node js hmac

var crypto = require('crypto');

var server_seed = 'ycspKWP0Rn3fm.bCmUdhpv88izrBGNC4.CAVPULqDYTR8s8VVmqcwoMcrHCY28lX3';
var client_seed = '0755766871111';
var nonce = 1;
var count = 1;

while (true) {
    var hash = crypto.createHmac('sha512', server_seed).update(client_seed + ':' + nonce.toString()).digest('hex').substring(0, 2);
    if (hash == '00') {
        console.log('  0', nonce);
        if (count++ == 1e3) break;
    } else if (hash == 'ff') {
        console.log('  1', nonce);
        if (count++ == 1e3) break;
    }

    nonce++;
}