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Hashing (How does it REALLY work?)


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Where can I read more about how Hashing REALLY works?

I have read most of it on Wikipedia, but I´m not satisfied.

Some questions:

1. Is there a lot of different Hashing soft wares?

2. If so, which does µTorrent 1.6.1 use?

3. Does µTorrent 1.6.1. transfer files, Bit by Bit or Packages?

4. If Packages, how many Bits/Package? (8, 16 or 32?)

5. Does Hashing check/detect on Bit level or Package level?

6. Does Hashing "Error correct" on Bit level or Package level?

7. In case of Error correction (Data lost during down load),

does Hashing reDown Load the incomplete package or does it repair it by CRC?

8. If using CRC (cyclic redundancy checks), which version does µTorrent 1.6.1 use? (CRC-1 to CRC-160)

Thank´s! :)

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1. Er... sure, there are lots of softwares that can find a hash of given data... but you don't specify which hash algorithm you speak of :o

2. I assume you're talking about what hashing algorithm µTorrent uses... in which case, it's mainly SHA-1 (though I'm not sure about other hash algorithms).

3. It divides the data into multiple pieces, each of which are divided into blocks, and transfers blocks/pieces in non-consecutive order.

4. For pieces, that depends on the piece size. For blocks, it's 16KiB/s (16384 bytes = 131072 bits) per block.

5. It checks per piece.

6. The hashes described in the .torrent files with which µTorrent compares hashes are also per-piece.

7. Damaged/corrupt pieces are redownloaded. Attempting to repair them would take ages, if at all possible.

8. It doesn't use CRC.

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Thank´s for all your answer! :)

Maybe I´m too old for this, so I gave and give you some silly questions and get some good answers in return. ;)

I have now read about SHA-1 at Wikipedia and I can´t say I understand all about it, but I can´t say I find it 100% secure when it comes to redownload data loss.

Simply: It looks like a soft ware RAID solution and we all know about that result. :(

So here comes some more questions...

5. It checks per piece.

5A. If Hash checks per Package (piece), how does Hash know what´s in the Package?

(Correct amount and order of ones and zeros? The amount of Bits could be correct, but that´s not the same thing)

It only knows that the Package have arrived or not.

7. Damaged/corrupt pieces are redownloaded.

7A. If Hash redownload per Package (piece), how does Hash know which Bits suppose to be in the Package?

It have only check the Package, not what´s inside the Package.

10. Hash checks per Package and redownload per Package and it is done by a mathematical formula.

How can I be sure that I receive ALL data bits, when Hash are done/finished?

10A. WinRAR check the "Volumes" per Bit when it is extracted, in case of data loss people have to redownload the incomplete Volume per Package and check again per Bit when reextracted.

11. WinRAR seems to be more secure to use than Hash, which always works on Package level.

12. To depend on Hash when it comes to "Data loss", feels like when people in the old days copy a VCR/VHS tape over and over again until it was so damage that they couldn´t watch it any more.

Simply: Today people lose some data Bits during every transfer (because of poor Internet connection quality), until they have lost so many Bits that the down loaded file become total corrupt.

So my main question is:

Do they need to use WinRAR side by side with Hash when they UL a file using µTorrent 1.6.1.?

Sorry for my poor English.

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RAID is totally different from SHA-1. RAID duplicates data, SHA-1 produces a hash sum that *shouldn't be* identical to the original data.

5. It doesn't know, but at the same time, it doesn't care. Hashing algorithms like SHA-1 take in whatever data, and convert it into a "message digest" that should, in theory, be unique to the input data -- if (and only if) you run the input data through the hash algorithm again, it should output the same message digest. It is that message digest that is stored in a .torrent file, and it is that message digest that µTorrent compares. Theoretically, if a chunk of data has the same hash as another chunk of data, then the two chunks of data are identical. In practice, though, it's not always the case, and collisions can be found for SHA-1. For the purpose it serves in the context of BitTorrent, though, SHA-1 works well, and it'd be VERY difficult for someone to engineer torrent contents consisting of entirely fake data with identical SHA-1 hashes, so collisions aren't really of concern.

7. As was described, if the hash doesn't match, then it's assumed to be different, and so the entire piece is tossed out.

10. Yeah... I think my response to question 5 kinda answers this. As for the question about WinRAR... I'm not sure what you mean. It's possible to modify data without changing its CRC checksum, so it's hardly as reliable as SHA-1 for detecting data corruption. BitTorrent needs to strike a balance between data verification and efficiency, and using hashes works fairly well to that end. Because of the need for efficiency, a checksum can't be created for each and every bit of data (as the resulting .torrent file would become larger than the original data), but at the same time, it can't hash an entire file in one pass, since it's slower, and impossible to fix without redownloading the entire file again (which can be large). As such, the data is split into the so-called "pieces," each of which are hashed individually when the .torrent file is created, are faster at being hashed than entire torrents, but at the same time, make it easier to detect where in the data the corruption is found.

11. I'm not sure what you're talking about here -- it isn't a really question of security when it comes to data verification in BitTorrent :o

12. µTorrent (and BitTorrent for that matter) doesn't rely on hashing to cope with data loss. It uses hashing to cope with data corruption. Your analogy does not work either, as redownloading is NOT analogous to re-recording. When you re-record, you record the defects, and the record is "lossy," so to speak. When you redownload, you're redownloading the piece(s) entirely, and they remain identical to the original data. It's like seamlessly cutting a part of the original VHS magnetic strip, and replacing the damaged part of your VHS magnetic strip (keyword being seamless -- you don't notice that there's a "cut" anywhere).

No, WinRAR isn't necessary, and is entirely irrelevant.

And your English is mostly understandable, so there's no need to apologize for it :P

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