That earth-shattering NSA crypto-cracking: Have spooks smashed RC4?

Ageing cipher at heart of HTTPS and VPNs fingered by experts


Analysis Fresh revelations from whistleblower Edward Snowden suggest that the NSA can crack TLS/SSL connections, the widespread technology securing HTTPS websites and virtual private networks (VPNs).

Although reports from the New York Times and its allied publications held off on the specifics, it may all mean that US spooks can reliably crack RC4, a popular encryption cipher, security experts fear.

As reported last night, the NSA and GCHQ are able to foil basic safeguards that supposedly ensure privacy on the web, allowing spooks to peek inside the encrypted contents of VPN traffic, online banking and shopping, and more. No wonder Blighty's intelligence agents shrugged off Brits' attempts to hide their data in SSL.

The UK and US governments can probably compromise HTTPS connections by gaining access to the root certificates and encryption keys, exploiting backdoors in equipment and algorithms, or otherwise allow the signals intelligence agencies to run man-in-the-middle attacks on encrypted traffic. GCHQ is alleged to have broken the security on some 30 VPN systems, and has plans to get into 300 by 2015.

The NSA's highly classified Bullrun programme allows the agencies to inspect data sniffed from submarine cables, and might involve, at least in part, collaboration with unnamed technology companies.

Tough ciphers, such as 256-bit AES, remain unbroken by the g-men, it's hoped. Snowden himself famously said "encryption works. Properly implemented strong crypto systems are one of the few things that you can rely on".

What the NSA appears to have done is circumvent or nobble the software and hardware that underpin widely used encryption systems, rather than all-out breaking the mathematical foundations of modern-day cryptography.

However, the agency's unspecified "groundbreaking cryptanalytic capabilities" could include a practical attack on RC4.

"Most major SSL-enabled websites use RC4, which was designed in 1987. This NSA crypto story should be a wake up call for the tech industry," said Christopher Soghoian, principal technologist and senior policy analyst at the ACLU in an update to his personal Twitter account.

Encryption guru Bruce Schneier concurred that an attack against RC4 was at least a more than plausible theory. "I don't know one way or the other, but that's a good speculation," Schneier said in blog post.

RC4 is an ageing but still widely used stream cipher that's often used as a component of Transport Layer Security (TLS) and Secure Sockets Layer (SSL) protocols of HTTPS to protect sensitive web traffic from snooping. Security researchers have shown data encrypted by the algorithm can be carefully analysed to silently extract information, such as an authentication cookie used to log into a victim's Gmail account.

An attack develop by security researchers at Royal Holloway, University of London and University of Illinois at Chicago and unveiled back in March relies on statistical flaws in the keystream generated by the RC4 algorithm. It relies on getting a victim to open a web page containing malicious JavaScript code that repeatedly tries to log into Google's Gmail, for example. This allows an attacker to get hold of a bulk of traffic needed to perform cryptanalysis.

This particular attack requires at least 16,777,216 captured sessions but it's easy to imagine that code breakers at the NSA and GCHQ are far further ahead of the game and have come up with a far more elegant, and therefore practical, attack.

RC4 was invented by Ron Rivest in 1987. Various attacks have been developed against RC4, allowing determined hackers to break the encryption, but the technology is still widely used - it's also used in Wi-Fi WEP protection.

It's understood about 50 per cent of all TLS traffic is protected using RC4 and its use is, if anything, growing after the discovery of various attacks (including BEAST and Lucky 13) against Cipher-block Chaining (CBC), a mode of encryption used by TLS.


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