authentic man in the middle

Jeffrey Altman jaltman at
Tue Feb 24 11:55:19 EST 2009


The problem you are facing is that you do not control the
key management of the Windows Domain.  Windows services
do not use fixed keys.  They use long term passwords that
are assigned to each account and which are then used with
the necessary enc-type and service principal name to compute
the appropriate key on-the-fly.  This permits an account to
have multiple names all of which do not need to be known
at account registration time.  For example, a mobile machine
that obtains a different hostname at each boot and registers
it with dynamic dns which in turn updates the machine's
entry in the active directory.

The passwords are also periodically updated.  Therefore,
even if you were to extract the machine's password from
the registry its lifetime would be limited.  You would
have to do it again whenever the password was replaced.

As a side note, this discussion really has nothing to do
with the development of MIT Kerberos.  Therefore, it is
my opinion that it should be held either on the kerberos at
mailing list or one of the Windows Security Groups.

Jeffrey Altman
Secure Endpoints Inc.

Nikhil Mishra wrote:
> Hi All,
> We have an issue with generating a valid keytab for windows based
> services which can be used on unix based machines to decrypt AP-REQ.
> I understand this issue is more on windows side but since I am trying
> to implement sort of man in the middle on MIT kerberos I think 
> someone could lend me some helping hand here.Any related references
> might also do some good to me.-
> Following is our setup :
> 1. Windows XP cifs client
> 2. Windows 2003 KDC and domain controller 64 bit
> 3. Windows XP cifs server 64 bit.
> 4. Linux FC7 machine with MIT kerberos 1.6.3
> We have the admin privileges for all the machines mentioned above.
> What we are trying to do ?
> 1. We request a kerberized traffic from cifs client to cifs server
>     which we want to route through linux box.
> 2. We want to do some processing with the AP-REQ.  Evidently  for
>     which we need to authenticate the client in AP_REQ on linux machine.
> 3. Now to authenticate the client in AP-REQ on linux machine we
>     propose to use GSSAPI calls using corresponding service keytab.
> The problem :
> 1. Our understanding is, all windows based services are registered
>     under corresponding computer name with their corresponding SPN.
> 2. This registration occurs whenever the machine joins the domain. So
>     basically , whenever the  server is up and running and is in domain
>     all its services are registered with windows domain controller and
>     are mapped to its computer name.
> 3. The exchange of long term keys for service between service and KDC
>     occurs at the same time.
> 4. We understand the definition of ktpass is "To generate keytab for
>     UNIX based services " but with no other option to generate a keytab,
>     we run ktpass for this windows based service which creates a new
>     long term service key for the service which  is not communicated back
>     to service.
>     When I use this keytab on linux machine through GSSAPI calls to
>     decrypt the AP-REQ , I get KRB5KRB_AP_ERR_BAD_INTEGRITY.
>     which is obvious since key used by KDC to encrypt the ticket for
>     service is different(Its the old key ) than what is in keytab.
> Questions :
> 1. Is there a way to bring KDC and  service in sync in terms of the
>     service key being used ? To be more precise , If I change the
>     service key for a service at KDC Is there a way to communicate 
>     this back to service so that the service starts using this new key
>     for all further requests ?
> 2. We understand ktpass is a tool to generate a keytab for unix based
>    services. Do we have any similar tool for windows based services ?
> 3. Since windows based service SPN's are registered under computer name
>     at the time of logon It can be mapped to some other user as well without
>     creating a duplicate SPN. Is it possible for a service to run under
>     a user account and obtain a service key in windows ?
> 4. We understand "man in the middle" is not possible with kerberos but 
>    when we own all components of traffic ( KDC , server , client , DC 
>    with admin privileges ) should't I be allowed to extract a service key
>    for the given SPN from KDC without disturbing the existing setup ? 
> Any help is deeply appreciated.
> Thanks & Regards
> Nikhil
> _______________________________________________
> krbdev mailing list             krbdev at
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