Newbie's question

[Jeffrey Hutzelman]

On Wednesday, March 30, 2005 04:33:21 PM -0800 kaiduan xie 
<kaiduanx at hotmail.com> wrote:

> Hi, all,
>
> I have an application where the client needs to authenticate to the
> server before carrying on further communications. The client will acquire
> ticket from KDC which runs in Microsoft's Domain Controller. After
> getting the ticket, the client will send the ticket and the authenticator
> together to the server for authentication. My question are:
>
> 1) Can I put the authenticator and ticket into MY application's message
> instead of Kerberos's Application Request message? For example, convert
> the authenticator and ticket into hex-coded string and put this string
> into the MY applicaiton's message.
>
> Credential: 0fde234567abedghi
>
> and at the server side, after receiving the authenticator and ticket from
> the application's message (NOT Kerberos's Application Request message),
> the server applies Kerberos algorithm to verify whether the credential is
> correct or not.

Well, you _can_ do whatever you want, but if you don't use Kerberos 
messages, then the resulting protocol is not Kerberos and cannot safely be 
assumed to have the security properties of Kerberos.  For example, in the 
Kerberos AP-REQ exchange, the authenticator is both encrypted and 
integrity-protected.  The algorithms used to provide these properties are 
carefully designed to avoid a number of attacks against both the crypto and 
the protocol as a whole; without the require properties, the protocol is 
not secure.

You should not expect Kerberos libraries to give you much help if you are 
not using Kerberos messages.  The simplest and most portable interfaces are 
designed around producing and consuming Kerberos protocol messages.  If you 
choose not to use those messages, you will end up writing a lot more code.


What you probably want to do instead is use the Kerberos messages, but 
transport them in an encoded form suitable for your application.  For 
example, your "Credential" example above could be the hex-encoded form of a 
Kerberos AP-REQ.  However, you should bear in mind that the actual messages 
will be considerably larger than suggested by your example.  If possible, 
you should consider using a more compact encoding, such as the base-64 
encoding defined in section 3 of RFC3548.

Also, it is worth noting that unless your application message is 
integrity-protected (and possibly encrypted) in a way that is 
cryptographically bound to your authentication exchange, you haven't 
actually gained much.  Sending a "Credential" containing a hex-encoded 
AP-REQ may prove the client's identity, but it doesn't prove that any of 
the rest of the message actually came from that client, as opposed to from 
some attacker who modified the request in transit.

-- Jeffrey T. Hutzelman (N3NHS) <jhutz+ at cmu.edu>
   Sr. Research Systems Programmer
   School of Computer Science - Research Computing Facility
   Carnegie Mellon University - Pittsburgh, PA