Non-doublespending offline digital money?

[matt]

Hi all.

Could anyone tell me if it is theoretically/physically possible to
have a digital cash system which is offline, and prevents double
spending?

Just thinking about it, it seems impossible. But maybe someone knows
some really tricky maths etc...?

Thanks,
Matt.

[Erwin Bolwidt]

matt wrote:

> Could anyone tell me if it is theoretically/physically possible to
> have a digital cash system which is offline, and prevents double
> spending?

Have a look at Bruce Schneiers "Applied Cryptography", it describes how
Chaums system works. It's offline in the sense that the seller doesn't
need to do an online check with a banking system.
It's a matter of chance; you can bring down the change of succesful
double spending to 1 in 2^n where you can make n as big as you want, but
it increases communication overhead lineairly with n.

> Just thinking about it, it seems impossible. But maybe someone knows
> some really tricky maths etc...?

What I want to know is if there is any non-patented scheme to do this?

Erwin Bolwidt

[David Hopwood]

-----BEGIN PGP SIGNED MESSAGE-----

matt wrote:
> Could anyone tell me if it is theoretically/physically possible to
> have a digital cash system which is offline, and prevents double
> spending?

It's possible to detect double spending in an off-line system (when
the cash is deposited with the bank), but not to prevent it at the
time of the transaction. You can make it more difficult (and reduce the
amount of resources needed to deal with it in practice) by using a
tamper-resistant user token, so that double spending can only occur
at deposit time if the user to which the cash was issued has broken
the tamper resistance of their token.

OTOH, with the cost of always-on access coming down significantly
in most countries, and with the introduction of wireless networks, I
doubt if the advantages of off-line cash would outweigh the added
complexity (which is considerable).

- --
David Hopwood <hop...@zetnet.co.uk>
PGP public key: http://www.users.zetnet.co.uk/hopwood/public.asc
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[Eric Norman]

matt wrote:
>
> Could anyone tell me if it is theoretically/physically possible to
> have a digital cash system which is offline, and prevents double
> spending?

If every offline transaction eventually leads to a subtraction
from their account, what double spending is there to worry about?

Eric Norman

"Congress shall make no law restricting the size of integers
that may be multiplied together, or the number of times that
an integer may be multiplied by itself, or the modulus by
which an integer may be reduced".

[Nick Tamer]

The answer is in quantum crypto systems: when you observe its state you
destroy it and thus you can't spend it again - unfortunately they aren't
yet widely used ;-)

matt wrote:

> Hi all.

>
> Could anyone tell me if it is theoretically/physically possible to
> have a digital cash system which is offline, and prevents double
> spending?
>

> Just thinking about it, it seems impossible. But maybe someone knows
> some really tricky maths etc...?
>

> Thanks,
> Matt.

[David A Molnar]

I seem to recall that digital cash was one of the first (maybe the first?)
application of quantum mechanics to cryptography. For exactly this
reason: try to copy a state and you end up destroying it.

I can't remember a reference. Does this ring a bell for anyone else?

Thanks,
-David

[the...@my-deja.com]

In article <
8b97bt$69d$1...@news.fas.harvard.edu>,
David A Molnar <dmo...@fas.harvard.edu>
wrote:

>
> I seem to recall that digital cash was one of the first (maybe the first?)
> application of quantum mechanics to cryptography. For exactly this
> reason: try to copy a state and you end up destroying it.
>
> I can't remember a reference. Does this ring a bell for anyone else?
>

Yes, I think you are referring to "quantum
money", an idea developed by Wiesner in the
late sixties which is the first instance of
theoretical quantum cryptography. He wrote a
paper on this in 1970 but it wasn't published
until 1983 due to a prior lack of interest in
the subject. There is a description of quantum
money and its origin in Singh's "The Code
Book". Note that quantum cryptography can
provide at minimum *partial* security for
quantum money but how much potential
security is attainable remains unknown (at
least to me).

Sent via Deja.com http://www.deja.com/
Before you buy.

[Johnny Bravo]

On Tue, 21 Mar 2000 16:27:32 -0600, Eric Norman <ejno...@doit.wisc.edu>
wrote:

>matt wrote:
>>
>> Could anyone tell me if it is theoretically/physically possible to
>> have a digital cash system which is offline, and prevents double
>> spending?
>

>If every offline transaction eventually leads to a subtraction
>from their account, what double spending is there to worry about?

Let's say I have 50,000 in my account at your bank. Then I spend
$1,000,000 over a week, resell what I bought for $.20 on the dollar.
Orders come in and go way over my account, you have no choice but to cover
the orders.
Then I burn my fake ID, leave the state $150,000 in cash ahead.

And you are now out $950,000.

This is why offline systems are a bad idea.

--
Best Wishes,
Johnny Bravo

"The most merciful thing in the world, I think, is the inability
of the human mind to correlate all it's contents." - HPL

[Niklas Frykholm]

>> Could anyone tell me if it is theoretically/physically possible to have a
>> digital cash system which is offline, and prevents double spending?
>

>Well, if you can copy it then you can reuse it... So what you have to do
>is to keep people from being able to copy it, which can be done if the
>digital cash is carried around inside hardware that rather destroys the
>data than let you access it the wrong way. You'd also have to encrypt
>the communication between the other unit and that hardware.
>
>It shouldn't be too hard to get such a system going which can be trust
>for smaller amounts, but you'd have to have a very active use of
>blacklists ready for the day that something, if ever, starts going
>wrong.

Proton Systems (www.protonworld.com) has developed a system along these
lines. It is used by most banks and retail stores in Sweden (and in many
other countries too), but customer response has been rather cold. After
all, if it is used just like cash, if it can be stolen just like cash then
what exactly is the advantage over cash?

// Niklas

[b...@myoffices.com]

Hi Matt guess who’s back.

[auxon0]

Ditto.

[Anonymous]

On Tuesday, March 21, 2000 at 3:00:00 AM UTC-5, matt wrote:
> Hi all.

> Could anyone tell me if it is theoretically/physically possible to
> have a digital cash system which is offline, and prevents double
> spending?

[Anonymous]

Today is the day almost 22 years later.To post on a former blog is amazing.
We look forward to knowing the truth when the truth is here to provide.

Best wishes and let the story of MyOffices guide you.