php - How can I create a shared-secret voucher code system between 2 independent servers? -

given workflow:

server a

1. user authenticates.
2. user purchases randomly generated unique voucher code using shared secret use application on on server b.

server b

1. user authenticates.
2. user inputs voucher code.
3. server b validates code legitimate using shared secret
4. server b grants access application.

i need way in php implement functions generatevouchercode , validatevouchercode shown below:

server a

$voucher = generatevouchercode("somesharedsecret"); 

server b

$isvalid = validatevouchercode($userinputtedcode, "somesharedsecret"); if($isvalid) {     // allow access application } 

validating legitimacy through shared secret hmacs for. can generate hmac in php through hash_hmac. workflow be:

1. server generates one-use code (in manner want) , calculates hmac. pair of code + hmac given user voucher code.
2. user presents voucher server b.
3. server b isolates one-use code voucher , independently calculates hmac using shared secret. if calculated hmac matches 1 in voucher voucher genuine.

example voucher generation:

$secret = '$uper$ecret$tring'; $code = 'a pet unicorn'; $voucher = $code.'/'.hash_hmac('sha512', $code, $secret);  echo 'your voucher '.$voucher'; 

example voucher verification:

$secret = '$uper$ecret$tring'; list ($code, $hmac) = explode('/', $voucher); $verify_hmac = hash_hmac('sha512', $code, $secret); if ($hmac === $verify_hmac) {     echo 'your voucher can redeemed '.$code'; } else {     echo 'invalid voucher, sorry'; }