Kerberos
Last updated
Identification, Authentication and Authorization
Last updated
Authenticate peers in a distributed environment.
Targeted for Athena (at MIT)
Distribute session keys for adding security to sessions between peers.
Authentication (the initial goal)
Confidentiality (optional)
Single Sign-On.
Only one password to remember
Daily use (typically)
A and B trust on a common Key Distribution Center (KDC).
KDC shares a key with every A and B. Central authentication authority.
KDC generates good (random) keys.
Directly imported by requesters
Indirectly obtained by targets
Authentication Service (AS)
Ticket Granting Server (TGS)
All have a secret shared with Kerberos (AS or TGS)
People: a key derived from a password:
Services/servers: key stored in some repository.
Requisites
Clocks (very well) synchronized.
Ticket: required to request a service.
Authenticator: proof of the identity of a requester.
Unforgeable piece of data.
It can only be interpreted by the target service.
Carries the identities of the client that can use it.
Carries a session key.
Carries a validity timestamp.
Carries a timestamp of the request.
Carries the identity of the client.
Proves that the client knows the session key.
Location of the Kerberos servers of the realm.
Authentication of user U by Kerberos (AS).
The TGT can be used to request other tickets needed by the user U to access every service S.
U requests Kerberos (TGS) a ticket for accessing S.
U uses TGT in the request.
U must prove that he is the owner of TGT.
Vulnerable to proactive dictionary attacks! (Kerberoasting).
Realms.
A kerberos server per realm.
Fundamental to allow a client from a realm to access a server in another realm.
Realms need to trust authentication performed by other realms.
Secret keys are shared between TGS servers of different realms.
Inter-realm key.
Each inter-realm key is associated with a trust path.
A client (user) needs to jump from TGS to TGS to get a ticket.
Not particularly user-friendly.
Secret keys, names, and network addresses.
name/instance@realm (user@ua.pt, ftp/ftp.ua.pt@ua.pt).
Timestamps in tickets (hours).
Timestamps in authenticators (seconds, minutes).
Nonces (in ticket distributions).
Timestamps / sequence numbers (in authenticators).
Key distribution in authenticators.
Options and authorizations in tickets.
Secret keys shared among TGS services, trust paths.
Ticket issuing from a TGS to another TGS.
Kerberos KDC can impersonate anyone. Needs maximum security in its administration.
Kerberos KDC may be a single point of failure. Replication is an option since stored keys are seldom updated.
A stolen user password allows others to impersonate the victim in every service of the realm. Stolen TGS credentials are less risky, as their validity is shortly limited (one day, usually).
Windows versions
Windows 2000 adopted Kerberos for inter-domain authentication.
Kerberos was modified to accommodate Windows credentials.
Components
Kerberos servers/daemons.
Libraries for “kerberizing” applications.
Support applications.
klogin, kpasswd, kadmin.
Kerberized applications (clients and servers).
= hash(password)
The user gets a Ticket Granting Ticket (TGT) and a session key () for interacting with another Kerberos service (TGS).
U gets a session key and a ticket to S .
U uses to make authenticated requests to S.
Server S uses to check the identity of U.
U must prove that he is the owner of .
