29.1 Lecture 29 Security I Based on the Silberschatz & Galvin’s slides And Stallings’ slides

29.2 Security The Security Problem Authentication Program Threats

29.3 The Security Problem Security must consider external environment of the system, and protect it from: –unauthorized access. –malicious modification or destruction –accidental introduction of inconsistency. Easier to protect against accidental than malicious misuse.

29.4 Types of Threats Interruption –an asset of the system is destroyed of becomes unavailable or unusable –Examples  destruction of hardware (hard disk)  cutting of a communication line  disabling the file management system

29.5 Types of Threats Interception –an unauthorized party gains access to an asset –Examples  wiretapping to capture data in a network  illicit copying of files or programs

29.6 Types of Threats Modification –an unauthorized party not only gains access but tampers with an asset –Examples  changing values in a data file  altering a program so that it performs differently  modifying the content of messages being transmitted in a network

29.7 Types of Threats Fabrication –an unauthorized party inserts counterfeit objects into the system –Examples  insertion of spurious messages in a network  addition of records to a file

29.8 Authentication User identity most often established through passwords, can be considered a special case of either keys or capabilities. Passwords must be kept secret. –Frequent change of passwords. –Use of “non-guessable” passwords. –Log all invalid access attempts.

29.9 Techniques for Learning Passwords Try default password used with standard accounts shipped with computer Exhaustively try all short passwords Try words in dictionary or a list of likely passwords Collect information about users and use these items as passwords`4

29.10 Observed Password Lengths Length Number Fraction of Total Purdue University. 54 machines user accounts

29.11 Techniques for Learning Passwords Try user’s phone numbers, social security numbers, and room numbers Try license plate numbers Use a Trojan horse to bypass restrictions on access Tap the line between a remote user and the host system

29.12 Password Cracked from a Sample Set of 13,797 Accounts [KLEI90] Type of Password Search Number Percentage of Size of Matches Passw. Match User/account name % Common names % Female names % Phrases and patterns % System dictionary % Machine names %

29.13 Effectiveness of guessing (W. Stallings) Personal information Words from various dictionaries (60000 words) Various permutations of words from the dictionaries) Capitalization permutations Test with 3 million words Thinking Machine Time to encrypt all these words: under one hour Success rate: 25%

29.14 Password Selection Strategies Computer generated passwords –users have difficulty remembering them –need to write it down –have history of poor acceptance Eliminate guessable passwords while allowing the user to select a password that is memorable

29.15 Password Selection Strategies Reactive password checking strategy –system periodically runs its own password cracker to find guessable passwords –system cancels passwords that are guessed and notifies user –consumes resources to do this –hacker can use this on their own machine with a copy of the password file

29.16 Password Selection Strategies Proactive password checker –the system checks at the time of selection if the password is allowable –with guidance from the system users can select memorable passwords that are difficult to guess

29.17 Program Threats (Needs Host Programs) Trojan Horse –Useful program (example a game) that contains hidden code that when invoked performs some unwanted or harmful function –can be used to accomplish functions indirectly that an unauthorized user could not accomplish directly  user may set file permission so everyone has access

29.18 Program Threats (Needs Host Programs) Trap Door –Entry point into a program that allows someone who is aware of trapdoor to gain access –used by programmers to debug and test programs  avoids necessary setup and authentication  method to activate program if something wrong with authentication procedure

29.19 Program Threats (Needs Host Programs) Logic Bomb –Logic embedded in a computer program that checks for a certain set of conditions to be present on the system  presence or absence of certain files  particular day of the week  particular user running application –When these conditions are met, it executes some function resulting in unauthorized actions