1. Authentication CS461/ECE422 Spring 2012

2. Readings Chapter 3 from text Rainbow tables – http://kestas.kuliukas.com/RainbowTables/ Chapter 10 from Handbook of Applied Cryptography http://www.cacr.math.uwaterloo.ca/hac/abo ut/chap10.pdf

3. Overview Basic elements of Authentication Password Systems Token Based Systems Biometrics

4. 4 IvanhoeIvanhoe, Sir Walter Scott Paraphrased: (Wamba gains entry to the castle dressed as a friar) Wamba: Take my disguise and escape, I will stay and die in your place. Cedric: I can’t possibly impersonate a friar, I only speak English. Wamba: If anyone says anything to you, just say “Pax vobiscum.” Cedric: What does that mean? Wamba: I don’t know, but it works like a charm!

5. 5 Basics Authentication: binding of identity to subject – Identity is that of external entity (my identity, the Illini Union Bookstore, etc.) – Subject is computer entity (process, network connection, etc.) – Two steps – Identification step: present identifier to security system. Registration – Verification step: Present or generate authentication information that corroborates the binding between entity and identifier

6. 6 Establishing Identity One or more of the following – What entity knows (e.g. password, private key) – What entity has (e.g. badge, smart card) – What entity is (e.g. fingerprints, retinal characteristics) – What entity does (e.g., voice pattern, handwriting, typing rhythm) – Where entity is (e.g. In front of a particular terminal) Example: scene from Ivanhoe Example: Credit card transaction Multi-factor authentication Use multiple elements to prove identity

7. Complementation Information User provides information to verify identity System stores a processed version of this information as the complementation information The complementation function maps from the user provided data to the system stored data

8. Password-based Authentication External entity is bound to system ID (user account) Authentication Step – External entity presents password – System compares with previously stored password – If password matches, system starts process with bound ID Later access control decisions made against ID Privilege decisions made against ID

9. Password Vulnerabilities Password systems widely used, but very vulnerable – Offline dictionary attack – Specific account attack – Workstation hijacking – Sticky notes – Password reuse – Social engineering – Electronic monitoring

10. 10 Password Storage Store as cleartext – If password file compromised, all passwords revealed Encipher file – Need to have decipherment, encipherment keys in memory – Reduces to previous problem Store one-way hash of password – If file read, attacker must still guess passwords or invert the hash

11. 11 Unix Password Hash Example Original UNIX system standard hash function – Hashes password into 13 char string – As authentication system: – Authentication information is strings of 8 characters or less – System stores hash with user’s identity in password file – Hash is complementation information – Verification function is hash on password and comparison with stored hash

12. 12 Salting Have a set of n hash functions – Randomly select one function when registering new authentication info – Store ID of function with registered info Attacker must try all n functions to see if his guess matches any password When does this help? When does it not?

13. 13 Examples Vanilla UNIX method – Use DES to encipher 0 message with password as key; iterate 25 times – Perturb E table in DES in one of 4096 ways 12 bit salt flips entries 0–11 with entries 24–35 E Table is per round expansion table Alternate methods – Use salt as first part of input to hash function

14. 14 Dictionary Attacks Trial-and-error from a list of potential passwords – Off-line (type 1): know functions and registered information, and repeatedly try different guesses g  A until the list is done or passwords guessed Examples: crack, john-the-ripper – On-line (type 2): have access to verification functions. Try guesses until one succeeds. – Examples: trying to log in by guessing a password

15. 15 Preventing Attacks How to prevent this: – Hide information so that either authentication input, authentication functions, or stored verification information cannot be found. Prevents obvious attack from above Example: UNIX/Linux shadow password files – Hides c’s – Block access to all verification methods Prevents attacker from knowing if guess succeeded Example: preventing any logins to an account from a network – Prevents knowing results of verification function or accessing verification function.

16. Rainbow Tables Rather than keeping dictionary list, could pre-compute the hashes of the dictionary values – For large dictionaries, still a lot of space – Particularly if using a smart lookup table Trade off space for time with hash chain – p1->H(p1)->R(H(p1))->H(R(H(p1))) – aaaaaa –h-> 281DAF40 –r-> sgfnyd –h-> 920ECF10 – Reduction function, r, picks another plaintext from the hash. It is not the inverse hash – Only store start and end of hash – Given hash to break, look for it in the end of the chains. – Apply reduction and hash if not found – Continue until found

17. Rainbow Tables Collisions are the problems with rainbow tables – Two passwords may hash to the same value – One of those values will be lost in the hash chain Could use many small tables with different reduction functions Could use a different reduction function for each column – Thus a “Rainbow” table

18. Rainbow tables Very effective against Lan Manager Hashes – Calculated by XP and up to Windows 2008 by default for “backwards compatibility” – Can download huge tables from a number of free sites Decent sized hashes make the rainbow table pre-computation space infeasible

19. 19 Using Time Anderson’s formula: P probability of guessing a password in specified period of time G number of guesses tested in 1 time unit T number of time units N number of possible passwords (|A|) Then

20. 20 Example Goal – Passwords drawn from a 96-char alphabet – Can test 10 4 guesses per second – Probability of a success to be 0.5 over a 365 day period – What is minimum password length? Solution – N ≥ TG/P = (365  24  60  60)  10 4 /0.5 = 6.31  10 11 – Choose s such that – So s ≥ 6, meaning passwords must be at least 6 chars long – What exactly does that equation mean?

