A (possibly silly?) idea would be to generate unique 64-bit values by using a 64-bit block cipher (e.g. DES) with a secret key, encrypting sequential numbers, like "int64 GetNewHandle() { static int64 counter = 0; return Encrypt(counter++, key); }". You can check that a handle is valid (i.e. has been generated in the past) by testing decrypt(handle, key) < counter. No handle will be repeated until the counter overflows. Nobody can generate a valid handle unless they break the encryption algorithm to determine your secret key, or unless they are lucky and choose a random value which happens to be a valid handle (and you could use 128-bit handles to decrease the chance of them being sufficiently lucky).

So you want a function that uniquley maps one integer value to another integer value?

eg:

#define PRIME_NUMBER_A 65521
#define PRIME_NUMBER_B 999721
#define PRIME_NUMBER_C 145709

unsigned int GetHandleForItemIndex( unsigned int in )
{
in =( PRIME_NUMBER_A*( in +PRIME_NUMBER_A ) );
in =( PRIME_NUMBER_A*( in +PRIME_NUMBER_B ) );
in =( PRIME_NUMBER_A*( in +PRIME_NUMBER_C ) );
in =( PRIME_NUMBER_B*( in +PRIME_NUMBER_A ) );
in =( PRIME_NUMBER_B*( in +PRIME_NUMBER_B ) );
in =( PRIME_NUMBER_B*( in +PRIME_NUMBER_C ) );
in =( PRIME_NUMBER_C*( in +PRIME_NUMBER_A ) );
in =( PRIME_NUMBER_C*( in +PRIME_NUMBER_B ) );
in =( PRIME_NUMBER_C*( in +PRIME_NUMBER_C ) );
return in;
}