Simple Encryption & Decryption in C#

//encryption
string output = "";
char[] readChar = InputText.ToCharArray();
for (int i = 0; i < readChar.Length; i++)
{
int no = Convert.ToInt32(readChar[i]) + 10;
string r = Convert.ToChar(no).ToString();
output += r;
}

//decryption

string output = "";
char[] readChar = InputText.ToCharArray();
for (int i = 0; i < readChar.Length; i++)
{
int no = Convert.ToInt32(readChar[i]) - 10;
string r = Convert.ToChar(no).ToString();
output += r;
}