authentic man in the middle

Nikhil Mishra nikhilm at
Tue Feb 24 05:39:32 EST 2009

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


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