shaka-player/tutorials/network_sample1.txt

/**
 * @param {string} schemeIdUri The ContentProtection's scheme ID URI.
 * @param {!Element} contentProtection The ContentProtection element.
 * @return {Array.<shaka.player.DrmInfo.Config>} An array of Config
 *     objects or null if the element is not understood by this application.
 */
function interpretContentProtection(schemeIdUri, contentProtection) {
  if (schemeIdUri != App.APP_SPECIFIC_SCHEME_ID_URI) {
    console.warn('Unrecognized scheme: ' + schemeIdUri);
    return null;
  }

  /**
   * @param {!shaka.player.DrmInfo.LicenseRequestInfo} requestInfo
   */
  var preProcessor = function(requestInfo) {
    // Encode the license request message as base64.
    var message = window.btoa(
        String.fromCharCode.apply(null, new Uint8Array(requestInfo.body)));

    // Encode the payload as JSON.
    var obj = {
      'app_data': App.clientData,  // Provide some arbitrary data.
      'message', message
    };
    requestInfo.body = JSON.stringify(obj);
    requestInfo.headers['Content-Type'] = 'application/json';
  };

  /**
   * @param {!Uint8Array} serverResponse
   * @return {!Uint8Array}
   */
  var postProcessor = function(serverResponse) {
    // serverResponse is a Uint8Array, so decode it into an object.
    var json = String.fromCharCode.apply(null, serverResponse);
    var obj = JSON.parse(json);

    // Update our arbitrary data.
    App.clientData = obj['app_data'];

    // obj['license'] is base64, so decode it into a string and then encode it
    // into a Uint8Array.
    var licenseString = window.atob(obj['license']);
    var license = new Uint8Array(licenseString.split('').map(
        function(ch) { return ch.charCodeAt(0); }));
    return license;
  };

  return [{
    'keySystem': 'com.widevine.alpha',
    'licenseServerUrl': App.APP_SPECIFIC_LICENSE_SERVER_URL,
    'licensePreProcessor': preProcessor,
    'licensePostProcessor': postProcessor
  }];
}