'Logjam' Vulnerability Threatens Encrypted Connections

An anonymous reader writes: A team of security researchers has revealed a new encryption vulnerability called 'Logjam,' which is the result of a flaw in the TLS protocol used to create encrypted connections. It affects servers supporting the Diffie-Hellman key exchange, and it's caused by export restrictions mandated by the U.S. government during the Clinton administration. "Attackers with the ability to monitor the connection between an end user and a Diffie-Hellman-enabled server that supports the export cipher can inject a special payload into the traffic that downgrades encrypted connections to use extremely weak 512-bit key material. Using precomputed data prepared ahead of time, the attackers can then deduce the encryption key negotiated between the two parties."

Internet Explorer is the only browser yet updated to block such an attack — patches for Chrome, Firefox, and Safari are expected soon. The researchers add, "Breaking the single, most common 1024-bit prime used by web servers would allow passive eavesdropping on connections to 18% of the Top 1 Million HTTPS domains. A second prime would allow passive decryption of connections to 66% of VPN servers and 26% of SSH servers. A close reading of published NSA leaks shows that the agency's attacks on VPNs are consistent with having achieved such a break." Here is their full technical report (PDF).

Comments

Opportunity to detect MITM attacks?

[Ungrounded Lightning]

I skimmed the start of the paper. If I have this right:

  - Essentially all the currently-deployed web servers and modern browsers have the new, much better, encryption.
  - Many current web servers and modern browsers support talking to legacy counterparts that only have the older, "export-grade", crypto, which this attack breaks handily.
  - Such a server/browser pair can be convinced, by a man-in-the-middle who can modify traffic (or perhaps an eavesdropper-in-the-middle who can also inject forged packets) to agree to use the broken crypto - each being fooled into thinking the broken legacy method is the best that's available.
  - When this happens, the browser doesn't mention it - and indicates the connection is secure.

Then they go on to comment that the characteristics of the NSA programs leaked by Snowden look like the NSA already had the paper's crack, or an equivalent, and have been using it regularly for years.

But, with a browser and a web server capable of better encryption technologies, forcing them down to export-grade LEAKS INFORMATION TO THEM that they're being monitored.

So IMHO, rather than JUST disabling the weak crypto, a nice browser feature would be the option for it to pretend it is unpatched and fooled, but put up a BIG, OBVIOUS, indication (like a watermark overlay) that the attack is happening (or it connected to an ancient, vulnerable, server):
  - If only a handful of web sites trip the alarm, either they're using obsolete servers that need upgrading, or their traffic is being monitored by NSA or other spooks.
  - If essentially ALL web sites trip the alarm, the browser user is being monitored by the NSA or other spooks.

The "tap detector" of fictional spy adventures becomes real, at least against this attack.

With this feature, a user under surveillance - by his country's spooks or internal security apparatus, other countries' spooks, identity thieves, corporate espionage operations, or what-have-you, could know he's being monitored, keep quiet about it, lie low for a while and/or find other channels for communication, appear to be squeaky-clean, and waste the tapper's time and resources for months.

Meanwhile, the NSA, or any other spy operation with this capability, would risk exposure to the surveilled time it uses it. A "silent alarm" when this capability is used could do more to rein in improper general surveillance than any amount of legislation and court decisions.

With open source browsers it should be possible to write a plugin to do this. So we need not wait for the browser maintainers to "fix the problem", and government interference with browser providers will fail. This can be done by ANYBODY with the tech savvy to build such a plugin. (Then, if they distribute it, we get into another spy-vs-spy game of "is this plugin really that function, or a sucker trap that does tapping while it purports to detect tapping?" Oops! The source is open...)