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[[!meta copyright="Copyright © 2007, 2008 Free Software Foundation, Inc."]]

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A capability is a protected reference.  It is a reference in that
it designates an object; it is protected in that in cannot be
forged.  A capabilities both designates the object it refers to and
carries the authority to manipulate it.

By binding [[designation]] and [[authorization]] together, capabilities
simplify [[delegation]].  Imagine that program instance A wants to
tell program B to use a particular file to store some data.
Further imagine that A and B are running in different [[trust_domains]]
(e.g., with different UIDs).  If A sends B just the name
of the file, B needs to first ensure that he does not accidentally
enable A to access the file on his own authority.  That is, B wants
to protect against A hijacking his authority.  (This problem is
refused to the [[confused_deputy]] problem.)  Also, since A likely
sent a string to identify the file to B, the identifier lacks a
[[naming_context]] and therefore may resolve to a different object
than A intended.  Be ensuring that [[designation]] and [[authorization]] are
always bound together, these problems are avoided.

[[Unix]] file descriptors can be viewed as capabilities.  Unix file
descriptors do not survive reboot, that is, they are not
[[persistent|persistency]].  To work around this, [[ACL]]s are used to
recover authority.