Re: reversible whole number license key
From: Mr X (iam.not_at_an.invalid)
Date: 08/05/03
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Date: Tue, 5 Aug 2003 08:32:58 +0300
"Thor Russell" <thor.russell@paradise.net.nz> wrote
> Hi,
> I am writing a network license for a software program, and want to use
> a serial code to specify the number of licenses a user has. What this
> means is that say the user buys 10 licenses, and it given the code
> 5634853215. They enter this into the computer, and it verifies that it
> is a correct code and informs them that they have 10 licenses. In
> order to do this I need an easily reversible mathematical operation
> that takes one ten digit number e.g. 5634853215 and converts it to
> 0000000010
Amateur advice:
10 -digit licence key does not sound like much. If the licence key can be
hundreds of digits long (in a file), the following might work:
You take some information specific to your customer, for example Company
Name, and the Number of Licences for that customer. Make a unique number
out of these ; no two customers have the same number.
Generate a RSA -key. For safety it has to be quite long, so we are not
talking about 10-digit numbers here.
Use the private RSA -key to calculate a digital signature (with padding).
You get a number, which is also quite big. This is the "serial code" for
the customer.
Verify the licence key ("serial number") with the public RSA -key in your
program. You get a number which is supposed to represent the Company Name
and Number of Licences together. Break down the number to its components
and check whether the Company Name is correct. If the name is OK, just
trust that the Number of Licences is also reliable?
Well yes, this can not be 100% secure, because we do not know what the
Number of Licences really should be. But the propability of success is
still high?
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