ACCESS CONTROL MANAGEMENT Project Progress (as of March 3) By: Poonam Gupta Sowmya Sugumaran

Kerberos Components

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_1234 Valid 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 command Output 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 NameDescription Ticket version number5 RealmDomain that issued ticket(mostly server’s) Server NameName of the server FlagsOptions that specify how & when to issue KeyClient-Server session key Client RealmRequestor’s domain name Client NameRequestor’s name Authentication TimeInitial authentication time Start timeTime after which ticket is valid End timeTicket’s expiration time Renewal TillMax end time that can be set with flag Client Address1 or more addresses Authorization DataContains 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: Verifying password – Master key:

Thank You..!!