Re: [Hipsec] Comments on hiccups draft

[Tobias Heer <[e-mail]>]

Hi Jan,

Am 28.07.2009 um 16:30 schrieb Jan Melen:

>> 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.
>>
>
> if you do send it as the receiving end doesn't have to generate the  
> actual parameter that was used to create MAC code in order to verify  
> the signature.

You must recalculate the digest anyway. Otherwise the receiver will  
only check that the signature covers the HMAC but the receiver will  
not check that the packet contents match the HMAC. In that sense only  
the HMAC but not the packet contents would be integrity-protected.  
Since you have to generate the digest over the whole packet anew  
anyway I do not see the advantage of sending it in the packet.

Best regards,

Tobias



--  

Dipl.-Inform. Tobias Heer, Ph.D. Student
Distributed Systems Group
RWTH Aachen University, Germany
tel: [phone]
web: http://ds.cs.rwth-aachen.de/members/heer