3 ms·

> In some cases though that is not possible, because at that timescale the signal they're after is degraded by noise that they cannot predict or control: operat

> In some cases though that is not possible, because at that timescale the signal they're after is degraded by noise that they cannot predict or control: operating system interrupts, memory traffic from other threads, etc. Wrong, those signals will average out over a long enough data collection time.

foota 9y ago
I'm not sure that's true in this case, if your timer doesn't have the resolution to ever detect a cache hit, how can you measure impact?
- deleted 9y ago
  [deleted]
- blattimwind 9y ago
  It's one of those things that feel utterly counter-intuitive. But if you have a signal overlaid with random noise [1], and you know what your signal's looking like and when it's happening, you can correlate. For example, a small delay that occurs at certain known points (or not), will introduce a bias into a timer measuring it, no matter how noisy or coarsely quantized that timer is. Similar techniques have been used in other fields for decades to pull useful signals from far below the noise floor (e.g. a lock in amplifier can go dozens of dB below the noise floor, because it, essentially, correlates frequency and phase and thereby eliminates all but a tiny sliver of noise. E.g. GPS signals are typically 20 dB below the noise floor. [1] It doesn't have to be random. —— So these mitigations just make the attacks harder, hopefully hard enough that they become not feasible to be exploited widely.
  - FreeHugs 9y ago
    Then why can't we make sharp high resolution photos of distant planets? Shouldn't we be able to average out all the noise for every pixel if we just collect light long enough?
    - contrarian_ 9y ago
      No, because there's correlated noise: all the stuff in between us and the planet.
      - FreeHugs 9y ago
        What type of stuff is between us and the planet and stays on the same pixel all the time? I would assume everything in the universe moves all the time. We move. The other planet moves. How can something block the same pixel of our view of the planet all the time?
        naasking 9y ago
        > We move. The other planet moves. Exactly. The same pixel isn't imaging the same location on the distant object. If it were, then what you say might be possible.
        contrarian_ 9y ago
        There's also the diffraction limit: https://en.wikipedia.org/wiki/Diffraction-limited_system https://en.wikipedia.org/wiki/Diffraction-limited_system
        sp332 9y ago
        I don't think other planets usually move enough to cross one pixel. They've mostly been detected by changes in brightness of their host star. https://en.wikipedia.org/wiki/Methods_of_detecting_exoplanets#Established_detection_methods https://en.wikipedia.org/wiki/Methods_of_detecting_exoplanet...
    - olegkikin 9y ago
      It's not the noise that prevents us from seeing distant planets, but the diffraction limits. https://en.wikipedia.org/wiki/Diffraction-limited_system https://en.wikipedia.org/wiki/Diffraction-limited_system

foota 9y ago

I'm not sure that's true in this case, if your timer doesn't have the resolution to ever detect a cache hit, how can you measure impact?

deleted 9y ago
[deleted]
blattimwind 9y ago
It's one of those things that feel utterly counter-intuitive. But if you have a signal overlaid with random noise [1], and you know what your signal's looking like and when it's happening, you can correlate. For example, a small delay that occurs at certain known points (or not), will introduce a bias into a timer measuring it, no matter how noisy or coarsely quantized that timer is. Similar techniques have been used in other fields for decades to pull useful signals from far below the noise floor (e.g. a lock in amplifier can go dozens of dB below the noise floor, because it, essentially, correlates frequency and phase and thereby eliminates all but a tiny sliver of noise. E.g. GPS signals are typically 20 dB below the noise floor. [1] It doesn't have to be random. —— So these mitigations just make the attacks harder, hopefully hard enough that they become not feasible to be exploited widely.
- FreeHugs 9y ago
  Then why can't we make sharp high resolution photos of distant planets? Shouldn't we be able to average out all the noise for every pixel if we just collect light long enough?
  - contrarian_ 9y ago
    No, because there's correlated noise: all the stuff in between us and the planet.
    - FreeHugs 9y ago
      What type of stuff is between us and the planet and stays on the same pixel all the time? I would assume everything in the universe moves all the time. We move. The other planet moves. How can something block the same pixel of our view of the planet all the time?
      - naasking 9y ago
        > We move. The other planet moves. Exactly. The same pixel isn't imaging the same location on the distant object. If it were, then what you say might be possible.
      - contrarian_ 9y ago
        There's also the diffraction limit: https://en.wikipedia.org/wiki/Diffraction-limited_system https://en.wikipedia.org/wiki/Diffraction-limited_system
      - sp332 9y ago
        I don't think other planets usually move enough to cross one pixel. They've mostly been detected by changes in brightness of their host star. https://en.wikipedia.org/wiki/Methods_of_detecting_exoplanets#Established_detection_methods https://en.wikipedia.org/wiki/Methods_of_detecting_exoplanet...
  - olegkikin 9y ago
    It's not the noise that prevents us from seeing distant planets, but the diffraction limits. https://en.wikipedia.org/wiki/Diffraction-limited_system https://en.wikipedia.org/wiki/Diffraction-limited_system