Secure RPC is fundamental to the Secure NFS system. The goal of Secure RPC is to build a system at least as secure as a time-sharing system (one in which all users share a single computer). A time-sharing system authenticates a user through a login password. With data encryption standard (DES) authentication, the same is true. Users can log in on any remote computer just as they can on a local terminal, and their login passwords are their passports to network security. In a time-sharing environment, the system administrator has an ethical obligation not to change a password to impersonate someone. In Secure RPC, the network administrator is trusted not to alter entries in a database that stores public keys.
You need to be familiar with two terms to understand an RPC authentication system: credentials and verifiers. Using ID badges as an example, the credential is what identifies a person: a name, address, birthday, and so on. The verifier is the photo attached to the badge: you can be sure the badge has not been stolen by checking the photo on the badge against the person carrying it. In RPC, the client process sends both a credential and a verifier to the server with each RPC request. The server sends back only a verifier because the client already "knows" the server's credentials.
RPC's authentication is open ended, which means that a variety of authentication systems can be plugged into it. Currently, there are two systems: UNIX, and DH.
When UNIX authentication is used by a network service, the credentials contain the client's host name, UID, GID, and group-access list, but the verifier contains nothing. Because there is no verifier, a superuser could falsify appropriate credentials, using commands such as su. Another problem with UNIX authentication is that it assumes all computers on a network are UNIX computers. UNIX authentication breaks down when applied to other operating systems in a heterogeneous network.
To overcome the problems of UNIX authentication, Secure RPC uses either DH authentication.
DH authentication uses the data encryption standard (DES) and Diffie-Hellman public-key cryptography to authenticate both users and computers in the network. DES is a standard encryption mechanism; Diffie-Hellman public-key cryptography is a cipher system that involves two keys: one public and one secret. The public and secret keys are stored in the name space. NIS stores the keys in the publickey map, and NIS+ stores the keys in the cred table. These maps contain the public key and secret key for all potential users. See the Solaris Naming Administration Guide for more information on how to set up the maps and tables.
The security of DH authentication is based on a sender's ability to encrypt the current time, which the receiver can then decrypt and check against its own clock. The timestamp is encrypted with DES. The requirements for this scheme to work are:
The two agents must agree on the current time.
The sender and receiver must be using the same encryption key.
If a network runs a time-synchronization program, the time on the client and the server is synchronized automatically. If a time-synchronization program is not available, timestamps can be computed using the server's time instead of the network time. The client asks the server for the time before starting the RPC session, then computes the time difference between its own clock and the server's. This difference is used to offset the client's clock when computing timestamps. If the client and server clocks get out of synchronization to the point where the server begins to reject the client's requests, the DH authentication system on the client resynchronizes with the server.
The client and server arrive at the same encryption key by generating a random conversation key, also known as the session key, and by using public-key cryptography to deduce a common key. The common key is a key that only the client and server are capable of deducing. The conversation key is used to encrypt and decrypt the client's timestamp; the common key is used to encrypt and decrypt the conversation key.
Kerberos is an authentication system developed at MIT. Encryption in Kerberos is based on DES. Kerberos support is no longer supplied as part of Secure RPC, but a client-side implementation is included with the Solaris 8 release. XREF
Be aware of the following points if you plan to use Secure RPC:
If a server crashes when no one is around (after a power failure for example), all the secret keys that are stored on the system are deleted. Now no process can access secure network services or mount an NFS file system. The important processes during a reboot are usually run as root, so these processes would work if root's secret key were stored away, but nobody is available to type the password that decrypts it. keylogin -r allows root to store the clear secret key in /etc/.rootkey, which keyserv reads.
Some systems boot in single-user mode, with a root login shell on the console and no password prompt. Physical security is imperative in such cases.
Diskless computer booting is not totally secure. Somebody could impersonate the boot server and boot a devious kernel that, for example, makes a record of your secret key on a remote computer. The Secure NFS system provides protection only after the kernel and the key server are running. Before that, there is no way to authenticate the replies given by the boot server. This could be a serious problem, but it requires a sophisticated attack, using kernel source code. Also, the crime would leave evidence. If you polled the network for boot servers, you would discover the devious boot server's location.
Most setuid programs are owned by root; if the secret key for root is stored in /etc/.rootkey, these programs behave as they always have. If a setuid program is owned by a user, however, it might not always work. For example, if a setuid program is owned by dave and dave has not logged into the computer since it booted, the program would not be able to access secure network services.
If you log in to a remote computer (using login, rlogin, or telnet) and use keylogin to gain access, you give access to your account. This is because your secret key gets passed to that computer's key server, which then stores it. This is only a concern if you do not trust the remote computer. If you have doubts, however, do not log in to a remote computer if it requires a password. Instead, use the NFS environment to mount file systems shared by the remote computer. As an alternative, you can use keylogout to delete the secret key from the key server.
If a home directory is shared with the -o sec=dh option, remote logins can be a problem. If the /etc/hosts.equiv or ~/.rhosts files are not set to prompt for a password, the login will succeed, but the users cannot access their home directories because no authentication has occurred locally. If the user is prompted for a password, the user will have access to his or her home directory as long as the password matches the network password.