Kerberos. What is Kerberos? Network authentication protocol Developed at MIT in the mid 1980s Available as open source or in supported commercial software.

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Presentation transcript:

Kerberos

What is Kerberos? Network authentication protocol Developed at MIT in the mid 1980s Available as open source or in supported commercial software

Why Kerberos? Sending usernames and passwords in the clear breech in the security of the network. Each time a password is sent in the clear, there is a chance for interception.

Firewall vs. Kerberos? Firewalls make a risky assumption: that attackers are coming from the outside. In reality, attacks frequently come from within. Kerberos assumes that network connections (rather than servers and work stations) are the weak link in network security.

Design Requirements Interactions between hosts and clients should be encrypted. Must be convenient for users (or they won’t use it). Protect against intercepted credentials.

Cryptography Approach Private Key: Each party uses the same secret key to encode and decode messages. Uses a trusted third party which can vouch for the identity of both parties in a transaction. Security of third party is imperative.

How does Kerberos work? Instead of client sending password to application server: ◦ Request Ticket from authentication server ◦ Ticket and encrypted request sent to application server How to request tickets without repeatedly sending credentials? ◦ Ticket granting ticket (TGT)

How does Kerberos work?: Ticket Granting Tickets

How does Kerberos Work?: The Ticket Granting Service

How does Kerberos work?: The Application Server

Applications Authentication Authorization Confidentiality Within networks and small sets of networks

Weaknesses and Solutions If TGT stolen, can be used to access network services. Only a problem until ticket expires in a few hours. Subject to dictionary attack. Timestamps require hacker to guess in 5 minutes. Very bad if Authentication Server compromised. Physical protection for the server.

The Competition: SSL

Susan Key Distribution Center Ticket Granting Service Authen- Tication Service XYZ Service Susan’s Desktop Computer Think “Kerberos Server” and don’t let yourself get mired in terminology.

Susan Key Distribution Center Ticket Granting Service Authen- Tication Service XYZ Service Susan’s Desktop Computer Represents something requiring Kerberos authentication (web server, ftp server, ssh server, etc…)

Susan’s Desktop Computer Susan Key Distribution Center Ticket Granting Service Authen- Tication Service XYZ Service “I’d like to be allowed to get tickets from the Ticket Granting Server, please.

Susan’s Desktop Computer Susan Key Distribution Center Ticket Granting Service Authen- Tication Service XYZ Service “Okay. I locked this box with your secret password. If you can unlock it, you can use its contents to access my Ticket Granting Service.”

Susan’s Desktop Computer Susan Key Distribution Center Ticket Granting Service Authen- Tication Service myPassword XYZ Service TGT

Because Susan was able to open the box (decrypt a message) from the Authentication Service, she is now the owner of a shiny “Ticket-Granting Ticket”. The Ticket-Granting Ticket (TGT) must be presented to the Ticket Granting Service in order to acquire “service tickets” for use with services requiring Kerberos authentication. The TGT contains no password information.

Susan’s Desktop Computer Susan Key Distribution Center Ticket Granting Service Authen- Tication Service XYZ Service “Let me prove I am Susan to XYZ Service. Here’s a copy of my TGT!” use XYZ TGT

Susan’s Desktop Computer Susan Key Distribution Center Ticket Granting Service Authen- Tication Service XYZ Service TGT Hey XYZ: Susan is Susan. CONFIRMED: TGS You’re Susan. Here, take this.

Susan’s Desktop Computer Susan Key Distribution Center Ticket Granting Service Authen- Tication Service XYZ Service TGT Hey XYZ: Susan is Susan. CONFIRMED: TGS I’m Susan. I’ll prove it. Here’s a copy of my legit service ticket for XYZ. Hey XYZ: Susan is Susan. CONFIRMED: TGS

Susan’s Desktop Computer Susan Key Distribution Center Ticket Granting Service Authen- Tication Service XYZ Service TGT Hey XYZ: Susan is Susan. CONFIRMED: TGS Hey XYZ: Susan is Susan. CONFIRMED: TGS That’s Susan alright. Let me determine if she is authorized to use me.

Authorization checks are performed by the XYZ service… Just because Susan has authenticated herself does not inherently mean she is authorized to make use of the XYZ service.

One remaining note: Tickets (your TGT as well as service-specific tickets) have expiration dates configured by your local system administrator(s). An expired ticket is unusable. Until a ticket’s expiration, it may be used repeatedly.

Susan’s Desktop Computer Susan Key Distribution Center Ticket Granting Service Authen- Tication Service XYZ Service TGT Hey XYZ: Susan is Susan. CONFIRMED: TGS ME AGAIN! I’ll prove it. Here’s another copy of my legit service ticket for XYZ. Hey XYZ: Susan is Susan. CONFIRMED: TGS use XYZ

Susan’s Desktop Computer Susan Key Distribution Center Ticket Granting Service Authen- Tication Service XYZ Service TGT Hey XYZ: Susan is Susan. CONFIRMED: TGS Hey XYZ: Susan is Susan. CONFIRMED: TGS That’s Susan… again. Let me determine if she is authorized to use me.