Книга: Distributed operating systems

7.2.2. Object Protection

7.2.2. Object Protection

The basic algorithm used to protect objects is as follows. When an object is created, the server picks a random Check field and stores it both in the new capability and inside its own tables. All the rights bits in a new capability are initially on, and it is this owner capability that is returned to the client. When the capability is sent back to the server in a request to perform an operation, the Check field is verified.

To create a restricted capability, a client can pass a capability back to the server, along with a bit mask for the new rights. The server takes the original Check field from its tables, EXCLUSIVE ORs it with the new rights (which must be a subset of the rights in the capability), and then runs the result through a one-way function. Such a function, y=f(x), has the property that given x it is easy to find y, but given only y, finding x requires an exhaustive search of all possible x values (Evans et al., 1974).

The server then creates a new capability, with the same value in the Object field, but the new rights bits in the Rights field and the output of the one-way function in the Check field. The new capability is then returned to the caller. The client may send this new capability to another process, if it wishes, as capabilities are managed entirely in user space.

The method of generating restricted capabilities is illustrated in Fig. 7-4. In this example, the owner has turned off all the rights except one. For example, the restricted capability might allow the object to be read, but nothing else. The meaning of the Rights field is different for each object type since the legal operations themselves also vary from object type to object type.


Fig. 7-4. Generation of a restricted capability from an owner capability.

When the restricted capability comes back to the server, the server sees from the Rights field that it is not an owner capability because at least one bit is turned off. The server then fetches the original random number from its tables, EXCLUSIVE ORs it with the Rights field from the capability, and runs the result through the one-way function. If the result agrees with the Check field, the capability is accepted as valid.

It should be obvious from this algorithm that a user who tries to add rights that he does not have will simply invalidate the capability. Inverting the Check field in a restricted capability to get the argument (C XOR 00000001 in Fig. 7-4) is impossible because the function f is a one-way function (that is what "one-way" means — no algorithm exists for inverting it). It is through this cryptographic technique that capabilities are protected from tampering.

Capabilities are used throughout Amoeba for both naming of all objects and for protecting them. This single mechanism leads to a uniform naming and protection scheme. It also is fully location transparent. To perform an operation on an object, it is not necessary to know where the object resides. In fact, even if this knowledge were available, there would be no way to use it.

Note that Amoeba does not use access control lists for authentication. The protection scheme used requires almost no administrative overhead. However, in an insecure environment, additional cryptography (e.g., link encryption) may be required to keep capabilities from being disclosed accidentally to wiretappers on the network.

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