C#

string HexToBinary(string hexString)
{
string result = "";
foreach(char c in hexString)
{

switch(c){
case '0':
 result += "0000";
case '1':
 result += "0001";
...
case 'F':
 result += "1111";
}
}

return result;
}

string BinaryToHex(string binString)
{
string result = "";
for(int i = 0; ; < binString.length / 4; i++)
{
    string temp = binString[0] + binString[1] + binString[2] + binString[3];
     binString = binString.Substring(4, binString.Length - 1);

  switch(temp){
case "0000":
   result += "0";
...

case "1111":
   result += "F";
}
}
return result;
}
string XORStrings(string s1, string s2)
{
   string bin1 = HexToBinary(s1);
   string bin2 = HexToBinary(s2);

string result = "";
   for(int i = 0; i < bin1.length; i++)
   {
     if(bin1[i] == bin2[i])//either both 0 or both 1
           result += "0";
     else //both are different i.e. 1 and 0 or 0 and 1
           result += "1";
   }
 return BinaryToHex(result);

}

private string HexToBinary(string hexString)
{
    string binaryval = String.Empty;
    for (int i = 0; i < hexString.Length; i = i + 2)
    {
       binaryval += Convert.ToString(Convert.ToInt32(hexString.Substring(i, 2), 16), 2).PadLeft(8, '0');
    }

    return binaryval;
}

private string BinaryToHex(string binString)
{
    string hexval = String.Empty;

    for (int i = 0; i < binString.Length; i = i + 8)
    {
       hexval += Convert.ToString(Convert.ToInt32(binString.Substring(i, 8), 2), 16).PadLeft(2, '0');
    }
    return hexval;
}

private string XORStrings(string s1, string s2)
{
    string bin1 = HexToBinary(s1);
    string bin2 = HexToBinary(s2);

    string result = String.Empty;
    for (int i = 0; i < bin1.Length; i++)
    {
         if (bin1[i] == bin2[i])//either both 0 or both 1
             result += "0";
         else //both are different i.e. 1 and 0 or 0 and 1
             result += "1";
    }
    return BinaryToHex(result);

}