Re: Unique code for every user

From:

"Mike Schilling" <mscottschilling@hotmail.com>

Newsgroups:

comp.lang.java.programmer

Date:

Tue, 6 Oct 2009 12:06:53 -0700

Message-ID:

<hag4gf$kr0$1@news.eternal-september.org>

Barry wrote:

HI,

I'm building a system where I wish to give my users a unique code
each
time they perform a transaction. On returning to my system, they can
then enter this code to retreive the data associated with it.

I number my transactions in assending order, 0, 1, 2 and so on, so I
need a function that will transform this value to a unique nine
digit
number. I also need a function that will transform this value back
again to the transaction number.

Something like this -

long codeTransactionNumber(long transactionNumber)
{
return transactionNumber + 100000000;
}

long uncodeTransactionNumber(long transactionNumber)
{
return transactionNumber - 100000000;
}

Thie problem with this though is that the user with the code
100-000-003 can easily guess that 100-000-004 is also a code for
another transaction. What would be a better way to generate this
number?

Also, I should point out that the number of clients that this system
has is very low - 5 per day max. Also, they enter the code using a
touch screen interface so entering many codes is difficult.

Figure out the number of unqiue transactions IDs you'll need before
you can re-use numbers. At five users a day, I'd guess 10,000 is
sufficient. Now:

1. Generate a short transaction ID 1-9999 (i.e. a four-digit number).
2. Write these digits into your long transaction code in some
non-obvious way, e.g. WXYZ -> nYnnWZnnX
3. Generate 4 more random digits PQRS.
4. Let T = 9 - ((4*P + 3*Q + 2*R + S ) % 10)

Your long transaction code is now PYQRWZSTX. You can recover the
short transaction ID by extracting W, X, Y, and Z, and you can check
that a transaction code is valid by seeing whether (4*P + 3*Q + 2*R +
S ) % 10 == 9

This was off the top of my head; some reflection will probably suggest
improvements.