LPM: Encryption Question

[Janine Ladick]

Does anyone know of a way to disguise a number so that it can be 
easily (as in, by eye) undisguised?  Here's the scenario:

My client assigns membership numbers to each of their customers.  
These numbers are unique.  They can also be used to obtain 
account information on the web, so they are "secret special" 
numbers.

When I send junk mail to these poor hopeless victims, I need to put 
some kind of unique record identifier in the mailing address.  As the 
junk mail travels through the postal system, if the mailing address 
corresponds to one in the USPS National Change of Address 
database then I'll receive a data file containing my unique identifier 
+ the new address for the unsuccessful escapee.  Then I append 
the membership number to the file and send it back to my client so 
they can update their database.

If the membership number wasn't so damned special, I could just 
use it and be done.  Unfortunately it *is* so damned special, so I'm 
looking for a way to encode it.  I don't want to use too fancy a 
conversion scheme, just something that can easily be decoded from 
memory (as in no complicated rules to remember).

The membership number is 6-12 digits long.  I was thinking of zero-
padding in the front to make each one exactly 12 digits, reversing 
the string, then adding 3 to each digit and mod-ing by 10.  (For 
example, that converts 1234567890 to 321098765433.)  It does the 
trick, but it's graceless.  My sense of aesthetics cries out for a more 
elegant solution.  Any ideas?

Thanks in advance for your suggestions!
Janine

"I work with junk mail.  This gives me an excuse to go postal."