mikeg's blog

While I usually read El Reg for teh lulz occasionally they come up with an interesting nugget, like their article on a "severe" OpenSSL vulnerability (quotes theirs). This vulnerability comes to us courtesy of the University of Michigan, and is tied to a simple oversight in the OpenSSL code: It doesn't perform a verification pass on signatures before sending them to a client.
So all you crypto-nerds, how many of you do a verify on your signatures before you send them? Hands please? Nobody? Not even me? -- Yeah, we all tend to trust that our math Just Works. I mean it's a computer, you put in fixed input through a deterministic algorithm and you get the same output ever time. Right? Nope.

Computers, as we all hopefully know, aren't perfect. Trillions of minor errors happen every day, and they're usually compensated for: ECC RAM, cross-checks of math, etc. But because OpenSSL doesn't do a cross-check it's possible for those minor errors to creep into a transmitted signature. As the good folks from UMichigan explain in their paper if you can induce some minor errors in the OpenSSL signature math and collect a good signature and a large enough set of flawed signatures you can eventually derive the private key used for the signing.

That is all well and good, except it relies on errors in the system generating the signatures. These errors are rare (and pretty random) in the real world, so no problem, right? Wrong again Sparky! These clever folks came up with something I wouldn't ever have thought of.

Those of you who have worked on old Commodore computers probably know one of the first symptoms of a power supply going wonky is that the system starts acting up: Lots of random crashes, video corruption, etc. Why does that happen? Because the voltage being supplied to the system is fluctuating. Fluctuating voltages (specifically down-swings) cause all the magical voltage regulation hardware to go out of regulation, which feeds out-of-spec voltages to the chips, which in turn start making mistakes in their math.

The bottom line for this attack is an extension of that well-known Commodore problem: By putting controlled voltage fluctuations into the power supply of real computers (SPARC boxen running Linux) the UMichigan folks were able to induce errors in the math used by the real-world OpenSSL code and recover an actual key.

Continue reading "Interesting (impractial) OpenSSL attack"