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Comment: Re:Speed penalty of encryption (Score 1) 119

by hankwang (#49175685) Attached to: Google Backs Off Default Encryption on New Android Lollilop Devices

if it happens on every mail sync, podcast download

In that case, the bottleneck is the data transfer over Wifi or 3G. At least, I'm pretty sure that I never reach 27 MiB/s (270 Mbit/s) data transfer rates. The wake time will not be affected in such cases. I think it's only activities such as app startup and media indexing that are affected by slow storage bandwidth.

And otherwise, encryption is really a must for me. With a custom ROM and bootloader (and no encryption), it's too easy for someone else to extract all personal data from the device, including credentials for my Gmail account and my banking app, both of which can have actual financial consequences.

Comment: Speed penalty of encryption (Score 1) 119

by hankwang (#49170835) Attached to: Google Backs Off Default Encryption on New Android Lollilop Devices

Whether in hardware or software, it's still a fair amount of computation, which means battery usage and latency.

I wouldn't be so sure about that. Android will only encrypt the /data partition, not /system. That's why you can still do a factory reset on an encrypted device. I'd guess that a lot of the I/O is in /system.

Anyway, here is a 100 MiB write test (Nexus 5, Cyanogenmod 11, Android 4.4, rooted), to the /data ("sdcard") partition and to /cache (not encrypted):

sync; sync
time sh -c 'dd if=/dev/zero of=/sdcard/foo.bin bs=1048576 count=100; sync; sync'
3.67 seconds
sync; sync
time sh -c 'dd if=/dev/zero of=/cache/foo.bin bs=1048576 count=100; sync; sync'
2.13 seconds

At 27 MiB/s versus 47 MiB/s it seems to be fast enough for me. Most apps are below 15 MB (apk size).

On a low-end, but fairly recent, LG L40/D165 phone, it's 11.2 seconds for a 100 MB encrypted write. (No root here, so I can't write to /cache for comparison). Still fast enough for me.

Comment: Re:FDE on Android doesn't work as of yet (Score 3, Informative) 119

by hankwang (#49170657) Attached to: Google Backs Off Default Encryption on New Android Lollilop Devices
Cyanogenmod 11 (Android 4.4.4) indeed has an option to set the encryption password separately. With stock Android this only possible on a rooted device (vdc cryptfs changepw [secretpassword1]), but the encryption password will be reset if you change the screen unlock code.

Comment: Re:chinese anti-satellite lasers (Score 1) 253

by hankwang (#49162771) Attached to: 20-Year-Old Military Weather Satellite Explodes In Orbit

But with your point about the coating, I'm only off by a factor of ten...

1000 times smaller beam diameter means 10^6 times higher irradiance. With 1% absorbance it's still a factor 10^4 off.

how much power do we really need on target to make it overheat given that it's designed to cycle between daylight and night

The solar panels are designed to receive 1500 W/m2 irradiance from the sun, of which maybe 500 W/m2 comes out as electricity and the other 1 kW/m2 is heat load. A black-body radiator against 4 K background temperature on one side and earth (279 K) on the other side will reach an equilibrium temperature of 330 K (57 C). To heat the solar panel to a damaging 150 C (just a guess), you'd need about 2.5 kW/m2 extra (on top of the solar irradiance).

It wouldn't surprise me if the back side of the solar panels is used as a radiative heat exchanger to get rid of the heat generated inside the body of the satellite. The gold-coated foil around the satellite body acts as a kind of thermos bottle to insulate the electronics inside from extreme temperature swings, but still the electronics need to get rid of its own dissipation, so you would not only destroy the solar panels but indirectly also the electronics.

I doubt that it's practical to aim specifically for the solar panels for an object that's travelling at 7 km/s, with a mechanism that can do that within 1 microrad. It might be more practical to have a 10 m diameter spot size, which would require 250 kW, which might be doable with a CO2 laser at 10.6 micrometer wavelength (the atmosphere seems to be fairly transparent at that wavelength).

If the solar panels have a mass of 15 kg/m2 (just a guess), then you would need to maintain this 2.5 kW/m2 for about 10 minutes, over which time the satellite will be travelling 4200 km. This doesn't sound easy...

