Application registration algorithm

Question

I'm writing a small shareware app that I would ideally like people to register. This is the classic, if they send in the registration code 'xxxxxx', then I can send the unlock code 'YYYYY' situation.
Three criteria:
1) I would like the registrtaion code xxxxx to be unique. I am toying with the idea of storing the date in millisecs of when the app was first run on the system. If anyone has any better suggestions, I would love to hear them.
2) I would really like to have the algorithm myself. i.e. I don't plan to make enough off of this to be worth purchasing a library.
3) This does not need to be incredibly secure. Just enough to keep the casual person from
Does anyone out there have an algorithm they are willing to share to accomplish this?
Thanks all. This falls into the 'I know it has to be a known problem, but I'm not sure where to start' category.

Answer 1

Unless I am misunderstanding, I think that I'm addressing what you mentioned. Let me recap and see if I can make it more clear.
1) Client sends you an identifier and saves it to the client's disk.
String start = "" + System.currentTimeMillis();
start = start.substring( start.length() - 5, start.length() );
//this pulls off the last 6 numbers from the first run time and converts to a string. 
// this is written to a file on the client side and then sent to me for registration.
2) You (the server) send back a hash of the identifier.
//registration app
String start = codeSentFromCustomer;
String superSecretDecoderString = "Secret Code";
start = start + superSecretDecoderString;
String result = MD5.getHashString( start );
result = result.substring( result.length() - 9, result.length() );
//this takes the 6 digits provided and appends "Secret Code".
//  i.e. if the customer's code is "333232", then "333232Secret Code" is fed into MD5.
//  I then send the customer the last 10 digits from my MD5 hash, say "1234567890" for simplicity sake
3) Clients types the unlock code ("1234567890") into the registration box, which is then saved to disk.
// to check on the unlock code's validity, I...
String superSecretDecoderString = "Secret Code";
String start = originalCodeFromAbove; //i.e. "333232" in this example
start = start + superSecretDecoderString;  //now start = "333232Secret Code"
String result2 = MD5.getHashString( start );
result2 = result2.substring( result2.length() - 9, result2.length());
// I compare the result2 to the stored unlock code to make sure they are equal
//  superSecretDecoderString is stored within the client and the registration prog's code.  i.e.
//  the customer never sees that string.
4) At each startup the client code checks the hash of sent ID against the return he has stored.
// run through the validation code detailed in #3.I think it addresses what you were referring to, but if it doesn't please tell me. It's my first crack at this sort of code, and I'd much rather discover any gaping holes now than later. :)
thanks again