Kerberos
Kerberos authentication scheme is the replacement of the NTLM discussed earlier, it’s
said to be a secure authentication scheme although it does a good job of preventing replay
attacks by using time stamps, does mutual authentication of the client and server and vice versa
it also uses better encryption and hashing algorithms than NTLM such as AES and SHA-256.
The process of kerberos is quite different from the one used in NTLM which was based upon
challenge-response mechanism, the process of Kerberos is based upon ticket-based
authentication. The steps of authentication are as follows:
1. The client sends its IDC, IDTGS and TS1 to the ticket granting server
2. The TGS verifies the data sent and sends a message encrypted the KC, which
only the client knows, after the client decrypts this message the following data will
be show:
a. KC,TGS: A key that only client and the ticket granting server know
b. IDC: The identifier of the client in the database
c. TS2: The second time stamp which should be more than TS1
d. Lifetime2: The time before this ticket expires
e. TicketTGS: A ticket to the TGS, which is encrypted with KTGS which only the
TGS know and is handled as a black-box by the client, this ticket
contains:
i. KC,TGS
ii. IDC
iii. ADC: The MAC address of the client's machine
iv. IDTGS: The ID of the ticket granting server
v. TS2
vi. Lifetime2
3. The client then sends these to the TGS:
a. IDV: The identifier of the service the client wants to access
b. TicketTGS
c. AuthenticatorC: An encrypted message with KC,TGS, contains:
i. IDC
ii. ADC
iii. TS3
4. The TGS replies back with an encrypted message with KC,TGS which contains:
a. KC,V: A key only known to the client and the service server
b. IDV
c. TS4
d. TicketV: A service granting ticket which is encrypted with KV,It contains:
i. KC,V
ii. IDC
iii. ADC
iv. IDV
v. TS4
vi. Lifetime4
5. Then the client will send the ticketV and the authenticatorC to the service server,
the authenticator in this step is different from the one used earlier it’s encrypted
with KC,V and has:
a. IDC
b. ADC
c. TS5
6. The service server will reply with an encrypted message with the key KC,V, the
message contains a modified version of TS5 that the service server and the client
agreed upon before the communication starts.
The use of timestamps stops replay attacks while the ADC acts as a simple origin
authentication mechanism although it’s easily avoidable by many tools to change the MAC
address of a machine virtually another point is the use of encryption throughout this process to
ensure the authentication process can’t be exposed to adversaries also, the use of a
mathematical function on TS5 authenticates the service server to the client, it’s worth noting that
the domain controller acts as the ticket granting server and the service server while the
database, which holds the data of the user to verify and authenticate, is the active directory.
This image has the steps in which I built my understanding upon, this protocol seems
secure but there’s an attack that could exploit a vulnerability in it that I didn’t talk about yet which
is the hashing process, like the NTLM, Kerberos suffers from hashing without salting which
allows the use of the hash digest without the need to find its corresponding password if the
protocol uses the hash of the password not the password itself, this is what we’re going to talk
about in the next topic which is Pass The Ticket attack.
