Capabilities in Distributed Systems

A server or manager issues capabilities to access the objects it controls, such as file servers managing object files

  
Figure 5: Capability in distributed systems

1. Check field encrypted version of permitted operations field and a random constant
2. Protects capability and prevents it being forged
3. When object created, a random number generated and stored in an internal reference table to the object
4. When an access request received, the object field used as an index into the table
5. Decrypting check field must yield the correct constant for the request to be permitted

• Creation, copying or modifying capabilities are protected OS operations
• Operations field indicates which of these operations are permitted on the capability itself
• Modifying operations field (usually to reduce access rights before passing on capability) performed by requesting the originator to issue a new one (by a special commutative ONE WAY function
• Capabilities permit VERY LIMITED access rights to be passed to other users
• Check capabilities can be more efficient than ACLs - so are used when checking is needed for each operation