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Ever wanted to be an Intermediate Certificate Authority?

Speaking at the 25th annual Chaos Communication Conference (25C3) early last week, security researchers demonstrated the first known application of a years-old theoretical attack against the MD5 hashing algorithm used by companies like Verisign and Thawte to issue SSL certificates.

SSL certificates use hash codes generated by a variety of algorithms, including MD5, to verify their issuer’s identity. The hash code is an important feature of public-key cryptography, which SSL is based upon, as it is essential to protecting the secret, private code that CAs use to sign SSL certificates.

By exploiting a weakness specific to hashes generated with the MD5 algorithm – namely, that they are prone to “collisions”, or multiple inputs producing the same output – an attacker could derive a working private key from a single, regular SSL certificate, and then use that key to sign future SSL certificates with the original CA’s signature.

Security experts have known about the possibility for MD5 collisions since at least 2004. Until now, however, the vulnerability was dismissed as a theoretical possibility due to the amount of CPU time needed to attack a single hash for collisions. The 25C3 presenters claim they were able to run the attack in only four weekends, using a network of 200 PlayStation 3 game consoles at a cost of $657.

For about $2,000, said the presenters, an attacker could pull off a similar attack using Amazon’s cloud-computing EC2 service, and the attack would take about a day.

A successful attack would allow attackers to appoint themselves as an Intermediate Certificate Authority, and then generate trusted certificates without having to contact a real CA. The spoofed certificates could then be used to add the appearance of legitimacy to a phishing site designed to steal bank account passwords, for example.

While many CAs have moved on to the more secure SHA-1 or SHA-2 algorithms, a handful of issuers have not. Of the brands still using MD5, the researchers found approximately 97% of those certificates to be signed by Verisign-owned low-cost CA RapidSSL. Other companies using MD5 include FreeSSL, Thawte, and Verisign.co.jp.

Verisign announced that it will replace RapidSSL customers’ certificates free of charge.

“This successful proof of concept shows that the certificate validation performed by browsers can be subverted and malicious attackers might be able to monitor or tamper with data sent to secure websites,” said security researcher Alexander Sotirov, who worked with others from the U.S., the Netherlands, and Switzerland.

Sotirov’s website includes a detailed explanation of the attack, as well as samples of a real certificate and the rogue signing certificate derived from it.

Extended-Validation SSL certificates are immune to the attack due to the fact that they are forbidden from using MD5.

Microsoft reportedly downplayed the threat, noting that the researchers withheld important information that renders the attack “not repeatable”.

A blog post from Verisign’s Tim Callahan says his company applauds the team’s research, noting that their work was so secret that not even Verisign had access to the information before the 25C3 presentation.

Customers holding an MD5-signed SSL certificate will need to contact their CA to acquire and install a new certificate on their servers.



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Just some numbers
By cbmeeks on 1/5/2009 8:00:00 AM , Rating: 1
OK, I bought a new PS3 (40gig) for $399. 200 x $399 = $79,800. Assuming he meant $657,000 like some of you think the op meant, then that would mean $657k / $399 = 1,646 PS3's. Assuming we are talking about the low-end PS3 because for something like this you need CPU more than hard drive (as in, no need for an 80 gig PS3, etc). So where does $657 or $657,000 come into play? Can you really rent 200 PS3's over a weekend? Seems like you would spend two weeks going around to video stores and explaining why you want to rent all of their PS3's.

As far as EC2, I would use the extra-high CPU instance @ $0.80/hour.

So for $2k / 24 hours (day) = 104 instances at 20 virtual compute units would give about 2083 compute units for one day for $2000. Not too bad.

On the other hand, $2k / 24 hours for the small instance (1 compute unit) would give you 833 instances with 833 compute units. So the extra-large instance would be the way to go. For that "$657", you could get 34 extra large instances and have 684 compute units for a day.




RE: Just some numbers
By abscoder on 1/5/2009 11:13:37 AM , Rating: 3
My initial thought was maybe they leased time on someones cluster or on an existing distributed network. <shrug>


RE: Just some numbers
By PrinceGaz on 1/5/2009 2:53:24 PM , Rating: 2
I doubt anyone interested in using this technique to produce a fake certificate would have to pay for the computing power needed as they probably have access to a botnet large enough to complete the task very quickly indeed.


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