c# - Result of RSA encryption/decryption has 3 question marks -

i using rsa encrypt , decrypt small notepad file 1 or 2 words. after processing file result has 3 question marks on begging of result.

for example, if encrypt , decrypt notepad file word "hello" in it, result "???hello". did 3 question marks come from?

this code:

    public partial class form1 : form {      private rsaparameters publickey;     private rsaparameters privatekey;      public string result;      public form1()     {         initializecomponent();         var rsa = new rsacryptoserviceprovider();         this.publickey = rsa.exportparameters(false);         this.privatekey = rsa.exportparameters(true);     }      private void button1_click(object sender, eventargs e)     {         openfiledialog1.showdialog();     }      private void openfiledialog1_fileok(object sender, canceleventargs e)     {         textbox1.text = openfiledialog1.filename;     }      private void button2_click(object sender, eventargs e)     {         filestream filestream = new filestream(textbox1.text, filemode.open);          byte[] buffer = new byte[filestream.length];         int len = (int)filestream.length;         filestream.read(buffer, 0, len);          var rsa = new rsacryptoserviceprovider();         rsa.importparameters(publickey);          var encrypted = rsa.encrypt(buffer, false);          result = convert.tobase64string(encrypted);         messagebox.show(result);     }      private void button3_click(object sender, eventargs e)     {          var rsa = new rsacryptoserviceprovider();         rsa.importparameters(privatekey);          byte[] todecode = convert.frombase64string(result);          var decrypted = rsa.decrypt(todecode, false);          string msg = encoding.ascii.getstring(decrypted);         messagebox.show(msg);     } } 

it's input file encoding utf8, , decoding ascii. try changing

string msg = encoding.ascii.getstring(decrypted); 

to

string msg = encoding.utf8.getstring(decrypted); 

the question marks generated byte order mark (bom) in front of text. uncommon utf-8 not require bom. more common utf-16 endianness issue, rest of plain text seems decode ascii, can not utf-16 encoded.

note ascii cannot show characters value 127 (7f in hexadecimals) or higher. .net platform seems silently replace bom values question marks.