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CSC 774 Advanced Network Security

CSC 774 Advanced Network Security. Topic 2.2 Hash-Based Primitives. Outline. Primitives based on cryptographic hash functions One-way hash chain Merkle hash tree Client puzzles Bloom filters Based on a different type of hash functions. F. F. F. F. F. F. K 0. K 1. K 3. K 2. K 4.

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CSC 774 Advanced Network Security

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  1. CSC 774 Advanced Network Security Topic 2.2 Hash-Based Primitives Dr. Peng Ning

  2. Outline • Primitives based on cryptographic hash functions • One-way hash chain • Merkle hash tree • Client puzzles • Bloom filters • Based on a different type of hash functions Dr. Peng Ning

  3. F F F F F F K0 K1 K3 K2 K4 Kn= R One-Way Hash Chain • Used for many network security applications • Example: S/Key • Good for authentication of the hash values commitment Ki=F(Ki+1), F: hash function Dr. Peng Ning

  4. Merkle Hash Tree • A binary tree over data values • For authentication purpose • The root is the commitment of the Merkle tree • Known to the verifier. • Example • To authenticate k2, send (m2, m3,m01,m47) • Verify m07= h(h(m01||h(f(k2)||m3)||m47) Dr. Peng Ning

  5. Bloom Filters • It’s used to verify that some data is not in the database (mismatch) • List of bad credit card numbers • Useful when the data consumes a very small portion of search space • A bloom filter is a bit string • k hash functions that map the data into n bits in the bloom filter Dr. Peng Ning

  6. A Simple Example • Use a bloom filter of 16 bits • H1(key) = key mod 16 • H2(key) = key mod 14 + 2 • Insert numbers 27, 18, 29 and 28 1 1 1 1 1 1 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 • Check for 22: • H1(22) = 6, H2(22) = 10 (not in filter) • Check for 51 • H1(51) = 3, H2(51) = 11 (false positive) Dr. Peng Ning

  7. Probability of False Positives • Consider an m-bit Bloom filter with k hash functions • After inserting n elements • Exercise Dr. Peng Ning

  8. Client Puzzles • Juels and Brainard client puzzle construction • Use pre-image of crypto hash functions • See T02.2.x-ClientPuzzles.ppt • Aura, Nikander, and Leiwo client puzzle construction • Use special image of crypto hash functions Dr. Peng Ning

  9. Client Puzzles w/ Special Images of Hash Functions • C  S: Hello • S  C: NS • C  S: C, NS, NC, X • S: verify h(C, NS, NC, X) has k leading zero’s Dr. Peng Ning

  10. Client Puzzles w/ Special Images of Hash Functions (Cont’d) • Exercise • What’s the expected cost of finding a puzzle solution? • Compare with the pre-image based puzzle construction. Dr. Peng Ning

  11. New Client Puzzle Outsourcing Techniques for DoS Resistance Brent Waters, Ari Juels, J. Alex Halderman and Edward W. Felten Dr. Peng Ning

  12. Motivation • Client puzzle mechanism can become the target of DoS attacks • Servers have to validate solutions which require resources • Puzzles must be solved online • User time is more important than CPU time Dr. Peng Ning

  13. Properties of the Proposed Solution • The creation of puzzles is outsourced to a secure entity, the bastion • Creates puzzle with no regard to which server is going to use them • Verifying puzzle solutions is a table lookup • Clients can solve puzzles offline ahead of time • A puzzle solution gives access to a virtual channel for a short time period Dr. Peng Ning

  14. Puzzle Properties • Unique puzzle solutions • Each puzzle has a unique solution • Per-channel puzzle distribution • Puzzles are unique per each (server, channel, time period) triplet • Per-channel puzzle solution • If a client has a solution for one channel, he can calculate a solution for another server with the same channel easily Dr. Peng Ning

  15. Pub: Y1 Server 1 Pub: Y2 Priv: X1 Virtual Channels Pub: Y3 c,t = gc,tX1 Server 2 Server 1: c,t = Y1f’(a) Priv: X2 Virtual Channels Server 2: c,t = Y2f’(a) c,t = gc,tX2 Server 3: c,t = Y3f’(a) Server 3 Priv: X3 Virtual Channels c,t = gc,tX3 Dr. Peng Ning

  16. System Description • Solutions for puzzles are only valid for the time period t. (Order of minutes) • Client: • During Ti, download puzzles for Ti+1 and solve • Check to see if server has a public key • If so append puzzle solutions to messages • Server: • During Ti, download and solve all puzzles for Ti+1 • If server is under attack only accept requests that have valid tokens • Checking puzzle solution is a simple table lookup Dr. Peng Ning

  17. Communication • Client uses option field in TCP SYN to relay the token • Only the first 48 bits of the solution is used • The server determines the virtual channel • Server limits new connection per channel Public key: Y Puzzle index: c Token: c,t Token: c,t+1 Virtual Channels c Dr. Peng Ning

  18. Resilience Against Attacks • 2.1 GHz Pentium can process 1024-bit DH key in 3.7ms. • With 5% recourse it can populate tokens for 16,000 virtual channels. • If s=2, every client can solve at least one puzzle and half of them can solve at least two • If attacker has 50 zombie machines, it can create 2*50*2 = 200 puzzle solutions occupying 1.25% of the channels • Probability of a benign user not getting a normal channel <.625% Dr. Peng Ning

  19. Experiment • Puzzle checking (table lookup) is implemented at kernel lvl • After the routing and before the packet reaches higher level protocols like TCP • Simulate conventional puzzles by replacing the lookup code with a SHA-1 hash computation • Simulate syncookies by allowing Linux to send an ACK packet back Dr. Peng Ning

  20. Dr. Peng Ning

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