C# – The input is not a valid Base-64 string as it contains a non-base 64 character

base64c

Here I am trying to decrpyt a string using private key and getting an error like:

The input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding characters, or a non-white space character among the padding characters.

The string(i.e string authenticationString) which i am trying to decrpyt is:

Z7W7ja7G+NB3+QsbNnsz7zCkt1xeZ1PQC606wZzQG2McjExT6WjFPDWNpVSqcw1X+K6TERZUK4677m5Z6x9TuxLxyA8h8LmB4dwcJsQZGoVg0mOLsxO6GZmdThLyQOxQgnA7sk4KHLv6DrVswtVzjM/gJouvDKHTC7+NZmjhWwA=

and my code is:

 internal virtual Credentials Extract(string basicAuthenticationCredentials)
      {
         string authenticationString = RemoveBasicFromAuthenticationString(basicAuthenticationCredentials);
         string privateKeyPath = @"D:\Bala\MVC\RestService\RestClient\Scripts\PrivateKey.xml";
         myRsa.LoadPrivateFromXml(privateKeyPath);  // Loading the Private key
         RSACryptoServiceProvider localRsa = new RSACryptoServiceProvider();
         localRsa.FromXmlString(File.ReadAllText(privateKeyPath));
         byte[] decMessage = Convert.FromBase64String(authenticationString);
         byte[] message = null;
         // Calling the right decryption method according to the user selection
         message = myRsa.PrivateDecryption(decMessage);
         string au = Encoding.UTF8.GetString(message);
         return extractor.Extract(decoder.Decode(au));
      }

and in string au I am geting a value like:

d��!���u�I|�3��iaȴ{ȱ��q��A��z��ta �i8?�-�[�#�*&��Y^l,�v������ā�\�f�$R�V����&g;�

and am getting this error for a particular username password only. Others are working fine.
Any suggestion?

EDIT:
The error is throwing in this line byte[] decodedStringInBytes = Convert.FromBase64String(encodedValue);

internal virtual string Decode(string encodedValue)
      {
         byte[] decodedStringInBytes = Convert.FromBase64String(encodedValue);
         return Encoding.ASCII.GetString(decodedStringInBytes);
      }

EDIT 2:

internal class DecodedCredentialsExtractor
   {
      internal virtual Credentials Extract(string credentials)
      {
         if (!string.IsNullOrEmpty(credentials))
         {
            string[] credentialTokens = credentials.Split(':');
            //string securityToken = string.Empty;
            if (credentialTokens.Length == 2)
            {
               return new Credentials(credentialTokens[0], credentialTokens[1]);
            }
         }

         throw new ArgumentException("The supplied credential string is invalid, it should comply to [username:password]", "credentials");
      }
   }

Best Answer

The fact that you are getting a value for au suggests that it isn't the base-64 decode in the question that is erroring. I'm going to make the assumption that is actually the last line that is failing, and is expecting base-64 - and you are using UTF-8 (backwards, I suspect, which isn't valid). Try:

     message = myRsa.PrivateDecryption(decMessage);
     string au = Convert.ToBase64String(message);
     return extractor.Extract(decoder.Decode(au));
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