file protection.59 to 60
TRANSCRIPT
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Recap
In the last class, you have learnt:
• Issues regarding deletion of files in a cyclic –
graph structure
• Advantages & disadvantage of a cyclic – graph
structure
• Understand the structure of general graph
directories & problems
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Objectives
On completion of this period, you would be able to
• Understand the concept of file protection
• Know types of access
• Understand Access Lists and Groups
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PROTECTION
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A computer file needs protection which is of two types
1. ReliabilityProtection from physical damageFile systems can be damaged by Hardware problems(errors in r/w)Power surges or failuresHead crashesDirt and temperatureBugs in file system software
2. SecurityProtection from improper access
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1. Reliability
• Provided by duplicate copies of files
• Take backup's at regular intervals
(daily/weekly/monthly)
A computer file needs protection which is of two types
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2. Security
• Protecting files from unauthorized access
• More important in a multi user system
• Provided by controlling access to files
A computer file needs protection which is of two types
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Need for protecting files
• Need for protection is due to the ability to
access files
• Two ways to tackle the problem
1. Prohibit access providing complete protection
2. Provide free access without protection
• Both approaches are too extreme for general
use
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Controlled Access
• Desirable
• Provided by limiting the types of file access that
can be made
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Types of Access
• Read : Read from the file
• Write : Write or rewrite the file
• Execute : Load file into memory and execute
• Append : Write new information at end of file
• Delete : Delete the file and free its space
• List : List the name and attributes of the file
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Protection Mechanisms
• Different mechanisms proposed
• Select mechanism depending on the application
Ex : A small computer used only by a few
members of a research group may not need the
same protection as a large corporate computer
used for research, finance and personnel
operations
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Access Lists and Groups
• A common approach to protection is provide
access based on user identity
• Associate with each file and directory an access list
• Access list specifies user name and types of
access allowed for each user
• Access to user is provided as per the access list
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Problem with access lists
• Length of the list
• If every user need to be given read access to a file,
we must list all users with read access
a) constructing such a list is tedious
b) The directory entry needs to be of
variable size, resulting in complicated
space management
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Classification of users
To condense the length of access list many systems recognize three classifications of users in connection with each file
• Owner : The user who created the file
• Group : A set of users sharing the file and need similar access
• Universe : All other users in the system constitute the universe
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Representing different users in UNIX
• Unix defines three fields of three bits each rwx, where
r controls read access
w controls write access
x controls execution
• A separate field is kept for the file owner, group users and other users
• 9 bits per file are needed
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A sample directory listing in Unix
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Consider the first field in the above listing -rwxrw-r--
• The first field describes the file’s protection
• A d as the first character indicates that the file is a
directory ; a – indicates an ordinary file
• 2nd,3rd & 4th bits owner’s access permissions
• 5th,6th & 7th bits group users’ permissions
• 8th,9th & 10th bits others’ permissions
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Other Protection Approaches
• Associate a password with each file
• Access to the file is controlled by the password
• Effective scheme in limiting access to a file if
passwords chosen randomly
passwords changed often
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Disadvantages of password protection
• If each file has a separate password, the no. of passwords a user needs to remember becomes large
• If all files have same password, then once it is discovered all files are accessible
• To deal with this problem, some systems allow a user to associate a password with a subdirectory rather than with a single file
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Summary
In this class, you have learnt:
• File protection
• Need for controlled access to files
• Types of access
• Access Lists
• User classification
• Password protection
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Frequently Asked Questions
1. Explain the concept of file protection
2. List the types of access to files
3. Explain access lists and groups
4. Explain disadvantages of password
protection
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Any questions ?
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Quiz
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1. File systems can be damaged by
a) Hardware problems
b) Power failures
c) Dirt
d) All of the above
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2. Which of following is desirable with regard to access to files?
a) Free access without protection
b) No access at all
c) Controlled access
d) None of the above
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3. Which is true about password protection ?
a) Passwords should be chosen randomly
b) Passwords must never be changed
c) Password should be changed often
d) Both a & c
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State whether the following are true or false
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State true or false
Reliability of files can be provided by duplicate copies
True
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State true or false
The problem with access lists is their length
True
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State true or false
Owner is the user who created the file
True
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State true or false
Unauthorized access to files is desirable
False
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