 4 ]
   7. Client and server use a symmetric and fair zero-knowledge proof
to verify that we both have the same hash without revealing the value
of the hash to one another. Imagine a two pan scale, and a secret of a
given weight of marbles in a bag. If we both place our bag of marbles
on the pans at the same time, if they come to an equilibrium we will
have verified our shared secret without revealing it to one another.
      For more information on this step please see the excellent paper
“A Fair and Efficient Solution to the Socialist Millionaires’
Problem“.
   8. Upon successful completion of the proof, the server allows the
session to proceed.

This method of authentication avoids nearly all of the current
pitfalls for current authentication schemes.  Things that cannot be
done against our authentication mechanism:

    * Cracking a hash from the wire: No hash is revealed!
    * Man in the middle attacks. Impossible– compared shared secret is
incorporating the server’s public key. If there’s an attacker in the
middle, client’s hash value y will be built with the wrong public key
and thus will fail zero-knowledge proof comparison.
    * Replay attacks. Underlying hash for proof is not revealed, and
nonced anyways.
    * Brute force of Socialist Millionaire protocol with a preloaded
Rainbow Table: This is doubly impossible.
         1. Even if (x – y) could be extrapolated, the nonce is built
with 16 bytes of random data. At least 2-3 bytes of the ticks value
are hard to predict, ending up with (18 + password length) bytes of
random data results in rainbow table size that is unfeasible to build.
         2. (x – y) cannot be extrapolated by brute force, as a new
nonce is built for every authentication attempt, and  potentially
meaningful data from (x – y) cannot be generated without multiple
attempts on the same shared secret value.

This can be accomplished in either the session layer or the
application layer– the latter allowing easy implementation atop
current infrastructure with trivial changes to clients.

THE ROADMAP FROM HERE:

There’s some barriers to implementation on this for HTTPS. Firstly,
there needs to be a javascript function that returns the current
public key (or at least the fingerprint) of the https server called to
load the page in the current DOM. There also needs to be some
mechanism in the GUI of the browser that can’t be mimicked by an
attacker to inform the client that the current login form implements a
Clench-like authentication mechanism. Because obviously an attacker
can just rewrite the form if they’re MitMing and trick the user into
sending plaintext.

SSH can be implemented much faster, it’d just need a PAM module.

"Hey, wait a second, doesn’t the passphrase have to be stored in
plaintext on the server?"
Well, possibly yes. The perception of this as being an insurmountable
flaw is largely the result of fallacious decisions in SysV in 1988.
They were good decisions at the time due to the fact that it was
ludicrously easy to break the security of a server then. However, the
network has become far more hostile than the server. There are two
major ways of ensuring  the safety of a plaintext password data store:

    * The authenticating server needn’t actually have the password in
plaintext, it merely needs access to a more device that has it. A
secure hashing device can be implemented on a PIC/Atmel/Xilinx– it’s
job is to generate the nonce, give it to the server with a cookie,
then when the server responds back with a userid and the cookie it
hashes together the nonce, the client’s password (which only the
secure device has access to) and the server’s known public key, taken
from a whitelist. It then passes the hash to the server. It is
trivially easy to build a device in hardware which can only provide
nonces, cookies and hashes and write new passwords without ever giving
stored passwords up in plaintext, and disallows reprogramming to do
anything otherwise. Or if not a hardware device, perhaps a
grsec-hardened machine running managed code with no network stack
exposed, doing a similar transaction raw over serial port or
Infiniband. If my sole goal of a machine is to hash things and keep a
file secure, I can confidently make it bulletproof without risk of
compromise.
      The potential implementations highlighted above are in
development, and will be aired at the first opportunity (provided I
evade unjust imprisonment, lol).
    * Make your users make two passphrases– the first of which will be
stored in plaintext, to assure no MitM, the second of which will be
stored shadowed as normal in case of server’s compromise. Telling
grandma she needs two different passwords to use her bank account is a
lot easier than teaching her to install a client certificate.

"Hey, there’s no way to tell the difference between an attempt at
man-in-the-middle and a mistyped password!"
With a user-specified password, no. If your initial shared secret has
a checksum or LUN check in it, however, the client can notify the user
of a potentially mistyped password.

"Okay, I’m tired of reading your shit. Where’s the codes?"
Rough implementation here: http://weev.datavibe.net/GoatseClench.tgz
Enjoy, and know that a meatier paper is on its way (provided I evade
unjust imprisonment long enough to do the peer review process for a
journal or conference) if you want to read this in academic tripe
format.

-weev
Analyst, Goatse Security

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