21. 21 Approaches: Password Selection Random selection – Any password from A equally likely to be selected – See previous example – Make sure it’s random! Pronounceable passwords User selection of passwords

22. 22 Pronounceable Passwords Generate phonemes randomly – Phoneme is unit of sound, e.g. cv, vc, cvc, vcv – Examples: helgoret, juttelon are; przbqxdfl, zxrptglfn are not ~ 440 possible phonemes 440 6 possible keys with 6 phonemes (12-18 characters long), about the same as 96 8 Used by GNU Mailman mailing list software (?)

23. 23 User Selection Problem: people pick easy-to-guess passwords – Based on account names, user names, computer names, place names – Dictionary words (also reversed, odd capitalizations, control characters, “l33t-speak”, conjugations or declensions, Torah/Bible/Koran/… words) – Too short, digits only, letters only – License plates, acronyms, social security numbers – Personal characteristics or foibles (pet names, nicknames, etc.) – Using the same password in multiple accounts

24. User Password Education Use the first letter of each word in a phrase – “My dog’s first name is Rex.” becomes “MdfniR”

25. Reactive Password Checking Have a password cracking program running in the background – Shut down account of passwords it can crack – CPU intensive – Shutting down active accounts is likely to annoy someone important eventually.

26. Proactive password checking Don’t let them pick a “bad” password in the first place Need to have a fairly fast test of the “goodness” of a password

27. Markov Model Reduce space requirements of bad password list Create model that represents the bad password database Created from trigrams (three letter segments) from words in bad password list

28. Bloom Filter Another way of encoding the bad password dictionary in a small cheap to check data structure Create N bit array Use k independent hash functions which hash into a space of 0 to N-1 For each bad password, – Compute every version of the hash, and set the corresponding bit in the hash table for every hash value To check a password – Computer every version of the hash, and check the corresponding bits in the array – If all bits are 1, then the password is bad Will allow some false positives – Passwords marked bad that weren’t in the original list But it will not generate any false negatives.

29. 29 Challenge-Response User and system share a secret function User proves knowledge of secret function by answering challenge user system request to authenticate user system random message r (the challenge) user system f(r) (the response)

30. 30 One-Time Passwords Password that can be used exactly once – After use, it is immediately invalidated Challenge-response mechanism – Challenge is one of a number of authentications; response is password for that particular number Problems – Synchronization of user, system – Generation of good random passwords – Password distribution problem

31. 31 S/Key One-time password scheme based on idea of Lamport h one-way hash function (MD5 or SHA-1, for example) User chooses initial seed k System calculates: h(k) = k 1, h(k 1 ) = k 2, …, h(k n–1 ) = k n Passwords are reverse order: p 1 = k n, p 2 = k n–1, …, p n–1 = k 2, p n = k 1

32. 32 S/Key Protocol user system { name } user system { i } user system { p i } System stores maximum number of authentications n, number of next authentication i, last correctly supplied password p i–1. System computes h(p i ) = h(k n–i+1 ) = k n–i+2 = p i–1. If match with what is stored, system replaces p i–1 with p i and increments i.

33. Token-based Authentication Something you have Memory Cards – No computation on the card – Need special reader to pull data off the card – Need pin to decrypt data off of card – E.g., ATM card or debit card By adding PIN (something you know) you get multi-factor authentication

34. Token Based Authentication Smart Card – Computation on the card – Plug in with USB or wireless communication (credit card) Authentication options – Static – equivalent to memory card – Dynamic password generator – generates a unique password every minute. – Challenge response

35. 35 Biometrics Automated measurement of biological, behavioural features that identify a person – Fingerprints: optical or electrical techniques Maps fingerprint into a graph, then compares with database Measurements imprecise, so approximate matching algorithms used – Voices: speaker verification or recognition Verification: uses statistical techniques to test hypothesis that speaker is who is claimed (speaker dependent) Recognition: checks content of answers (speaker independent)

36. 36 Other Characteristics Can use several other characteristics – Eyes: patterns in irises unique Measure patterns, determine if differences are random; or correlate images using statistical tests – Faces: image, or specific characteristics like distance from nose to chin Lighting, view of face, other noise can hinder this – Keystroke dynamics: believed to be unique Keystroke intervals, pressure, duration of stroke, where key is struck Statistical tests used

37. 37 Biometric Physical characteristics encoded in a template – The C or complement information User registers physical information (S) – Generally with multiple measurements The verification function takes a measurement and tries to line up with template

38. 38 Authentication vs Identification Used for surveillance – Subject is motivated to avoid detection Used for authentication – Subject is motivated to positively identify – Perhaps pick up other's characteristics False positives vs false negatives

39. 39 Biometric Cautions These can be fooled! – Assumes biometric device accurate in the environment it is being used in! – Transmission of data to validator is tamperproof, correct (remember pax vobiscum) Physical characteristics change over time Some people may not be able to identify via specific characteristics – Albinos and iris scans

40. Biometric Cautions Where are the biometric templates stored? What if your biometric template data is stolen?

41. Key Points Passwords are the reality for now Multi-factor authentication is must stronger Biometrics can help, but not a silver bullet yet