diff --git a/lisp.py b/lisp.py
index 5679b20..d395dd4 100755
--- a/lisp.py
+++ b/lisp.py
@@ -13,7 +13,7 @@
     archive for more details.
 """
 
-import socket,struct,random,netifaces,sys
+import socket,struct,random,netifaces,sys,hmac,hashlib
 from string import ascii_letters
 from scapy import *
 from scapy.all import *
@@ -46,6 +46,12 @@
     "lcaf" : 16387 
 }
 
+_KEY_LENGTH = {
+    0 : 0,
+    1 : 20,
+    2 : 32
+}
+
 """ nonce_max determines the maximum value of a nonce field. The default is set to 18446744073709551615, since this is the maximum possible value (>>> int('f'*16, 16)). TODO - see about the entropy for this source"""
 nonce_max = 16777215000
 nonce_min = 15000000000
@@ -99,6 +105,7 @@ def guess_payload_class(self, payload):
 """
 
 class LISP_AddressField(Field):
+    __slots__ = ["fld_name", "_ip_field", "_ip6_field"]
     def __init__(self, fld_name, ip_fld_name):
         Field.__init__(self, ip_fld_name, '0')
 
@@ -225,20 +232,41 @@ class LISP_MapReply(Packet):
 class LISP_MapRegister(Packet):
     """ map reply part used after the first 16 bits have been read by the LISP_Type class"""
     name = "LISP Map-Register packet"
+    authentication_key = "password" # the key to use in the HMAC SHA computation
     fields_desc = [ 
         BitField("ptype", 0, 4),
-        FlagsField("register_flags", None, 1, ["proxy_map_reply"]),
-        BitField("p3", 0, 18), 
-        FlagsField("register_flags", None, 1, ["want-map-notify"]),
+        FlagsField("register_flags", None, 4, ["proxy_map_reply", "lisp_sec", "itr_id_present", "rtr"]),
+        BitField("p3", 0, 13),
+        FlagsField("additional_register_flags", None, 3, ["merge", "something", "want-map-notify"]),
         FieldLenField("register_count", None, "register_records", "B", count_of="register_records", adjust=lambda pkt,x:x/16 - 1),
         XLongField("nonce", random.randint(nonce_min, nonce_max)),
 	ShortField("key_id", 0),
         ShortField("authentication_length", 0),
             # authentication length expresses itself in bytes, so no modifications needed here
         StrLenField("authentication_data", None, length_from = lambda pkt: pkt.authentication_length),
-        PacketListField("register_records", None, LISP_MapRecord, count_from=lambda pkt:pkt.register_count + 1)
+        PacketListField("register_records", None, LISP_MapRecord, count_from=lambda pkt:pkt.register_count + 1),
+        ConditionalField(XLongField("xtr_id_high", 0), lambda pkt:pkt.register_flags & 2 == 2),
+        ConditionalField(XLongField("xtr_id_low", 0), lambda pkt:pkt.register_flags & 2 == 2),
+        ConditionalField(XLongField("site_id", 0), lambda pkt:pkt.register_flags & 2 == 2)
     ]
 
+    def post_build(self, p, pay):
+        key_length = _KEY_LENGTH[self.key_id]
+        if self.key_id == 0:
+            # no HMAC
+            return p
+        # add authentication field with the correct length and bytes set to zero
+        self.authentication_data = '\x00' * key_length
+        p = p[:14] + struct.pack("!H", key_length) + self.authentication_data + p[16:]
+        if self.key_id == 1:
+            # compute HMAC-SHA1 checksum
+            self.authentication_data = hmac.new(self.authentication_key, msg=str(p), digestmod=hashlib.sha1).digest()
+        elif self.key_id == 2:
+            # compute HMAC-SHA256
+            self.authentication_data = hmac.new(self.authentication_key, msg=str(p), digestmod=hashlib.sha256).digest()
+        p = p[:16] + self.authentication_data + p[(16+key_length):]
+        return p
+
 class LISP_MapNotify(Packet):
     """ map notify part used after the first 16 bits have been read by the LISP_Type class"""
     name = "LISP Map-Notify packet"