Encrypt and Decrypt string in C#

public string Encrypt(string strInputVal)
{
string strOutputVal = "";
try
{
for (int i = 0; i < strInputVal.Length; i++)
{
strOutputVal = strOutputVal + Convert.ToChar(Encoding.ASCII.GetBytes(strInputVal.Substring(i, 1))[0] - 30);
}
if (strOutputVal != "")
strOutputVal = strOutputVal.Replace("'", "''");
return strOutputVal;
}
catch { return ""; }
}
public string Decrypt(string strInputVal)
{
string strOutputVal = "";
try
{
for (int i = 0; i < strInputVal.Length; i++)
{
strOutputVal = strOutputVal + Convert.ToChar(Encoding.ASCII.GetBytes(strInputVal.Substring(i, 1))[0] + 30);
}
return strOutputVal;
}
catch { return ""; }
}