ACCESS CONTROL MANAGEMENTProject Progress (as of March 3)

By: Poonam Gupta Sowmya Sugumaran

Kerberos Components

http://technet.microsoft.com/en-us/library/cc772815(WS.10).aspx

Getting the Tickets

• kinit –forwards request for TGT to KDC• KDC encrypts TGT with pswrd and sends back• kinit has following options - l(lifetime) - f(forwardable tickets) -r(renewable life)

Listing the Tickets

• klist – lists the tickets of the authenticated user.

Eg of the output of an unsuccessful authentication is: klist: No credentials cache file found (ticket cache /tmp/krb5cc_1234)

Contd..

• klist provides:– Information of all tickets– Expiration time of each ticket– Flags that apply to the ticket

Example:Ticket cache: /tmp/krb5cc_1234Valid starting Expires 29 Jul 98 11:25:47 30 Jul 98 12:25:42

Destroying the Tickets

• Destroyed automatically on logging out• Destroying by hand:– Using kdestroy commandOutput generated by klist when all the tickets are destroyed:klist: No credentials cache file found

Changing Kerberos Password

• Kpasswd is used for changing Kerberos passwords

– kpasswd: Changing password – Old password: your_old_password – kpasswd:your_new_password – New password (again): your_new_password– Kerberos password changed

• password is controlled by the policy default, which requires a minimum of 6 characters from at least 2 classes (the five classes are lowercase, uppercase, numbers, punctuation, and all other characters).

What is in a Ticket?Field Name Description

Ticket version number 5

Realm Domain that issued ticket(mostly server’s)

Server Name Name of the server

Flags Options that specify how & when to issue

Key Client-Server session key

Client Realm Requestor’s domain name

Client Name Requestor’s name

Authentication Time Initial authentication time

Start time Time after which ticket is valid

End time Ticket’s expiration time

Renewal Till Max end time that can be set with flag

Client Address 1 or more addresses

Authorization Data Contains access restrictions

What happens when tickets expire?

• KDC doesn’t notify client when the ticket is about to expire

• Tickets-for authenticating new connections• Ongoing operations are not interrupted

CLIENT SERVER

Expired service ticket

Error message

Renewable TGTs

• Only session keys are refreshed, without issuing new tickets every time.

• When Renewable policy is permitted, KDC sets a “Renewable” flag-R in the ticket.

• Sets 2 expiration time– (i)limits life of current instance of ticket– (ii)limit on the cumulative lifetime of all instances

KDC configuration

RFC 1510 recommends the following values:

Configuration Element RFC 1510 Recommendation

Active Directory Domain Default Setting

Maximum ticket lifetime One day 600 minutes (10 hours)

Maximum renewable lifetime One week Seven days

 

The Authenticator

• The client includes an authenticator whenever it sends ticket to the server(either TGS or service server)

• Authenticator – verifies that the destination in the ticket is really the ticket’s source.

Why is an Authenticator necessary

• The server trusts the ticket-ticket is encrypted using server’s secret key

• Server doubts about the sender• The ticket could be stolen and then sent by

the imposter

How does the Authenticator work• The authenticator is encrypted with the session key created by the KDC

to be used between the client and the target server. Only the client and the target server can access the session key.

• The target server uses its secret key to decrypt the ticket, finds the session key inside the ticket, and uses it to decrypt the authenticator.

• If the target server can successfully decrypt the authenticator and if the authenticator's data is accurate, then the target server will trust the source of the ticket.

Authenticator’s Timestamp

• Important piece of data• Kerberos policy requires that authenticator’s

timestamp be within minutes of the time on the server

• This prevents replay attack

Setting up KDC Hardware

• KDC – holds database with passwords and all information

• KDC must be as secure as possible:– Put the server machine into a physically secured

location, to which only a very few people have access. – Do not run any network applications on it except the

KDC. – It is probably a good approach to install a minimal

system first then check the list of installed packages and remove any unneeded packages.

Clock Synchronization

• All clocks within the organization must be synchronized

• Very important – Protects against replay attack• Possible solution:– Installing time server on one machine and having

all clients synchronize their clocks with this machine

Setting the Master Key

• Database master key – protects from accidental disclosure

• Derived from pass phrase and stored in stash file

• Don’t back up stash file while making backups of database in a tape– Master key:<enter pass phrase>

Verifying password – Master key:<enter pass phrase again>

Thank You..!!