Category Archives: Cryptography

SSL/TLS RFC updated against CVE-2009-3555

A solution has been finally brought up to fix CVE-2009-3555 and the temporary solution that broke client authentication.

At least, the IETF agreed on a fix as Marsh Ray informs us, though it will still take some weeks for the whole validation process to complete.

Moreover, as it requires both the servers and the clients to be patched, it will take months before the patches can be applied and one can have a working client authentification architecture. The longest will be the client side, of course, so I feel sorry for those who have a large park to manage.

As far as I am concerned, fortunately, I will just have a few browsers that I manage directly to update. Anyway, still more patience is needed !

OpenSSL : CVE-2009-3555 security fix and mod_ssl client authentication breakage

A security advisory on OpenSSL has recently been published. Details are there and there.

It is vulnerable to a MiTM attack where the attacker can intercept and retrieve the credential to a trusted HTTPS website, by intercepting the session cookie sent back to the client.

A proof of concept of an attack against Twitter was made.

Fine. But so far, the answer was to just disable any renegociation.

This actually causes some issues with SSL session timeout and totally broke client authentication.

I got into problems because of the latter. I am using client authentication for some location of my web server, and I recently could not connect anymore to these with the following log in apache :

[Tue Nov 24 16:56:15 2009] [debug] ssl_engine_kernel.c(1912): OpenSSL:Exit: error in SSLv3 read client hello A
[Tue Nov 24 16:56:15 2009] [error] [client x.x.x.x] Re-negotiation handshake failed: Not accepted by client!?

I first was not aware of the openssl patch and tried almost anything possible. My focus was, of course, on the certificate and the client.
But, a nice guy on IRC #suse, Stittel, had a good hunch and suggested me to look at the CVE-2009-3555 fix.

After more tests, it was quickly confirmed to work well with older versions of OpenSSL (as shipped in Debian Lenny).
Finally, I downgraded the OpenSSL version on my openSUSE box to a version prior to the CVE-2009-3555 fix and it just worked fine.

Then, I dig into it and found a lot of interesting reports there and there. So far it is a real mess.
In short, the breakage will stay as long as browsers don’t also include a patch to avoid renegotiation.
So far, I could not find a browser that does include a patch.
If anyone reading it knows a version that does it, please let me know.

Meanwhile, you have actually the choice between :

  • low security by deactivating client authentication on your server
  • low security by keeping a vulnerable version of OpenSSL

As my server is not very exposed, I chose the latter, but that’s not satisfying.  It is not recommended, but if like me you need to use client authentication with mod_ssl on openSUSE 11.2, do :

% zypper install --from repo-oss openssl openssl-certs libopenssl0_9_8 libopenssl0_9_8-32bit

where repo-oss is the alias to the 11.2 release (without updates) on your system.

What a brutal way to fix an issues without much notification and consideration to the users ! Even the log message is wrong and just confusing the administrator…

PS 1 : thanks again to Stittel for the good hint (I hope you will come by here) and to the always nice and helpful #suse channel in general ;)

PS 2 : bug reported on openSUSE bugzilla

SHA-1 vulnerable : consider SHA-2

Not long after md5, the computation progress has made another victim.

Last week, it was made public that the SHA-1 hash function should be now considered vulnerable. The discovery bring up that the computation to create a collision hash has been dramaticaly reduced. As a consequence, the SHA-1 function can’t warranty anymore the uniquity – and so the integrity – of the hashed object.

There are a few ressources :

This is the natural evolution of cryptography. Stronger cryptography is required against stronger CPU power. In this race, it is now suggested to migrate to SHA-2 whose digest lenghts range from 224 to 512 bits (instead of 160 bits for SHA-1). And until next time, where we will be urged to migrate to SHA-3 ;)

Debian, whose infrastructure was based on SHA-1 (package management, development) is quickly migrating all its keys right now. The procedure the Debian people should follow implies a transition period, where the new key will be signed with the former key, until the latter is revoked. Thus until all peers are updated too, to avoid breaking all the signing infrastruture.

Myself, I ugraded my keys from DSA/1024 bits to RSA/4096 bits (with SHA-2 hashing), so it should be ok for a while. You will find my new key here or linked from the contact page.

MD5 in your SSL certificate ? No need to panic !

MD5 was found vulnerable a few years ago. Recently, a team succeeded in producing a fake CA SSL certificate.

MD5 or SHA-1 is the algorithm used to authenticate the peer in SSL messages. If it gets compromised, and using various combined technics, it becomes possible to do a MiM attack.

But too much noise has been made about it. There is a nice reaction.

Indeed, it still requires a lot of efforts and conditions for the attack to be possible. And the CPU power is still huge : the researchers used not less than a cluster of 200 PS3 to drive the attack. Even with that hardware and engineering, it took until 3 days of intensive computation.

Not everyone can afford it, nor would have much motivation to attack a single user like this.

Security has always been a compromise between usuability and risk. Today, the risk concerning MD5 is still low enough to stop this wind of panic.

Let’s begin the migration to SHA-1 quietly.

How to stop Firefox from prompting for the client certificate

I am using a client certificate to authenticate against some Apache HTTPS website.

By default, Firefox 3 has a very annoying setting : it will prompt you with a box to select your certificate, every time the browser access to a file.

I quickly realized that there is not setting in the preference tab to change this behavior. That sucks, really !

Fortunately, it is possible to tweak it within the about:config page. Set the security.default_personal_cert entry with Select Automatically instead of Ask Every Time.

But what a dumb behavior !

It is like the alert page that Firefox displays every time a self-signed certificate is used. I am now wondering if the developers really understood well what a certificate is !

Setting Firefox properly for Client certificate

Setting Firefox properly for Client certificate

Cold boot attack, not a threat to Full disk encryption (FDE)

Since the new cold boot attack hack is on the news, touching most of the software encryption solutions, I have wondered if it had any chance to concern also hardware encryption.

Hardware encryption is provided by a few laptop makers, generally on high-range an business models.

It has much less performance impact than software encryption, and protect the data independently from your system configuration and its partitions.

Full disk encryption is the so called hardware encryption technology used by Lenovo on my Thinkpad.

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