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1Lecture 9Page 1CS 239, Winter 2005AuthenticationCS 239Computer SecurityFebruary 14, 2005Lecture 9Page 2CS 239, Winter 2005Outline• Introduction• Basic authentication mechanisms• Authentication on a single machine• Authentication across a networkLecture 9Page 3CS 239, Winter 2005Introduction• Much of security is based on good access control• Access control only works if you have good authentication• What is authentication?Lecture 9Page 4CS 239, Winter 2005Authentication• Determining the identity of some entity–Process–Machine–Human user• Requires notion of identity• And some degree of proof of identityLecture 9Page 5CS 239, Winter 2005Proving Identity in the Physical World• Most frequently done by physical recognition–I recognize your face, your voice, your body• What about identifying those we don’t already know?Lecture 9Page 6CS 239, Winter 2005Other Physical World Methods of Identification• Identification by recommendation– You introduce me to someone• Identification by credentials– You show me your driver’s license• Identification by knowledge– You tell me something only you know• Identification by location– You’re behind the counter at the DMV• These all have cyber analogs2Lecture 9Page 7CS 239, Winter 2005Differences in Cyber Identification• Usually the identifying entity isn’t human• Often the identified entity isn’t human, either• Often no physical presence required• Often no later rechecks of identityLecture 9Page 8CS 239, Winter 2005Identifying With a Computer• Not as smart as a human– Steps to prove identity must be well defined • Can’t do certain things as well– E.g., face recognition• But lightning fast on computations and less prone to simple errors– Mathematical methods are acceptableLecture 9Page 9CS 239, Winter 2005Identifying Computers and Programs• No physical characteristics–Faces, fingerprints, voices, etc.• Generally easy to duplicate programs• Not smart enough to be flexible–Must use methods they will understand• Again, good at computations Lecture 9Page 10CS 239, Winter 2005Physical Presence Optional• Often must be identified over a network or cable• Even if the party to be identified is human• So authentication mechanism must work in face of network characteristics–E.g., active wiretappingLecture 9Page 11CS 239, Winter 2005Identity Might Not Be Rechecked• Human beings can make identification mistakes• But they often recover from them– Often quite easily• Based on observing behavior that suggests identification was wrong• Computers and programs rarely have that capability– If they identify something, they believe itLecture 9Page 12CS 239, Winter 2005Authentication Mechanisms• Something you know– E.g., passwords• Something you have– E.g., smart cards or tokens• Something you are– Biometrics• Somewhere you are– Usually identifying a role3Lecture 9Page 13CS 239, Winter 2005Passwords• Authentication by what you know• One of the oldest and most commonly used security mechanisms• Authenticate the user by requiring him to produce a secret– Known only to him and to the authenticator– Or, if one-way encryption used, known only to him Lecture 9Page 14CS 239, Winter 2005Problems With Passwords• They have to be unguessable–Yet easy for people to remember• If networks connect terminals to computers, susceptible to password sniffers• Unless fairly long, brute force attacks often work on themLecture 9Page 15CS 239, Winter 2005Proper Use of Passwords• Passwords should be sufficiently long• Passwords should contain non-alphabetic characters• Passwords should be unguessable• Passwords should be changed often• Passwords should never be written down• Passwords should never be sharedLecture 9Page 16CS 239, Winter 2005Passwords and Single Sign-On• Many systems ask for password once–Resulting authentication lasts for an entire “session”• Unless other mechanisms in place, complete mediation definitely not achieved• Trading security for convenienceLecture 9Page 17CS 239, Winter 2005Handling Passwords• The OS must be able to check passwords when users log in• So must the OS store passwords?• Not really – It can store an encrypted version• Encrypt the offered password – Using a one-way function• And compare it to the stored versionLecture 9Page 18CS 239, Winter 2005Standard Password HandlingLogin: GrouchoWe6/d02,Password: swordfishHarpo 2st6’sG0Zeppo G>I5{as3Chico w*-;sddwKarl sY(34,ee,Groucho We6/d02,Gummo 3(;wbnP]The Marx Brothers’Family Machine4Lecture 9Page 19CS 239, Winter 2005Is Encrypting the Password File Enough?• What if an attacker gets a copy of your password file?• No problem, the passwords are encrypted–Right?• Yes, but . . .Lecture 9Page 20CS 239, Winter 2005Dictionary Attacks on an Encrypted Password FileDictionaryaardvark340jafg;Now you can hack the Communist Manifesto!Harpo 2st6’sG0Zeppo G>I5{as3Chico w*-;sddwKarl sY(34,ee,Groucho We6/d02,Gummo 3(;wbnP]sY(34,eeRats!!!!aardwolfK]ds+3a,abaca sY(34,eeLecture 9Page 21CS 239, Winter 2005A Serious Issue• All Linux machines use the same one-way function to encrypt passwords• If someone runs the entire dictionary through that function, –Will they have a complete list of all encrypted dictionary passwords?Lecture 9Page 22CS 239, Winter 2005Illustrating the Problembeard^*eP6la-beard^*eP6la-aardvark 340jafg;aardwolf K[ds+3a,abaca sY(34,ee. . .beard ^*eP61a-Lecture 9Page 23CS 239, Winter 2005The Real Problem• Not that Darwin and Marx chose the same password• But that anyonewho chose that password got the same encrypted result• So the attacker need only encrypt every possible password once• And then she has a complete dictionary usable against anyoneLecture 9Page 24CS 239, Winter 2005Salted Passwords• Combine the plaintext password with a random number–Then run it through the one-way function• The random number need not be secret• It just has to be different for different users5Lecture 9Page 25CS 239, Winter 2005Did It Fix Our Problem?beard beardD0Cls6&)#4,doa8aardvark 340jafg;aardwolf K[ds+3a,abaca sY(34,ee. . .beard ^*eP61a-Lecture 9Page 26CS 239, Winter 2005Protecting the Password File• So it’s OK to leave the encrypted version of the password file around?• No, it isn’t• Why make it easy for attackers?• Dictionary attacks against single accounts can still work• Generally, don’t


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UCLA COMSCI 239 - Lecture 9

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