Object Storage Service：.NET

private static string GetPolicyToken()
{
  //expireTime
  var expireDateTime = DateTime.Now.AddSeconds(expireTime);

  // example of policy
  //{
  //  "expiration": "2020-05-01T12:00:00.000Z",
  //  "conditions": [
  //    ["content-length-range", 0, 1048576000]
  //    ["starts-with", "$key", "user-dir-prefix/"]
  //  ]
  //}

  //policy
  var config = new PolicyConfig();
  config.expiration = FormatIso8601Date(expireDateTime);
  config.conditions = new List<List<Object>>();
  config.conditions.Add(new List<Object>());
  config.conditions[0].Add("content-length-range");
  config.conditions[0].Add(0);
  config.conditions[0].Add(1048576000);
  config.conditions.Add(new List<Object>());
  config.conditions[1].Add("starts-with");
  config.conditions[1].Add("$key");
  config.conditions[1].Add(uploadDir);

  var policy = JsonConvert.SerializeObject(config);
  var policy_base64 = EncodeBase64("utf-8", policy);
  var signature = ComputeSignature(accessKeySecret, policy_base64);

  //callback
  var callback = new CallbackParam();
  callback.callbackUrl = callbackUrl;
  callback.callbackBody = "filename=${object}&size=${size}&mimeType=${mimeType}&height=${imageInfo.height}&width=${imageInfo.width}";
  callback.callbackBodyType = "application/x-www-form-urlencoded";

  var callback_string = JsonConvert.SerializeObject(callback);
  var callback_string_base64 = EncodeBase64("utf-8", callback_string);

  var policyToken = new PolicyToken();

  policyToken.accessid = accessKeyId;
  policyToken.host = host;
  policyToken.policy = policy_base64;
  policyToken.signature = signature;
  policyToken.expire = ToUnixTime(expireDateTime);
  policyToken.callback = callback_string_base64;
  policyToken.dir = uploadDir;

  return JsonConvert.SerializeObject(policyToken);
}

public void DoGet()
{
  Console.WriteLine("DoGet request: {0}", this.httpURL);
  var content = GetPolicyToken();
  this.swResponse.WriteLine("HTTP/1.0 200 OK");
  this.swResponse.WriteLine("Content-Type: application/json"); 
  this.swResponse.WriteLine("Access-Control-Allow-Origin: *");
  this.swResponse.WriteLine("Access-Control-Allow-Method: GET, POST");
  this.swResponse.WriteLine($"Content-Length: {content.Length.ToString()}");
  this.swResponse.WriteLine("Connection: close");
  this.swResponse.WriteLine("");
  this.swResponse.WriteLine(content);
}

public bool VerifySignature()
{
  // Get the Authorization Base64 from Request
  if (this.httpHeadersDict["authorization"] != null)
  {
    this.strAuthorizationRequestBase64 = this.httpHeadersDict["authorization"].ToString();
  } else if (this.httpHeadersDict["Authorization"] != null) {
    this.strAuthorizationRequestBase64 = this.httpHeadersDict["Authorization"].ToString();
  }
  if (this.strAuthorizationRequestBase64 == "")
  {
    Console.WriteLine("authorization property in the http request header is null. ");
    return false;
  }

  // Decode the Authorization from Request
  this.byteAuthorizationRequest = Convert.FromBase64String(this.strAuthorizationRequestBase64);

  // Decode the URL of PublicKey
  this.strPublicKeyURLBase64 = this.httpHeadersDict["x-oss-pub-key-url"].ToString();
  var bytePublicKeyURL = Convert.FromBase64String(this.strPublicKeyURLBase64);
  var strAsciiPublickeyURL = System.Text.Encoding.ASCII.GetString(bytePublicKeyURL);

  // Get PublicKey from the URL
  ServicePointManager.ServerCertificateValidationCallback = new RemoteCertificateValidationCallback(validateServerCertificate);
  HttpWebRequest request = (HttpWebRequest)WebRequest.Create(strAsciiPublickeyURL);
  HttpWebResponse response = (HttpWebResponse)request.GetResponse();
  StreamReader srPublicKey = new StreamReader(response.GetResponseStream(), Encoding.UTF8);
  this.strPublicKeyBase64 = srPublicKey.ReadToEnd();
  response.Close();
  srPublicKey.Close();
  this.strPublicKeyContentBase64 = this.strPublicKeyBase64.Replace("-----BEGIN PUBLIC KEY-----\n", "").Replace("-----END PUBLIC KEY-----", "").Replace("\n", "");
  this.strPublicKeyContentXML = this.RSAPublicKeyString2XML(this.strPublicKeyContentBase64);

  // Generate the New Authorization String according to the HttpRequest
  String[] arrURL;
  if (this.httpURL.Contains('?'))
  {
    arrURL = this.httpURL.Split('?');
    this.strAuthSourceForMD5 = String.Format("{0}?{1}\n{2}", System.Web.HttpUtility.UrlDecode(arrURL[0]), arrURL[1], this.httpBody);
  }
  else
  {
    this.strAuthSourceForMD5 = String.Format("{0}\n{1}", System.Web.HttpUtility.UrlDecode(this.httpURL), this.httpBody);
  }

  // MD5 hash bytes from the New Authorization String 
  var byteAuthMD5 = byteMD5Encrypt32(this.strAuthSourceForMD5);

  // Verify Signature
  System.Security.Cryptography.RSACryptoServiceProvider RSA = new System.Security.Cryptography.RSACryptoServiceProvider();
  try
  {
    RSA.FromXmlString(this.strPublicKeyContentXML);
  }
  catch (System.ArgumentNullException e)
  {
    throw new ArgumentNullException(String.Format("VerifySignature Failed : RSADeformatter.VerifySignature get null argument : {0} .", e));
  }
  catch (System.Security.Cryptography.CryptographicException e)
  {
    throw new System.Security.Cryptography.CryptographicException(String.Format("VerifySignature Failed : RSA.FromXmlString Exception : {0} .", e));
  }
  System.Security.Cryptography.RSAPKCS1SignatureDeformatter RSADeformatter = new System.Security.Cryptography.RSAPKCS1SignatureDeformatter(RSA);
  RSADeformatter.SetHashAlgorithm("MD5");

  var bVerifyResult = false;
  try
  {
    bVerifyResult = RSADeformatter.VerifySignature(byteAuthMD5, this.byteAuthorizationRequest);
  }
  catch (System.ArgumentNullException e)
  {
    throw new ArgumentNullException(String.Format("VerifySignature Failed : RSADeformatter.VerifySignature get null argument : {0} .", e));
  }
  catch (System.Security.Cryptography.CryptographicUnexpectedOperationException e)
  {
    throw new System.Security.Cryptography.CryptographicUnexpectedOperationException(String.Format("VerifySignature Failed : RSADeformatter.VerifySignature Exception : {0} .", e));
  }

  return bVerifyResult;
}

public void DoPost()
{
  this.GetPostBody();

  // Verify Signature
  try
  {
    if (this.VerifySignature())
    {
      Console.WriteLine("\nVerifySignature Successful . \n");

      // do something accoding to callback_body ... 

      this.HttpResponseSuccess();
    }
    else
    {
      Console.WriteLine("\nVerifySignature Failed . \n");
      this.HttpResponseFailure();
    }
  }
  catch
  {
    Console.WriteLine("\nVerifySignature Failed . \n");
    this.HttpResponseFailure();
  }
}