AUTHENTICATION. Slides are from Andreas Pfitzmann, Andreas Westfeld, Sebastian Clauß, Mike Bergmann and myself (Stefan Köpsell)
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1 1 AUTHENTICATION Slides are from Andreas Pfitzmann, Andreas Westfeld, Sebastian Clauß, Mike Bergmann and myself (Stefan Köpsell)
2 Lookahed: Why autentication: Admission and access control 2 Admission control communicate with authorized partners only user process reference monitor check authorization; log author and operation before access to data or programs data, programs Access control subject can only exercise operations on objects if authorized.
3 Identification of human beings by IT-systems 3? What one is has hand geometry finger print picture hand-written signature retina-pattern voice typing characteristics paper document metal key magnetic-strip card smart card (chip card) calculator eid-card knows password, passphrase answers to questions calculation results for numbers
4 New German eid Card 4 PIN protects access to chip
5 Identification of IT-systems by human beings 5? What it is casing seal, hologram pollution knows password answers to questions calculation results for numbers Where it stands
6 Identification of IT-systems by IT-systems 6? What it knows password answers to questions calculation results for numbers cryptography Wiring from where
7 Password based authentication 7 Simple approach yes grant access User (dog,bone) Server no deny access Login dog Password bone
8 Password based authentication 8 Simple approach security problems yes grant access User (dog,bone) Server no deny access Attacker might get access! Login dog Password bone
9 Password based authentication 9 Enhanced approach using one way (hash) functions yes grant access User (dog,bone) Server calculates h(bone) no deny access Login dog Password h(bone)
10 One-way functions cryptographic hash functions 10 One-way function f: calculating f(x)=y is easy calculating f -1 (y)=x is hard computation / storage open question: Do one-way functions exist? Cryptographic hash function h might have different properties depending on the use case collision resistance: it is hard to find h(y)=h(x) with y x note: h is usually not collision free, because h(x) x preimage resistance / one-way function / secrecy given h(x) it is hard to find x second-preimage resistance / weak collision resistance / binding given h(x) it is hard to find h(y)=h(x) with y x Note: h is not necessarily a random extractor only one of secrecy and binding can be information theoretic secure
11 Examples for cryptographic hash functions 11 MD5 Message-Digest Algorithm developed by Ronald Rivest (April 1992) produces 128 bit hash values can process arbitrary long inputs today MD5 is broken! SHA-1 Secure Hash Standard published 1993 as FIPS PUB 180 by US NIST produces 160 bit hash values today SHA-1 is insecure! SHA-2 set of hash functions, with hash values of 224, 256, 384, 512 bit published 2001 as FIPS PUB by NIST SHA-2 hash functions are believed to be secure SHA-3 result of the NIST Cryptographic Hash Algorithm Competition started November selection rounds, 5 finalists result 2 nd October 2012: Keccak (pronounced [kɛtʃak], like ketchak )
12 MD5 Hash in the Wild 12 United States Cyber Command (USCYBERCOM) mission statement:= USCYBERCOM plans, coordinates, integrates, synchronizes and conducts activities to: direct the operations and defense of specified Department of Defense information networks and; prepare to, and when directed, conduct full spectrum military cyberspace operations in order to enable actions in all domains, ensure US/Allied freedom of action in cyberspace and deny the same to our adversaries.
13 MD5 Hash in the Wild 13 mission statement:= USCYBERCOM plans, coordinates, integrates, synchronizes and conducts activities to: direct the operations and defense of specified Department of Defense information networks and; prepare to, and when directed, conduct full spectrum military cyberspace operations in order to enable actions in all domains, ensure US/Allied freedom of action in cyberspace and deny the same to our adversaries.
14 mission statement:= USCYBERCOM plans, coordinates, integrates, synchronizes and conducts activities to: direct the operations and defense of specified Department of Defense information networks and; prepare to, and when directed, conduct full spectrum military cyberspace operations in order to enable actions in all domains, ensure US/Allied freedom of action in cyberspace and deny the same to our adversaries. MD5(mission statement)= 9ec4c12949a4f31474f299058ce2b22a (Remember: MD5 is broken find other interesting mission statements)
15 Password based authentication 15 Enhanced approach using one way (hash) functions yes grant access User (dog,bone) Server calculates h(bone) no deny access Login dog Password h(bone)
16 Password based authentication 16 Enhanced approach using one way (hash) functions yes grant access User (dog,bone) Server calculates h(bone) no deny access Slightly reduced risk, if attacker gets access. Login dog Password h(bone)
17 Remaining problems of password based authentication based one way functions Brute Force attack function h() is public value of h(x) is known to the attacker try all possible values for x Considerations: usually >> 1 Mio. h(x)/s on ordinary hardware assumption: password uses only small letters password length = 8 time needed: h first countermeasures (against remote attacks): limit false attempts first password rules: use a large alphabet (small and capitalised letters, numbers, specials) use a long password Login dog Password h(bone) 17
18 Remaining problems of password based authentication based one way functions 18 first password rules: use a large alphabet (small, capitalised letters, numbers, specials) ( ) time needed: a use a long password Login dog Password h(bone) remaining possible attacks: increase in computation power distributed approach GPU Moore s law pre-computation: attacker creates lockup table search time (example above): ld <53 comparisons
19 Remaining problems of password based authentication based one way functions 19 remaining possible attack: pre-computation: attacker creates lockup table search time (example above): ld <53 comparisons problem: storage space Example: size of MD5 hash: 128 bit number of hashes: = storage space: bit TByte price: 2 Mio solution: storage space / computation tradeoff Login Martin E. Hellman: A Cryptanalytic Time Memory Trade-Off dog Password h(bone)
20 Cryptanalytic Time Memory Trade-Off 20 main idea: store only certain parts of the lookup table regenerate the missing parts on demand requires reduce function f f: H P (H: set of hash values, P: set of passwords) note: f is NOT the inverse of h general procedure: calculate a chain of hash and reduce function calls p h() f() h() f() h () f() p store first and last value in a table sort by the last value length of chain influences Time Memory trade-off
21 Cryptanalytic Time Memory Trade-Off 21 Example: sonne MD5 97ad856de10a64018f15e8e325ab1d0d reduce f boosa MD5 61bd9c7e4ae81f59c4db6c7f954a5144 reduce f aitha MD a288fa5e7c3dd28b41b65302cdb reduce f caing
22 Cryptanalytic Time Memory Trade-Off 22 2 nd example breaking of PINs h(x):= (x 7807) mod f(x) := x mod mod mod PIN-table: mod mod mod mod table entry: 1339 : 6399
23 Cryptanalytic Time Memory Trade-Off PIN-table: Breaking a PIN: h(pin i ) Goal: find PIN for given hash value h(pin) Algorithm: 1. hash / reduce until value is in the right column 2. take left column value 2. hash / reduce until PIN is found no h(pin) reduce f PIN i PIN i in table (right col.)? reduce f(h(pin j )) yes no take PIN j from table (left col.) h(pin j ) = h(pin)? 23 yes found PIN = PIN j!
