The attack is the latest to expose serious fractures in the system that virtually all online entities use to protect data from being intercepted over insecure networks and to prove their website is authentic rather than an easily counterfeited impostor. Over the past few years, Moxie Marlinspike and other researchers have documented ways of obtaining digital certificates that trick the system into validating sites that can't be trusted.
Earlier this month, attackers obtained digital credentials for Google.com and at least a dozen other sites after breaching the security of disgraced certificate authority DigiNotar. The forgeries were then used to spy on people in Iran accessing protected GMail servers.
By contrast, Duong and Rizzo say they've figured out a way to defeat SSL by breaking the underlying encryption it uses to prevent sensitive data from being read by people eavesdropping on an address protected by the HTTPs prefix.
“BEAST is different than most published attacks against HTTPS,” Duong wrote in an email. “While other attacks focus on the authenticity property of SSL, BEAST attacks the confidentiality of the protocol. As far as we know, BEAST implements the first attack that actually decrypts HTTPS requests.”
Duong and Rizzo are the same researchers who last year released a point-and-click tool that exposes encrypted data and executes arbitrary code on websites that use a widely used development framework. The underlying “cryptographic padding oracle” exploited in that attack isn't an issue in their current research.
Instead, BEAST carries out what's known as a plaintext-recovery attack that exploits a vulnerability in TLS that has long been regarded as mainly a theoretical weakness. During the encryption process, the protocol scrambles block after block of data using the previous encrypted block. It has long been theorized that attackers can manipulate the process to make educated guesses about the contents of the plaintext blocks.
If the attacker's guess is correct, the block cipher will receive the same input for a new block as for an old block, producing an identical ciphertext.
At the moment, BEAST requires about two seconds to decrypt each byte of an encrypted cookie. That means authentication cookies of 1,000 to 2,000 characters long will still take a minimum of a half hour for their PayPal attack to work. Nonetheless, the technique poses a threat to millions of websites that use earlier versions of TLS, particularly in light of Duong and Rizzo's claim that this time can be drastically shortened.
In an email sent shortly after this article was published, Rizzo said refinements made over the past few days have reduced the time required to under 10 minutes.