Re: Unique code for every user

Daniel Pitts <>
Tue, 06 Oct 2009 11:00:45 -0700
Barry wrote:


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

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.

Thanks for your advice,


You could try encrypting the transactionId and a hash-code. Its
important to store the hash-code and check it on decrypting, otherwise
they may still be able to find other transactions.


public String calcSecureCode(long transactionId) {
    return transactionId + "-" + calcHash(transactionId) ;
String encodeTransactionNumber(long transactionId) {
    String toEncrypt = calcSecureCode(transactionId);
    return encrypt(toEncrypt);

Long decodeTransactionNumber(String encrypted) {
    String decrypted = decrypt(encrypted);
    long transactionId =
Long.valueOf(StringUtils.substringBefore(decrypted, "-"));
    if (calcSecureCode(transactionId).equals(decrypted)) {
       return transactionId;
    // Failed validation.
    return false;

make sure "encrypt" and "decrypt" use secure encryption, as the
transaction+hash is still vulnerable if the outside user figures out
your hash algorithm.

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