Issuing objectGUID as an ADFS Claim
The objectGUID in AD will in a base64 format when issued from an AD attribute store. To get the actual GUID value, you must decode and convert it. You can use the StringPrcoessing custom attribute store and extend it using something like:
static private string ConvertBase64ToGuid(string myData)
{
byte[] encodeAsBytes = System.Convert.FromBase64String(myData);
string returnValue = new Guid(encodeAsBytes).ToString();
return returnValue;
}