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Invisible wrote:
> Now, here's a thought. MD5 produces a 128-bit hash value. So what are
> the changes of two random datums having the same hash?
>
> 2^128 = 3.40 * 10^38
And yet, it's not possible to just keep a record of MD5 hashes and
backward-compute the file that generated it (or even a reasonably small
list and a note as to which one should be used) (and save an awful lot
of storage space).
:(
--
Tim Cook
http://empyrean.freesitespace.net
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Tim Cook wrote:
> Invisible wrote:
>> Now, here's a thought. MD5 produces a 128-bit hash value. So what are
>> the changes of two random datums having the same hash?
>>
>> 2^128 = 3.40 * 10^38
>
> And yet, it's not possible to just keep a record of MD5 hashes and
> backward-compute the file that generated it (or even a reasonably small
> list and a note as to which one should be used) (and save an awful lot
> of storage space).
>
> :(
Indeed. For although the number of possible MD5 hashes may be utterly
vast, the number of possible 4GB files is so MIND-PARALYSINGLY GIGANTIC
that you can't shoe-horn all the members of one set into the other.
--
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*
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Tim Cook <z99### [at] gmail com> wrote:
> Invisible wrote:
> > Now, here's a thought. MD5 produces a 128-bit hash value. So what are
> > the changes of two random datums having the same hash?
> >
> > 2^128 = 3.40 * 10^38
> And yet, it's not possible to just keep a record of MD5 hashes and
> backward-compute the file that generated it (or even a reasonably small
> list and a note as to which one should be used) (and save an awful lot
> of storage space).
Well, rather obviously. An MD5 has can be used to store at most 64 bytes
of data (meaning that all possible combinations of 64 bytes can be stored
there, but no more).
--
- Warp
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Warp wrote:
> Well, rather obviously. An MD5 has can be used to store at most 64 bytes
> of data
Did you mean 16, or am I missing something? Even if the internals track 64
bytes, a 128-bit resultant hash is only 16 bytes.
--
Darren New, San Diego CA, USA (PST)
Forget "focus follows mouse." When do
I get "focus follows gaze"?
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Orchid XP v8 wrote:
> Indeed. For although the number of possible MD5 hashes may be utterly
> vast, the number of possible 4GB files is so MIND-PARALYSINGLY GIGANTIC
> that you can't shoe-horn all the members of one set into the other.
However, there's a practical difference between possible files and files
that actually exist. And, to a certain extent, a central database of
files and their associated hashes would allow client computers to just
have the hash, and ask the server for the associated file.
...leaving the central server vulnerable, defeating the notion of
widespread data redundancy.
XD
--
Tim Cook
http://empyrean.freesitespace.net
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Tim Cook wrote:
> However, there's a practical difference between possible files and files
> that actually exist.
Notionally, of all the 4GB files that can hypothetically exist, only a
vanishingly tiny fraction of them will ever exist in our universe. Even
assuming that every star system in the entire observable universe has a
sentiant lifeform that has built binary computers with secondary storing
using file-based organisation...
...and yet, you can still create any such file you want. Weird, isn't it?
> And, to a certain extent, a central database of
> files and their associated hashes would allow client computers to just
> have the hash, and ask the server for the associated file.
>
> ...leaving the central server vulnerable, defeating the notion of
> widespread data redundancy.
>
> XD
Yay, destruction! :-D
Also: Isn't this almost exactly what Amazon S3 could be considered to be
doing? (Or, in fact, The Internet as a whole?)
--
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*
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On Tue, 12 Jan 2010 13:31:34 +0000, Invisible wrote:
> 2^64 = 1.844 * 10^19.
Start looking at tetration if you want big numbers. :-)
Jim
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Darren New <dne### [at] sanrrcom> wrote:
> Warp wrote:
> > Well, rather obviously. An MD5 has can be used to store at most 64 bytes
> > of data
> Did you mean 16, or am I missing something? Even if the internals track 64
> bytes, a 128-bit resultant hash is only 16 bytes.
For some reason I misremembered MD5 as being 512 bits long, even though
it really is only 128 bits. The "512 bits" might have been mentioned in
another context in this thread, which is why I got confused.
--
- Warp
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Orchid XP v8 wrote:
> Tim Cook wrote:
>> ...leaving the central server vulnerable, defeating the notion of
>> widespread data redundancy.
> Also: Isn't this almost exactly what Amazon S3 could be considered to be
> doing? (Or, in fact, The Internet as a whole?)
Well, the internet as a whole is based around the idea that data gets
spread out and copied over various computers all over the world...the
exact opposite of the central-data-repository idea. Haven't heard of
Amazon S3.
--
Tim Cook
http://empyrean.freesitespace.net
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>> 2^64 = 1.844 * 10^19.
>
> Start looking at tetration if you want big numbers. :-)
Constructing huge numbers is easy. I'm looking at big numbers that have
some sort of real-world significance.
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