Transcript KERBOROSE SYSTEM - Villanova University

KERBEROS SYSTEM
Kumar Madugula
What is Kerberos?
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A secure network authentication protocol.
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Uses trusted key distribution center
Developed at MIT in 80’s
What it does?
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Authenticates the client
Distributes a shared session key between
client and application server programs.
User enters the password only once. No
need to enter password when ever user
opens an application.
Terminology
Principle
 Authentication Server (AS)
 Ticket Granting Server (TGS)
 Application Server
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Ticket Granting Ticket (TGT)
 Ticket
 Session Key
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Terminology
Authentication
Server
Ticket Granting
Server
TGT
Ticket
Client
Application
Server
Working
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Client steps
Obtains Ticket Granting Ticket from the
Authentication Server
Obtains Ticket from the ticket granting
server to required application server.
Communicate with the application server
Client and Authentication Server (AS)
interaction
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Client sends user name and a request for a
ticket to access TGS.
Name, TGS, nonce
Client
Authentication
Server
Client and Authentication Server
interaction
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The authentication server looks up the client in its database.
Generates a session key (KCT) for use between the client and the
TGS.
AS encrypts the KCT using the client’s secret key (Kuser).
The authentication server also uses the TGS’s secret key to create
and send the user a ticket-granting ticket (TGT).
SK1,TGT
Client
Authentication
Server
SK1={KCT ,nonce} Kuser
TGT={user,TGS,t1,t2, KCT} KTGS
Client Ticket Granting Server Interaction
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Client uses his password to decrypt SK1 to obtain session key.
then uses it to create an authenticator containing the user’s
name, IP address and a time stamp.
The client sends this authenticator, along with the TGT, to the
TGS, requesting access to the application server (S).
AUTH1, TGT, Server, nonce
Client
Ticket Granting
Server
AUTH1={user, ipaddress, timestamp} KCT
TGT={user,TGS,t1,t2, KCT} KTGS
Client and Ticket granting server interaction
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The TGS decrypts the TGT, then uses KCT inside the TGT to
decrypt the authenticator. It verifies information in the
authenticator (AUTH1)
Then the TGS creates a new session key (KCS) for the client and
application server to use, encrypts it using KCT. It also creates a
new ticket encrypted with the target server’s secret key (KServer)
SK2, TK
Client
Ticket Granting
Server
SK2={KCS, nonce} KCT
TK={user,server,t1,t2, KCS} KServer
Client and Server Interaction
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Client decrypts SK2 get KCS.
creates a new authenticator encrypts with KCS and sends it with
ticket to the application server.
AUTH2,TK,request,nonce
Client
Application
Server
AUTH1={user, ipaddress, timestamp} KCS
TK={user,server,t1,t2, KCS} KServer
The application server decrypts and checks the ticket then decrypts
the authenticator and verifies the user.
From now client and server uses KCS as a shared secret key to
communicate.
Advantages and Weaknesses
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User's passwords are never sent across the
network, encrypted or in plain text
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A user need only authenticate to the Kerberos
system once
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Kerberos v5 can use any private key encryption
algorithm
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Windows 2000 uses a modified version of
Kerberos which uses public key certificates
instead of shared secret keys for initial
authentication.
Disadvantages
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Trusting trusted party (TGS and AS)
Kerberos was designed for use with single-user
client systems
All the existing software's must be Kerberos
compatible.
vulnerable to brute-force attacks against TGS or
AS