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     8 \section*{Handout 4 (Unix-Style Access Control)}

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    10 Access control is essentially about deciding whether to grant

    11 access to a resource or deny it. This sounds easy. Right? Well

    12 it turns out that things are not as simple as seem at first.

    13 Let us study as a case how access is organised in Unix-like

    14 systems (Windows systems have generally similar access

    15 control, although the details might be quite different).

    16  

    17 Following the Unix-philosophy that everything is considered as

    18 a file, even memory or ports, access control is organised

    19 around 11 Bits that specify how a file can be accessed. There

    20 are three modes for access \textbf{r}ead, \textbf{w}rite and

    21 e\textbf{x}ecute. Moreover there are .... owner, group and

    22 everybody else.

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