Introduction to SEAM

This chapter provides an introduction to Sun Enterprise Authentication Mechanism (SEAM).

• What Is SEAM?

• How SEAM Works

• SEAM Security Services

• SEAM Releases

What Is SEAM?

SEAM is a client/server architecture that provides secure transactions over networks. SEAM offers strong user authentication, as well as data integrity and data privacy. Authentication guarantees that the identities of both the sender and the recipient of a network transaction are true. SEAM can also verify the validity of data being passed back and forth (integrity) and encrypt the data during transmission (privacy). Using SEAM, you can log on to other machines, execute commands, exchange data, and transfer files securely. Additionally, SEAM provides authorization services, which allows administrators to restrict access to services and machines. Moreover, as a SEAM user, you can regulate other people's access to your account.

SEAM is a single-sign-on system, which means that you only need to authenticate yourself to SEAM once per session, and all subsequent transactions during the session are automatically secured. After SEAM has authenticated you, you do not need to authenticate yourself every time you use a SEAM-based command such as ftp or rsh, or access data on an NFS file system. Thus, you do not have to send your password over the network, where it can be intercepted, each time you use these services.

SEAM is based on the Kerberos V5 network authentication protocol that was developed at the Massachusetts Institute of Technology (MIT). People who have used Kerberos V5 should therefore find SEAM very familiar. Since Kerberos V5 is a de facto industry standard for network security, SEAM promotes interoperability with other systems. In other words, because SEAM works with systems that use Kerberos V5, it allows for secure transactions even over heterogeneous networks. Moreover, SEAM provides authentication and security both between domains and within a single domain.

Note - Because SEAM is based on, and designed to interoperate with, Kerberos V5, this manual often uses the terms "Kerberos" and "SEAM" more or less interchangeably, for example, "Kerberos realm" or "SEAM-based utility." Moreover, "Kerberos" and "Kerberos V5" are used interchangeably. The manual draws distinctions when necessary.

SEAM allows for flexibility in running Solaris applications. You can configure SEAM to allow both SEAM-based and non-SEAM-based requests for network services such as the NFS service, telnet, and ftp. As a result, current Solaris applications still work even if they are running on systems on which SEAM is not installed. Of course, you can also configure SEAM to allow only SEAM-based network requests.

Additionally, applications do not have to remain committed to SEAM if other security mechanisms are developed. Because SEAM is designed to integrate modularly into the Generic Security Service (GSS) API, applications that make use of the GSS-API can utilize whichever security mechanism best suits its needs.

How SEAM Works

The following is an overview of the SEAM authentication system. For a more detailed description, see How the Authentication System Works.

From the user's standpoint, SEAM is mostly invisible after the SEAM session has been started. Commands such as rsh or ftp work pretty much in their usual fashion. Initializing a SEAM session often involves no more than logging in and providing a Kerberos password.

The SEAM system revolves around the concept of a ticket. A ticket is a set of electronic information that serves as identification for a user or a service such as the NFS service. Just as your driver's license identifies you and indicates what driving permissions you have, so a ticket identifies you and your network access privileges. When you perform a SEAM-based transaction (for example, if you rlogin in to another machine), you transparently send a request for a ticket to a Key Distribution Center, or KDC. The KDC accesses a database to authenticate your identity and returns a ticket that grants you permission to access the other machine. "Transparently" means that you do not need to explicitly request a ticket; it happens as part of the rlogin command. Because only the authenticated client can get a ticket for a specific service, another client cannot use rlogin under an assumed identity.

Tickets have certain attributes associated with them. For example, a ticket can be forwardable (which means that it can be used on another machine without a new authentication process), or postdated (not valid until a specified time). How tickets are used (for example, which users are allowed to obtain which types of ticket) is set by policies that are determined when SEAM is installed or administered.

Note - You will frequently see the terms credential and ticket. In the greater Kerberos world, they are often used interchangeably. Technically, however, a credential is a ticket plus the session key for that session. This difference is explained in more detail in Gaining Access to a Service Using SEAM.

The following sections further explain the SEAM authentication process.

Initial Authentication: the Ticket-Granting Ticket

Kerberos authentication has two phases: an initial authentication that allows for all subsequent authentications, and the subsequent authentications themselves.

The following figure shows how the initial authentication takes place.

Figure 13-1 Initial Authentication for SEAM Session

1. A client (a user, or a service such as NFS) begins a SEAM session by requesting a ticket-granting ticket (TGT) from the Key Distribution Center (KDC). This request is often done automatically at login.

   A ticket-granting ticket is needed to obtain other tickets for specific services. Think of the ticket-granting ticket as similar to a passport. Like a passport, the ticket-granting ticket identifies you and allows you to obtain numerous "visas," where the "visas" (tickets) are not for foreign countries but for remote machines or network services. Like passports and visas, the ticket-granting ticket and the other various tickets have limited lifetimes. The difference is that "Kerberized" commands notice that you have a passport and obtain the visas for you. You don't have to perform the transactions yourself.

   Another analogy for the ticket-granting ticket is that of a three-day ski pass which is good at four different ski resorts. You show the pass at whichever resort you decide to go to (until it expires) and you receive a lift ticket for that resort. Once you have the lift ticket, you can ski all you want at that resort. If you go to another resort the next day, you once again show your pass, and you get an additional lift ticket for the new resort. The difference is that the SEAM-based commands notice that you have the weekend ski pass, and get the lift ticket for you, so you don't have to perform the transactions yourself.

2. The KDC creates a ticket-granting ticket and sends it back, in encrypted form, to the client. The client decrypts the ticket-granting ticket by using the client's password.

3. Now in possession of a valid ticket-granting ticket, the client can request tickets for all sorts of network operations, such as rlogin or telnet, for as long as the ticket-granting ticket lasts. This ticket usually lasts for a few hours. Each time the client performs a unique network operation, it requests a ticket for that operation from the KDC.

Subsequent Authentications

After the client has received the initial authentication, each individual authentication follows the pattern that is shown in the following figure.

Figure 13-2 Obtaining Access to a Service

1. The client requests a ticket for a particular service (say, to rlogin into another machine) from the KDC by sending the KDC its ticket-granting ticket as proof of identity.

2. The KDC sends the ticket for the specific service to the client.

   For example, suppose user joe wants to access an NFS file system that has been shared with krb5 authentication required. Since he is already authenticated (that is, he already has a ticket-granting ticket), as he attempts to access the files, the NFS client system automatically and transparently obtains a ticket from the KDC for the NFS service.

   For example, suppose the user joe uses rlogin on the server boston. Since he is already authenticated (that is, he already has a ticket-granting ticket), he automatically and transparently obtains a ticket as part of the rlogin command. This ticket allows him to rlogin into boston as often as he wants until it expires. If joe wants to rlogin into the machine denver, he obtains another ticket, as in Step 1.

3. The client sends the ticket to the server.

   When using the NFS service, the NFS client automatically and transparently sends the ticket for the NFS service to the NFS server.

4. The server allows the client access.

These steps make it appear that the server doesn't ever communicate with the KDC. The server does, though; it registers itself with the KDC, just as the first client does. For simplicity's sake, we have left that part out.

The SEAM Remote Applications

What are the SEAM-based (or "Kerberized") commands that a user such as joe can use? They are:

• ftp

• rcp

• rlogin

• rsh

• telnet

These applications are the same as the Solaris applications of the same name, except that they use Kerberos principals to authenticate transactions, thereby providing Kerberos-based security. (See Principals for information on principals.)