Getting error ASN1 bad tag value met. in C#

Question

I am integrating thirdparty API and tryed to decrpt thirdparty result using private key

third party user using java to encrypt the result using public key . i am using C#.net

application to decrypt the data. third party user has given below instructions to decrypt the

text .

 

1)Those fields are in Base64URL format. So the first thing we need to do is to convert it to Base64.

 

2) After obtaining the Base64 values, we need to decode them. To do that, we used "base64 --decode" linux command.

 

3) After decoding all parts, we decrypt the secondary key with Merchant's private key.

below is my code which i am trying

 

key ="e4yDH3QAhnYtMRLPX-iRFJVn2Q1NGVRG6Rs5DEqb6F9tsMyyAggny831leDV-xXxm50LDlgO4EMdb_7iLdsoV2KyeRMBkYn8doARZDMu24Y8j55mQqaAB_na-ju2LiWSSNlPImUWtn1Peoh3esh01ia3mUWN2HKkg3sPzEa20-E";

string dummyData = key.Replace("-", "+");

dummyData= dummyData.Replace("_", "/");

dummyData= dummyData.Replace(" ", "+") ;

dummyData = dummyData.PadRight(dummyData.Length + (4 - dummyData.Length % 4) % 4, '=');

var convertbase64 = Convert.FromBase64String(dummyData);

//string decodedString = Encoding.UTF8.GetString(convertbase64);

string decodedString = Convert.ToBase64String(convertbase64);

string path = ConfigurationManager.AppSettings["Certpath"].ToString();

doDecrypt(decodedString.ToString());

private PKCS7 sntqPKCS7 = new PKCS7();

public string doDecrypt(string str2Decrypt)

{

string strAfterDecrypt = "";

string password = "Password";

try

{

log.Info("start Decrypt.");

var certificate = new X509Certificate2(ConfigurationManager.AppSettings["Certpath"].ToString(),

password);

strAfterDecrypt = sntqPKCS7.decryptMessage(str2Decrypt, certificate);

log.Info("strAfterDecrypt :" + strAfterDecrypt);

log.Info("doDecrypt Success.");

}

catch (Exception exp)

{

log.Info("doDecrypt (Err) : " + exp.Message);

}

return strAfterDecrypt;

 

Error :System.Security.Cryptography.CryptographicException: ASN1 bad tag value met.

 

Kindly help on this.