[Hipsec] Comments on hiccups draft

[Tobias Heer <heer at cs.rwth-aachen.de>]

Some comments on the hiccups draft. In general I find the approach  
interesting and worth pursuing. Below are some questions that mostly  
focus on security:

Conceptual comments and questions:
----------------------------------

Section 3: Existence of the HMAC in the packet:
The hiccups draft states that "[the payload is] protected by a  
PAYLOAD_HMAC parameter". To me it is unclear how such protection can  
possibly work. Since there is no previous handshake there are no keys  
for use in the HMAC. Jan explained that the HMAC is merely used as a  
way to create a digest over the packet for making the signature more  
efficient. However, if it is only used for creating the digest, I  
wonder why it is actually transmitted in the packet because without a  
secret included in the packet, the digest can easily be calculated and  
transmitting the digest in a packet seems to be a unnecessary waste of  
space. Am I missing something here? It would be nice if the draft was  
more precise about the nature and the use of the HMAC.

Section 4.3: Sending R1 if receiver suspects an attack:
I guess that in this case, the receiver drops the data packet and its  
content should be retransmitted (by a higher-layer mechanism?)? If  
yes, this could be mentioned somewhere.

Replay protection:
The draft talks about a immediate replays of DATA packets in the  
context of DoS attacks targeted at ACK signature generation. However,  
it does not talk about replays after a longer time (e.g. to mess with  
upper-layer state machines).
How can hosts shelter against replays of valid but old hiccups  
packets? The sequence number only works from the second packet on and  
still complete sequences of HIP DATA packets can be replayed. Caching  
packets or keeping state for every ever-received packet is probably  
not feasible. Is there a solution? If not, this should also be stated  
in the text and the security considerations.

Various sections: DoS resistance:
The draft briefly mentions that the protocol is susceptible to DoS  
attacks. This statement is rather vague and only mentions the absence  
of "half-stateless DoS protection nature of the base exchange" and  
immediate replays. However, CPU targeted DoS attacks (verification of  
PK-sigantures without a puzzle or working HMAC to shelter against  
floods of HIP DATA packets - not necessarily replays) seem much more  
realistic to me than state a space exhaustion attacks. Using HIP DATA  
packet creates a computational asymmetry between the attacker and the  
victim (receiver). This imbalance could be mentioned in a more  
explicit way.


Editorial comments:
-------------------

Section 4.3: "The host MAY responds" -> "The host MAY respond"

Best regards,
Tobias






--  

Dipl.-Inform. Tobias Heer, Ph.D. Student
Distributed Systems Group
RWTH Aachen University, Germany
tel: +49 241 80 207 76
web: http://ds.cs.rwth-aachen.de/members/heer