c# - problem with hexadecimal values in xml document -

i have simple c# function takes 1 string encode , return it:

    public static string encodestring(string input)     {         byte[] bchipertext = null;          rijndaelmanaged rp = new rijndaelmanaged();         rp.key = utf8encoding.utf8.getbytes("!lb!&*w_4xc54_0w");         rp.iv = utf8encoding.utf8.getbytes("6&^fi6s5saks_ax6");          icryptotransform re = rp.createencryptor();          byte[] bcleartext = utf8encoding.utf8.getbytes(input);         memorystream mstm =  new memorystream();          cryptostream cstm = new cryptostream(mstm, re, cryptostreammode.write);          cstm.write(bcleartext, 0, bcleartext.length);         cstm.flushfinalblock();          bchipertext = mstm.toarray();          cstm.close();         mstm.close();           return system.text.asciiencoding.ascii.getstring(bchipertext);    }  

after call function parameter "hello" xml file this:

  <?xml version="1.0" encoding="utf-8"?>   <users>       <user name="user1" password="?v?py????%??&#x13;?9?"/>   </users> 

everithing fine when open xml file in visual studio 2010 receive warning this:

error 1 character ' ', hexadecimal value 0x13 illegal in xml documents.

can tell have done wrong?can ignore warnings?
thanks

this problem:

return system.text.asciiencoding.ascii.getstring(bchipertext); 

you're converting arbitrary binary data text treating if ascii. it's not. don't that.

the safest approach use base64:

return convert.tobase64string(bchipertext); 

of course, client need same in revert, e.g. using convert.frombase64string.