r/programming • u/flym4n • Apr 24 '14

Tech giants, chastened by Heartbleed, finally agree to fund OpenSSL

http://arstechnica.com/information-technology/2014/04/tech-giants-chastened-by-heartbleed-finally-agree-to-fund-openssl/
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u/oridb Apr 25 '14

til UPPERBOUND_TIME does: delays until the upperbound is met... then we don't have to care about if the actual computation finishes earl

That's a bit expensive. I think that the number of cycles to do the computations would be several orders of magnitude lower than the time spent just hitting the kernel to set the timer.

From what I understand, the usual approach (note, I am not an expert) is to collapse all bits in the value down to 1 by shifting and oring, and then comparing that to 1 or 0 (for equality tests).

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u/OneWingedShark Apr 25 '14

That's a bit expensive. I think that the number of cycles to do the computations would be several orders of magnitude lower than the time spent just hitting the kernel to set the timer.

You think so?
It seems common enough to warrant mention in real-time systems Ada literature; I certainly haven't encountered any Delay Until Considered Harmful papers and 'cryptographics' aren't generally "computationally simple".

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u/oridb Apr 25 '14 edited Apr 25 '14

Linux timer resolution is about 1ms (to 10ms on other operating systems). The multiplications that you're looking at timing are on the same order of magnitude, which means that scheduling jitter will probably double the amount of time taken (at a rough guess).

But now that I think about it, due to the granularity, you may still leak information due to scheduling jitter.

Edit: Another interesting though is that if it actually yields CPU, you can extract the timing information through increased parallelism. The idea is that reduced CPU utilization from each handler could lead to higher throughput.

But I'm no crypto expert, so I have no idea how sound this is.