Consider the diagram in https://en.wikipedia.org/wiki/Kerberos_(protocol)#/media/File:Kerberos_protocol.svg depicting the Kerberos protocol. I'm wondering how the authentication server (AS) is useful.. Couldn't we drop the messages A, B, C, D and simply use the client key K_c (green) instead of K_C-TSG (red) to encrypt message F? I.e., the client would directly request the service from the TGS and authentication and authorization is directly handled in one message, instead of 3.

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Theoretically, the AS and the TGS can be set up together (even on the same physical machine), and I think that in small systems you may spare some messages. This can also be seen in the image along the dashed line. Also see the Needham–Schroeder protocol [1] that uses a two-way handshake with the AS (Kerberos is based on this protocol and improves it).

However, the TGS meets the essential requirements that Kerberos provides:

  • Scalability - The system must support a large number of servers and clients. Only one server is a bottleneck.
  • Separation of duties - the AS shares a key with the clients (which is usually derived from the client's password), the TGS holds a key with the servers. The AS verifies the identity of the client and the TGS "links" it with the server.
  • Hierarchical structure - typical organizations have hierarchical structure. Kerberos allows you to have a single AS and multiple TGSs, which fits a typical organizational structure.

[1] https://en.wikipedia.org/wiki/Needham%E2%80%93Schroeder_protocol

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