Comment: Re:chinese anti-satellite lasers (Score 4, Informative) 253

by hankwang (#49160223) Attached to: 20-Year-Old Military Weather Satellite Explodes In Orbit

"A 1um laser fired out of a 1m aperture spreads to an 800m circle at an altitude of 800km"

You're off by a factor 1000. The divergence is about 1e-6 rad, which makes 0.8 m diameter at 800 km.

Now another issue is that satellites tend to be wrapped in gold-coated foil, which will reflect 99% of the light at 1 micrometer. It would be difficult to overheat the body of the satellite, although the solar panels might be damaged more easily.

Comment: Re:Yes, it's a conflict of interest. (Score 1) 448

by hankwang (#49104557) Attached to: How One Climate-Change Skeptic Has Profited From Corporate Interests

Having a conflict of interest is understandable; hiding a conflict of interest is problematic.

The strange thing is that his management (Alcock and Kress) is claiming that they were unaware of his funding. The research grants pass through the accounting department of the institute, right? Large sums of money deposited onto or withdrawn from the institute's bank account need to have proper paperwork in place and be signed off by the management? His management must have been actively looking the other way.

Comment: Etching does the same thing (Score 2) 133

by hankwang (#49099639) Attached to: Crystal Pattern Matching Recovers Obliterated Serial Numbers From Metal

It has been possible for decades to recover serial numbers by chemical etching, which is sensitive to changes in the crystal structure. All you need is a polishing implement and a bottle of etching gel. What is the added value of this EBSD technique? I can see a big disadvantage: you need equipment (a scanning electron microscope) worth a few hundred kEUR and the object with the erased serial number needs to fit into the vacuum chamber of said equipment.

The article mentions that etching techniques don't always work, but they don't state that their technique does work on samples for which the etching method doesn't work...

Comment: Re:You're both right, and both wrong. (Score 1) 311

by hankwang (#49068525) Attached to: Nuclear Plant Taken Down In Anticipation of Snowstorm

What makes you both partially correct is that there isn't a delay in fission, but a delay in neutron release.

From reading the linked Wikipedia page, I get that the delayed neutrons are responsible for 0.6% of all neutrons emitted from fission, and most of them with half-life times between 2 and 20 seconds. While this makes for some nice control bandwidth (you can afford to be 0.6% off with the control rods without getting an instantaneous nuclear explosion), I don't see how these delayed neutrons can be responsible for the majority of the 7% decay heat.

Comment: Re:Nuclear plants don't like sudden shutdowns (Score 2) 311

by hankwang (#49064637) Attached to: Nuclear Plant Taken Down In Anticipation of Snowstorm

"Normally there is some time between neutron capture and actual nuclear fission (I have heard a figure of 15 minutes)."

The fact that you can detonate a nuclear bomb by bringing together two subcritical pieces of U-235 shows that this can't be true.

In a nuclear reactor, 7% of the heat output is from the decay of the fission products (alpha and beta decay). This 7% will continue to be generated regardless of control rods or neutron absorbers. It will last hours to weeks, depending on where you put the threshold for "finished". Remember Fukushima: it became a disaster when the water circulation backup pumps failed 12 hours after the reactor shutdown.

Comment: Re:The Greater Danger (Score 1) 180

by hankwang (#49030321) Attached to: US Gov't To Withdraw Food Warnings About Dietary Cholesterol

"until the discovery of the chirality (and chirality affects a lot of things) of saturated fats, they probably got mixed together (they're still saturated fats, after all) and the original problem was because of the hidden trans- version."

I think you are mixing up things. Trans and cis fats are both unsaturated fats, with double bonds. Saturated fatty acids have no chirality. (Well... Triglycerides are technically often chiral (the middle carbon atom of the glycerol backbone), but that's not relevant for the cis/trans/saturated discussion.)

Comment: Re: Be careful not to justify government corruptio (Score 1) 215

by hankwang (#49015265) Attached to: Silk Road Drug Dealer Pleads Guilty After Federal Sting

"I recently heard about a case called "Kids For Cash", where a judge was sending lots of juveniles to a privatized detention center and getting paid for each."

Actually, two judges:

Quote: For example, Ciavarella adjudicated children to extended stays in youth centers for offenses as minimal as mocking a principal on Myspace, trespassing in a vacant building, or shoplifting DVDs from Wal-mart.

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