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Weakness In Internet Security Uncovered

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Researchers in California and at Centrum Wiskunde & Informatica (CWI) in the Netherlands, EPFL in Switzerland, and Eindhoven University of Technology (TU/e) in the Netherlands have discovered a weakness in the Internet digital certificate infrastructure. The defect may allow hackers to create false "certificates" that are fully trusted by all commonly used web browsers.

The flaw makes it possible to impersonate secure websites and email servers and to perform virtually undetectable phishing attacks. So visiting secure websites is not safe. The results were presented at the 25C3 security congress in Berlin on the 30th of December and the experts hope to increase the adoption of more secure cryptographic standards on the Internet and therewith increase the safety of the internet.

When you visit a website whose URL starts with "https", a small padlock symbol appears in the browser window. This indicates that the website is secured using a digital certificate issued by one of a few trusted Certification Authorities (CAs). To ensure that the digital certificate is legitimate, the browser verifies its signature using standard cryptographic algorithms. The team of researchers has discovered that one of these algorithms, known as MD5, can be misused.

In 2004 at the annual cryptology conference "Crypto", weakness in the MD5 algorithm was presented by a team of Chinese researchers. They successfully completed a "collision attack" and were able to create two different messages with the same digital signature. The Chinese researcher’s attack was limited, and so a much stronger collision construction was announced by the researchers from CWI, EPFL and TU/e in May 2007. Their method showed that it was possible to have almost complete freedom in the choice of both messages. The team of researchers has now discovered that it is possible to create a rogue certification authority (CA) that is trusted by all major web browsers by using an advanced implementation of the collision construction and a cluster of more than 200 commercially available game consoles.

The demonstration shows that a critical part of the Internet’s infrastructure is not safe. A rogue CA, in combination with known weaknesses in the DNS (Domain Name System) protocol, can open the door for virtually undetectable phishing attacks.

For example, without being aware of it, users could be redirected to malicious sites that appear exactly the same as the trusted banking or e-commerce websites they believe to be visiting. The web browser could then receive a forged certificate that will be erroneously trusted, and users’ passwords and other private data can fall in the wrong hands. Besides secure websites and email servers, the weakness also affects other commonly used software.

"The major browsers and Internet players – such as Mozilla and Microsoft – have been contacted to inform them of our discovery and some have already taken action to better protect their users," reassures Arjen Lenstra, head of EPFL’s Laboratory for Cryptologic Algorithms. "To prevent any damage from occurring, the certificate we created had a validity of only one month – August 2004 – which expired more than four years ago. The only objective of our research was to stimulate better Internet security with adequate protocols that provide the necessary security."

According to the researchers, their discovery shows that MD5 can no longer be considered a secure cryptographic algorithm for use in digital signatures and certificates. Currently MD5 is still used by certain certificate authorities to issue digital certificates for a large number of secure websites. "Theoretically it has been possible to create a rogue CA since the publication of our stronger collision attack in 2007," says cryptanalyst Marc Stevens (CWI). "It’s imperative that browsers and CAs stop using MD5, and migrate to more robust alternatives such as SHA-2 and the upcoming SHA-3 standard," insists Lenstra.