24 Remaining problems of password based authentication based one way functions 24 remaining possible attack: pre-computation: attacker creates lockup table search time (example above): ld <53 comparisons problem: storage space Example: size of MD5 hash: 128 bit number of hashes: = storage space: bit TByte price: 2 Mio solution: storage space / computation tradeoff Login Martin E. Hellman: A Cryptanalytic Time Memory Trade-Off dog Password h(bone) today: use the power of the Internet / Cloud
25 Remaining problems of password based authentication based one way functions 25 remaining possible attack: pre-computation countermeasure: salt! h(x) h(salt,x) salt: long (e.g. 128 bit) random value Login Password some part is unique for the system (i.e. 104 bit) some part is randomly chosen by the system for each entry in the password table (i.e. 24 bit) NOT stored at the system verification: iterate over all possible salt values dog h(bone) pre-computation has to be done for each possible salt
26 Remaining problems of password based authentication based one way functions remaining possible attack: dictionary attack problem: people do not chose passwords randomly often names, words or predictable numbers are used attacker uses dictionaries for brute force attack prominent program: John the Ripper supports dictionary attacks and password patterns possible solutions: Login enforce password rules consider usability pre-check passwords (e.g. using John) train people to generate good passwords Example: sentence password This is the password I use for Google mail TitpIu4Gm dog Password h(salt,bone) 26
27 Password based authentication 27 Simple approach security problems yes grant access User (dog,bone) Server no deny access Attacker might get access! Login dog Password h(salt,bone)
28 Password based authentication 28 Simple approach security problems yes grant access User (dog,bone) Server no deny access Might be the attacker! Login dog Password h(salt,bone)
29 The Server as Attacker 29 a new Web 2.0 service for people which like city journeys find cool cities and places like shops, restaurants, hotels etc.... information from globe-trotters for globe-trotters they can share their knowledge after secure login So that s wrong? It collects (username,password) and tries to login into other popular services like Gmail, Twitter, ebay, Amazon etc. password rule: never reuse passwords!
30 Password based authentication 30 Simple approach security problems yes grant access User (dog,bone) Server no deny access Attacker might get access! Login dog Password h(salt,bone)
31 Password based authentication 31 security problems User (dog,bone) Server possible solution: encrypt communication remaining problems: not always possible replay attack
32 Password based authentication 32 security problem replay attack User Enc(dog,bone) Server Enc(dog,bone)
33 Password based authentication 33 security problem replay attack possible solution: challenge-response protocol random challenge c User response f(c, secret) Server tries to ensure freshness remaining problems: Man-in-the-middle attacks parallel protocol runs
34 Password based authentication 34 security problem MITM / parallel protocol runs 1.1 login request (dog) User 2.1 login request (dog) Server 1.2 random challenge c 2.2 random challenge c 1.2 random challenge c 1.3 response f(c, secret) 2.3 response f(c, secret)
35 Password based authentication 35 security problem MITM / parallel protocol runs possible solutions: disallow parallel login protocol runs for the same user make protocol runs distinguishable
36 Password based authentication 37 security problem MITM / parallel protocol runs possible solutions: disallow parallel login protocol runs for the same user make protocol runs distinguishable remaining security problems: (ok I will stop here if you are interested in many more problems / solutions I recommend: Colin Boyd, Anish Mathuria: Protocols for Authentication and Key Establishment, Springer, 2003.)
37 Password based authentication 39 (non protocol related) security problems: phising, i.e. faked UI for entering secret information today: mostly Internet based attacks but: local attacks possible as well faked login / lock screen solution: trusted path / Secure Attention Key The TCB [Trusted Computing Base] shall support a trusted communication path between itself and user for initial login and authentication. Communications via this path shall be initiated exclusively by a user. [Department of Defense: Trusted Computer System Evaluation Criteria, CSC-STD , 15. August 1983 called Orange Book ] well known implementations: Windows: Ctrl+Alt+Del Linux: Ctrl+Alt+Pause could be freely chosen in principle [ ikipedia.org/w iki/file:orange-book-s mall. PNG]
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