One-way Hash Chain and Its Applications

2. One-way Hash Chain and Its Applications Presenter: Hoa Nguyen Ngoc

3. 4. One way hash chain in RFID system 2. Merkle Hash Tree 3. Radio Frequency Identification (RFID) 1. One-way Hash Chain Outline

4. 1. One Way Hash Chain

5. Good for authentication of the hash values Used for many network security apps h: Cryptographically strong hash function One Way Hash Chain 2 3 1 Example: S/Key, RFID authentication, micropayment systems H0 = x Hn = h(Hn-1) = h(h(h(…h(x)))) One-way Hash Chain

6. S/Key • Setup • Server generates hash chain: H100(s), H99(s), H98(s),……., H(s) • User prints out list • Server stores H101(s) • Authentication • At time 0: User  Server: H100(s) • Server verfifies h(H100(s)) = H101(s) if true • Server stored H100(s), user crosses one off each time • At time 1: User  Server: H99(s)…

7. 2. Merkle Hash Tree

8. Merkle Hash Tree

9. 3. Radio Frequency Identification (RFID)

10. Radio Frequency Identification • Architecture communication interface & protocol middleware tag reader

11. How does RFID work? 02.3DFEX4.78AF51 EasyToll card #816 Radio signal (contactless) Range: ~5m Tags (transponders) Attached to objects, call out their (unique) name and/or static data on a special radio frequency Reader (transceiver) Reads data of the tags without direct contact Database Matches tag IDs to physical objects

12. Security and Privacy Threats within RFID • Spoofing • Imitating the behavior of a genuine tag • Denial of Service • Man in the middle attack • Modify the response of the tag to the reader or vice versa • Replay Attack • Eavesdrop message from the tag (reader) & re-transmit the message to the legitimate reader (tag). • Traffic Analysis • Monitoring of comm. between reader & tag allows adversary to perform traffic analysis & generate statistical data.

13. 4. 4. One way hash chain in RFID system

14. OSK: RFID Scheme design • Hash chain model in RFID apps • How it works • A tag has initial s1. • At i-th transaction with the reader, the RFID will do 2 things: • Send ai = G(si) to the reader, • Renews si+1 = H(si) • The reader send ai to back-end

15. OSK Scheme • Goal: Enable reader to identify the RFID tag, change tag identifier on each read Database Reader Tag Query Ai=G(Si) Ai=G(Si) Compute Hash Chain Si+1=H(Si) Tag ID

16. OSK (Cont.) PROS • Different random like values on every read operation prevents tracking • Forward Security ensured due to one way hash property • Tag needs to store only 2 hash implementations, hence low cost • Minimal number of transmissions CONS • Not scalable for large scale applications due to brute force search • Motivates reducing computation time at reader/backend • Susceptible to DoS attacks • May lead to problem due to hash collisions.

17. “Who are you?” metaID key “My real ID is…” Hash Lock • Goal: Authentication reader to the RFID tag Reader RFID tag Compute hash(key) and compare with stored metaID Stores metaID=hash(key) Stores key; hash(key) for any tag Unique key for each tag

18. Hash Lock (cont.) PROS • Relatively cheap to implement : Tag has to store hash function implementation and metaID • Security based on weak collision-resistance of hash function • Scalable due to low key look-up overhead CONS • Constant tag output – enables traceability • Motivates Randomization • Too many messages/rounds • Requires reader to know all keys

19. “Who are you?” R, hash(R,IDk) “You must be IDk” Randomized Hash Lock • Goal: Authentication reader to the RFID tag Reader RFID tag Generate random R Compute hash(R,IDi) for every known IDi and compare Stores its own IDk Stores all IDs: ID1, … ,IDn

20. Randomized Hash Lock PROS • Randomized response prevents tracking • Tag needs to store hash implementation and pseudo-random number generator CONS • Inefficient brute force key look-up • No Forward security • Motivates updating tag ID on each read • Security Flaw - Adversary can impersonate tag by learning a valid tag response.

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