var verifier = require('gamecenter-identity-verifier');

// identity from client.
// Reference:  https://developer.apple.com/library/ios/documentation/GameKit/Reference/GKLocalPlayer_Ref/index.html#//apple_ref/occ/instm/GKLocalPlayer/generateIdentityVerificationSignatureWithCompletionHandler

var identity = {
  publicKeyUrl: 'https://valid.apple.com/public/timeout.cer',
  timestamp: 1460981421303,
  signature: 'PoDwf39DCN464B49jJCU0d9Y0J',
  salt: 'saltST==',
  playerId: 'G:1111111',
  bundleId: 'com.valid.app'
};

verifier.verify(identity, function (err, token) {
  if (!err) {
    // use token in here.
    console.log(token);
  }
});