[ig1]

It's about verifiability, you can verify if a CPU is adding incorrectly, there's no way of verifying if the random source is generating randomness correctly.

Integers form a closed group (in the group theory sense) under multiplication and addition, so if you were concerned about specific calculations being compromised you could verify the answer via alternative calculations and testing for consistency.

Faking consistency is likely impossible without causing a huge amount other calculations (which the CPU will do as part of day-to-day operations) to fail. Making the CPU alter calculations only under very specific circumstances and in an undetectable way would require a huge amount of complexity.

On the other hand we know several fairly trivial ways of faking reversable randomness using standard crypto algos that would be statistically undetectable without taking the hardware apart.

[patio11]

Tell you what: You write executable code which tests whether, e.g., Ruby's Fixnum addition is actually adding or not, without using introspection on the implementation. (i.e. You're allowed to test any combination of inputs and outputs, but cannot look within the black box.) Feel free to use number theory, statistics, or a web service which wraps Cthulu in JSON.

I will provide a script which monkeypatches Fixnum such that your test script still reports "Yep, it's still adding correctly" and which will predictably cause 1 + 1 to equal the string "LOL I AM THE NSA" under attacker-chosen conditions.

There are many, many ways I could do this as the crafty adversary, but I anticipate my implementation would be roughly seven lines long and trivially implementable in hardware. (At complexity somewhere between a half-adder and the circuits which we had to hand-draw for our finals in hardware design, which -- obviously -- are a drop in the ocean of complexity on a modern CPU.)

[ig1]

Say you wanted your backdoor to kick-in when I was calculating the fibonacci sequence, will you also detect when I was calculating it via the golden ratio, or via continued fractions, etc. If I can write a test that can show that there's a backdoor because it's not behaving consistently then your attack is significantly weaker then a RNG attack.

You can't just break one calculation under specific circumstances, you have to break any way of verifying it which means compromising pretty much every calculation.

(not to mention you'd have to figure out that it's the fibonacci sequence being calculated and not some other calculation that happens to start with 1+1)

Remember you don't have an interactive attack, you have one chance to build something and ship it out in hardware, and it then has to compromise software that's going to be written in the future.

The beauty of the RNG attack is that it's undetectable, introduces a backdoor into a huge number of systems and it only makes the system vulnerable to the attacker and not to anyone else.

[patio11]

Super double-secret squirrel sources have said there exists a file "nsa_does_fixnum.rb" which is 1006 bytes long and the SHA512 hash of 896e12642f5720d830572a8b11e4c729da3a20c57402758d22a25c46c3c2e6 bb4b09de9030ad591ecb51b8a9a4d4c119cefbf4d51a72db219d94150fb63a 3623. (Concat them -- I broke it into a few lines to avoid breaking the comment page on low-resolution devices.) Our intrepid squirrel didn't get a good look at what it is doing but claims "It doesn't use any external library and doesn't do anything even remotely tricky." They claim that it secretly modifies any Ruby1.8 interpreter to cause 1 + 1 to equal "LOL THIS IS THE NSA" under some circumstances.

Quick ig1, you are freedom's last, best hope. Write actual computer code which can, by adding numbers together and inspecting their output, determine whether your Ruby interpreter has been compromised by the NSA. You're lucky, since the NSA has already shipped their exploit (or did they?), they can't modify it in response to your detection code. Bad news, though: if your detection code fails and an interpreter which includes the backdoor can, after passing your detection code, still get the wrong answer for 1 + 1, an innocent user fails to find the backdoor and suffers a FatalHitByCruiseMissileError. You don't get to say "OK, so in hindsight, now that I see the backdoor addressing it was pretty darn easy. Mop up the mess and I'm sure to win round two."

[acqq]

Sorry, I don't see what you're trying to argument here, since CPUs aren't Ruby interpreters with libraries. CPUs have much stronger limitations under which they have to operate. Remember the Pentium DIV bug? It was discovered.

I'm writing assembly code, I've spent time also to measure the time of the machine operations etc, so I simply can't imagine the valid argument which ignores exact limitations present in actual hardware.

[rst]

People in this thread seem to be arguing past each other. Patrick is specifically disputing the claim (a few posts up) that you can always tell if the hardware is adding incorrectly. His argument is that if it's deliberately manufactured to add almost correctly, except in certain carefully tailored cicrumstances, the chance of that being detectable by a third party are pretty slim. The Ruby stuff is just a metaphor.

(Google for "Illinois Malicious Processors" for what an attacker might actually do. A few extra transistors here and there can lay you wide open.)

[acqq]

I have referred to

http://en.wikipedia.org/wiki/Pentium_FDIV_bug

I still don't see how adding "almost incorrectly" would have not be detected in practice. In practice you also have to implement "almost incorrectly" in a way that doesn't affect the performance of your CPU. We are talking about processors, not Ruby interpreters.

[jsnell]

Can you please break that hash on multiple lines or quote it? It breaks the layout of the comment page.

[ig1]

Your attack relies upon security by obfuscation, hoping that no-one will notice that the behaviour is incorrect (we obviously disagree about the chance that such an attack will be spotted but I think to some degrees that's irrelevant).

The RNG attack has correct behaviour under every statistical and blackbox analysis. The only way you can break it externally is if you have the key.

If you were designing a cryptosystem you'd pick the one where the security lies in the key and not in obfuscation. The same applies when designing an attack you want to keep secret.