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Chapter 4 Protection in General Purpose
Operating Systems

• Protection features provided by general-purpose
  operating systemsprotecting memory, files, and
  the execution environment
• Controlled access to objects
• User authentication

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Protected Objects and Methods of Protections

• 1rst OS were simple utilities executives
• Multiprogramming OS required monitors which
  oversaw each programs execution
• Protected objects
• Memory
• Sharable I/O devices (disks)
• Serially reusable devices (printers)
• Shareable programs subprocedures
• Networks
• Shareable Data

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Security Methods of Operating Systems

• Physical Separation (different processes use
  different objects)
• Temporal Separation (processes executed at
  different times)
• Logical Separation (process appears to be alone)
• Cryptographic Separation (processes conceal data
  and computations)

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Security Methods of Operating Systems

• Want to be able to share resources without
  compromising security
• Do not protect
• Isolate different processes
• Share all or nothing
• Share via access limitation (granularity)
• Share by capabilities
• Limit use of an object

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Memory Address Protection

• Fence confines user to one side of boundary
• Use predefined memory addresses
• Can protect OS, but not one user from another
• Relocation changes all addresses of program
  using offset
• Base/Bounds Registers
• Uses variable fence register (base register) to
  provide lower bound
• Uses bounds register for upper address

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Memory Address Protection

• Tagged Architecture
• Every word of machine memory has extra bits to
  indicate access rights (expensive)
• Segmentation (program divided into pieces)
• Each segment has name offset
• Each address reference is checked for protection
• Different classes of data can be assigned
  different levels of protection
• Users can share access to segments
• User cannot access an unpermitted segment
• Paging (program uses equal sized pages memory
  divided into equal sized page frames)

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Control of Access to General Objects

• Memory
• File/data set
• Program in memory
• Directory of files
• Hardware device
• Data structure (stack)
• Operating system table
• Instructions (privileged)
• Passwords / user authentication mechanism
• Protection mechanism

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Goals in protecting objects

• Check every access
• Enforce least privilege
• Verify acceptable usage

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Directory mechanism

• Each user (subject) has a file directory, which
  lists all files accessible by user
• List can become too large if many shared objects
• Cannot revoke rights of everyone to an object
• File names for different owners may be different

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Access Control List

• One list for each object with list showing all
  subjects their access rights
• Can use wildcards to limit size of ACL
• Access Control Matrix
• Rows for subjects
• Columns for objects
• Sparse matrix of triples ltsubjects, objects,
  rightsgt

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Capability

• Unforgeable token that gives possessor rights to
  an object
• Predecessor of Kerberos
• Can propagate capabilities to other subjects
• Capabilities must be stored in inaccessible memory

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Procedure-Oriented Access Control

• Procedure that controls access to objects
  including what subjects can do to objects

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File Protection Mechanisms

• All-None Protection
• Lack of trust
• All or nothing
• Timesharing issues
• Complexity
• File listings

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File Protection Mechanisms

• Group Protection
• User cannot belong to two groups
• Forces one person to be multiple users
• Forces user to be put into all groups
• Files can only be shared within groups

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File Protection Mechanisms

• Single Permissions
• Password/Token for each file
• Can be lost
• Inconvenient
• Must be protected (if changed, must notify all
  users)
• Temporary Acquired Permission
• UNIXs set userid (suid)

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User Authentication

• Something the user knows (password, PIN,
  passphrase, mothers maiden name)
• Something the user has (ID, key, drivers
  license, uniform)
• Something the user is (biometrics)

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Use of Passwords

• Mutually agreed-upon code words, assumed known
  only to user and system
• First line of defense
• Loose-Lipped Systems
• WELCOME TO XYZ COMPUTING
• ENTER USER ID summers
• INVALID USER NAME
• ENTER USER ID

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Attack on Passwords

• Ask the user
• Search for the system list of passwords
• Find a valid user ID
• Create a list of possible passwords (encrypt if
  needed)
• Rank the passwords from high to low probability
• Try each password
• If attempt fails, try again (don't exceed
  password lockout)

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Attack on Passwords

• Exhaustive Attack (brute-force)
• 18,278 passwords of 3 letters or less
• 1 password / millisecond would take 18 seconds (8
  minutes for 4 letters, 3.5 hours for 5 letters)
• Probable passwords (dictionary attack)
• 80,000 word dictionary would take 80 seconds
• Expanded dictionary

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Attack on Passwords

• UK Study (http//www.cnn.com/2002/TECH/ptech/03/13
  /dangerous.passwords/?related)
• 50 passwords were family names
• Celebrities/soccer stars 9 each
• Pets 8
• 10 reflect a fantasy
• Only 10 use cryptic combinations

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Attack on Passwords

• Look on desk
• Try no password
• Try user ID
• Try users name
• Common words (password, private, secret)
• Short dictionary
• Complete English word list
• Common non-English dictionaries
• Dictionary with capitalization and substitutions
  (0 for o and 1 for i)
• Brute force (lowercase alphabet)
• Brute force (full character set)

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Attack on Passwords

• Plaintext System Password List (MS Windows)
• Encrypted Password List 1-way (/etc/passwd)
• Shadow Password List (/etc/shadow)
• Salt 12-bit number formed from system time and
  process id concatenated to password

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Password Selection Criteria

• Use characters other than A-Z
• Choose long passwords
• Avoid names and words
• Choose unlikely password
• Change password regularly (dont reuse)
• Dont write it down
• Dont tell anyone
• http//www.mit.edu/afs/sipb/project/doc/passwords/
  passwords.html
• One-time passwords

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Authentication

• Should be slow (5-10 seconds)
• Should only allow a limited of failures (e.g.
  3)
• Challenge-Response Systems
• Impersonation of Login
• Authentication Other than Passwords