Re: [TICTOC] [ntpwg] security ID submitted for review

[[e-mail]]

Hello Dave and Kurt,

thanks for your comments:

As Kurt already pointed out. Currently the ID specifies only the design of 
the security protocol. We believe that we need to be clear about the 
design questions before specifying the technical details. 

The private key of the client is not needed for the MAC. Instead the 
calculation of the MAC includes the cookie and the whole NTP packet (i.e. 
the extension fields also). The cookie is calculated from the server seed 
and the client's public key, which the client attaches to each time 
request (which requires a new extension field as pointed out in the 
draft). During cookie exchange the server encrypts the cookie with the 
client's public key before transmission. An adversary is therefore not 
able to abuse a cookie which he requests in behalf of the client. 

As Kurt pointed out the certificate exchange works as in a standard PKI. I 
emphasize that this is different to autokey. In autokey the chain of trust 
and time are equal whereas in this ID chain of trust and time might be 
disjunct.

Regarding the broadcast mode. I agree with Dave that the use of TESLA for 
the broadcast mode adds additional complexity.  It also has the 
disadvantage that the authenticity of a packet can only be verified in 
retrospect. But I don't see security vulnerabilities. Dave's suggestion to 
use a combination of interleave mode with asymmetric cryptography is 
interesting. We should consider this. The argument that asymmetric 
cryptography might be feasible because of modern power of CPUs is okay for 
traditional computer systems but I'm not sure whether this assumption also 
applies for small devices with constrained resources. 

Regards
Dieter


----------------------------------
Physikalisch-Technische Bundesanstalt
Dr. Dieter Sibold
Q.42 - Serversysteme und Datenhaltung
QM-Verantwortlicher der Stelle IT-Infrastruktur
Bundesallee 100 
D-38116 Braunschweig
Tel: [phone]
E-Mail: [e-mail]



Von:    Kurt Roeckx <[e-mail]>
An:     "David L. Mills" <[e-mail]>
Kopie:  NTP Working Group <[e-mail]>, "[e-mail]" 
<[e-mail]>
Datum:  03.08.2013 20:04
Betreff:        Re: [ntpwg] security ID submitted for review
Gesendet von:   ntpwg-bounces+dieter.sibold=[e-mail]



On Thu, Aug 01, 2013 at 08:42:08PM +0000, David L. Mills wrote:
> Guys,
> 
> Here are some general comments on the ID.  I apologize if they are a
> little rough as I have to dictate this message instead of my usual
> polished wordsmithing.
> 
> format.
> The ID does not specify an extension field format.

The draft only seems to specify the the general idea of how it
should work and doesn't seem to go into detail how this should
work at the protocol level.  I assume he first wants to know if
the solution is something that can work and that others see
as a good solution before he works out the details.

> In particular, the private key for the message
> authentication code (MAC) , if used, must be known beforehand.  This
> is where the "cookie snatcher" issue comes up as described in
> previous messages.  In particular, an important constraint for
> normal operation is that extension fields are not used unless
> required for cookie determination.

I'm not sure I understand your remark.  As far as I know the
"cookie snatcher" you talk about is the normal "man in the middle"
attack.  As I understand we would change the verification by
replacing it by the normal X.509 verification as done for instance
SSL/TLS.

I assume you meant public key and note private key.

> Glaring vulnerability exists when using extension fields, as the key
> value is public and must be known in advance.  This invites a
> cut-and-paste attack, as well as header changes, even if
> subsequently detected by the on-wire protocol.

I've seen this cut-and-paste attack mentioned before, but I've
never really understood it.  As I understand things, the MAC
is generated over the NTP + extensions.  I don't see how you
can take the extension from 1 packet into an other and still
have a valid MAC.  Or are you saying that you do something
with a packet even when it doesn't have a valid MAC?

> certificate.
> Both RFC 5906 and the ID require the server certificate.  The server
> certificate can be conveyed, either off-line, or directly or
> indirectly via a certificate trail as described in RFC 5906 or the
> ID.  If obtained via the trail, there is a hazzard due to a
> middle-man attack and masquerade.

I really don't understand how such a man in the middle attack
can work.  Either the certificate + the chain validate, or they
don't.  It is of course important to you check that the CommonName
in the certificate matches the server you're trying to reach,
and that the root CA is in your list of trusted CAs.  But there
really isn't anything new or hard about this.

> In broadcast mode, the protocols described in RFC 5906 and the ID
> are messy and with several possible vulnerabilities.  Here is a
> suggestion that might be useful.  Define a new broadcast packet
> format as the NTP header followed by one extension field
> representing the MAC.  The extension field represents the signature
> of the header computed using the private key of the server and can
> be verified by the client using the public key in the server
> certificate.  Originally, this was considered an unacceptable
> dilution of accuracy due to the variability of the signature
> computing time and the length of the signature.  However, at least
> in modern NTP, the interleaved broadcast mode can be used to correct
> for this overhead.  This assumes that the overhead for either the
> server or client is reduced using fast modern processors.

I think that in all cases where it's no problem to keep track of
all clients you should use the interleaved mode, and we can use
something like the Diffie-Hellman key agreement for those.

But there clearly is also a need for servers that can't keep
the state of each client, and we need something else for that.
So maybe we need to define 2 ways of working.



Kurt

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