= Kerberos =

== Terms ==

|| authentication ||||<(> ||
|| authentication server||||<(> ||
|| Kerberos||||<(> ||
|| Kerberos realm||||<(> ||
|| Propagating cipher block chaining (PCBC) mode||||<(> ||
|| Public-key certificate||||<(> ||
|| Realm||||<(> ||
|| Sequence number||||<(> ||
|| Subkey||||<(> ||
|| Ticket||||<(> ||
|| Ticket-granting server (TGS)||||<(> ||
|| X.509 Certificate||||<(> ||

== Review Questions ==

'''14.1 What problem was Kerberos designed to address?'''

Authenticating requests and users in an open distributed environment. p402

'''14.2 What are three threats associated with user authentication over a network or Internet?'''

   1. Impersonating the user by gaining access to a workstation (physically or using malware)
   1. Change workstation address so that you are impersonating the machine.
   1. Replay attack by eavesdropping on exchanges to disrupt operations.

'''14.3 List three approaches to secure user authentication in a distribted environment.'''

   1. Rely on each individual client workstation to assure the identity of its user or users and rely on each server to enforce a security policy based on user ID.
   1. Require that client systems authenticate themselves to servers, but trust the client system concerning the identity  of its user.
   1. ''Require the user to prove identity for each service invoked. Also require that servers prove their identity to clients.''

'''14.4 What four requirements were defined for Kerberos?'''

   1. Secure
   1. Reliable (available - so that multiple systems can support the functionality needed).
   1. Transparent - except for entering a password.
   1. Scalable

'''14.5 What entities constitute a full-service Kerberos environment?'''

Kerberos Server, Clients and application servers.

'''14.6 In the context of Kerberos, what is a realm?'''

   * A Kerberos Server containing all the user ids and hashes of their passwords - we say the users are registered with the Kerberos server.
   * A Kerberos server must share a secret key with the application servers - that is they are registered with the Kerberos server. 

The above defines the concepts related to Kerberos realms

   * To interoperate between realms, kerberos servers in different realms share a secret key and are said to be registered with each other.

'''14.7 What are the principal differences between version 4 and version 5 of Kerberos?'''

   1. v4 uses DES, v5 can use any encryption technique
   1. v4 uses IPv4, v5 can use different protocols
   1. No standard was used for byte order encoding, in v5 ASN.1 and BER are used.
   1. v4 Ticket lifetime was limited to just over 21 hours, v5 has explicit begin and end times.
   1. Double encryption was not needed, and the second one is dropped in v5.
   1. DES mode PCBC is not used in v5, instead CBC mode is used.
   1. Subsession keys are used between a client and server to protect against replay attacks.
   1. Both versions are vulnerable to a password attack see p415.

'''14.8 What is the purpose of the X.509 standard?'''

'''14.9 What is a chain of certificates?'''

'''14.10 How is an X.509 certificate revoked?'''