1. 07/16/2013 Attila Altay Yavuz Robert Bosch Research and Technology Center Pittsburgh, PA 15203, USA attila.yavuz@us.bosch.com Practical Immutable Signature Bouquets (PISB) for Authentication and Integrity in Outsourced Databases 6 th ACM Conference on Security and Privacy in Wireless and Mobile Networks 27th Annual IFIP WG 11.3 Working Conference on Data and Applications Security and Privacy (DBSec '13)

2. Motivation Data outsourcing is beneficial, especially for small-medium business Reduces the cost via continuous service, expertise, maintenance/upgrade; Database as a Service (DAS) [1]: Data owners outsource their data to a database service provider (e.g., IBM). Database service provider offers a reliable maintenance and access for the hosted data. Despite its benefits, DAS also brings security and privacy challenges: Privacy versus utilization (e.g., searchable encryption) Access privacy (e.g., ORAM [2]) Authentication and integrity: Immutable digital signatures (e.g., [3,4]) 2 DBSec 2013

3. A DAS Model and Limitations (I) Model of Hacigumus et al. [1] extended by Mykletun et al. in [3] 3 DBSec 2013 M 1,…,Mn S1, …., Sn - Each tuple in database is M1,…, Mn - Compute signatures S1,…,Sn M 1,…,Mn S1, …., Sn High bandwidth - Semi-trusted entity - Honest service, but compromise? Return tuples {M1,…,Mk} along with S1,…,Sk Verify S1,…,Sk before accepting results - Bandwidth, battery and/or Computation limited queriers A query related to some tuples on the server Each query: O(K) signature transmission Better verification efficiency?

4. Digression: Aggregate Signatures Given multiple individual signatures and corresponding public key(s), output a single compact (verifiable) signature Condensed-RSA (C-RSA) [3]: Aggregate signatures with the same private key Boneh Lynn Shacham (BLS) [5]: Cryptographic pairing-based, signatures under the different private keys can be aggregated 4 DBSec 2013

5. A DAS Model and Limitations (II) DAS model of Mykletun et al. in [3] 5 DBSec 2013 M 1,…,Mn S1, …., Sn M 1,…,Mn S1, …., Sn - Each tuple in database is M1,…, Mn - Compute individual signatures S1,…,Sn with Agg High bandwidth - Semi-trusted entity - Bandwidth, battery and/or Computation limited queriers A query related to some tuples on the server Given tuples{M1,…,Mk}, Select corresponding S1,…,Sk S = Agg(S1,…,Sk) Return S as aggregate signature Verify S before accepting results O(1) signature transmission Batch signature verification Problem: Aggregate signatures are mutable

6. Problem Statement: Signature Mutability Given two C-RSA signatures, it is possible to derive a new valid signature 6 DBSec 2013 Access Control Applications: Colluding clients can elevate access privileges Paid Database Services: Online authorized music album distributor (server), store large database of digitally signed songs. Colluding clients can act as “album distributor” without paying, by mix-matching songs and their signatures. Steal profit of actual distributor. The same applies to BLS signature scheme (aggregation is modular addition).

7. Limitations of Existing Immutable Signatures Mykletun et al. developed immutable signature schemes in [3,4]. Immutable Condensed RSA (IC-RSA): Hide C-RSA signature Guillou-Quisquater [6] based scheme : Use zero-knowledge to hide C-RSA signature. (+) It is the most computationally efficient variant proposed in [3,4]. (-) Interaction introduces communication overhead and delay, (-) A signature scheme is supposed to be non-interactive! Skroot based scheme: Use “Signature of Knowledge” [7] to hide C-RSA signature (+) Non-interactive, more communication efficient than GQ-based scheme (-) High computational cost and storage cost Immutable BLS Signatures (iBLS) : BLS signature  on m’=(m1,…,ml). Compute a secondary protection signature  on m’, and aggregate  on . (+) Non-interactive and small signature (-) The most computationally costly alternative (due to crypto pairing): Verifier side 7 DBSec 2013

8. Practical Immutable Signature Bouquets (PISB) (i) PISB Condensed Sequential RSA (PISB-CSA-RSA); (ii) PISB-Generic. Non-Interactive Immutability: Communication efficiency, PISB-CSA-RSA requires 1 KB overhead, while GQ-based in [3,4] requires 9 KB overhead. High Computational Efficiency: PISB-CSA-RSA is up to 40 times faster than iBLS, skroot and GQ based schemes in [3,4]. PISB-Generic offers pre-computability, which is ideal for server to handle requests at peak times. Small Signature Sizes: PISB-CSA-RSA is more communication efficient than GQ and skroot based schemes in [3,4]. PISB-Generic is more efficient than PISB-CSA-RSA, and it is comparable to iBLS [3,4]. Low End-to-End Delay: Much faster response time based on the above properties. Provable Security: PISB schemes are only immutable signatures with formal proofs. 8 DBSec 2013

9. PISB-CSA-RSA Scheme (Intuition) Recall iBLS signatures [3,4]: Server computes a protection signature  over queried data items, and aggregate  on the original aggregate signature . Limitation of IC-RSA: IC-RSA cannot aggregate signatures of data owner and clients. The same modulo n cannot be shared among multiple signers (expose key [8]). Objective: Server and data owner jointly compute a single compact RSA signature, such that server can aggregate C-RSA signature and his protection RSA signature. Observation: Sequential Aggregate RSA (SA-RSA) [9] can help! (Simplified below) 9 DBSec 2013

10. PISB-CSA-RSA Scheme (Detailed) 10 DBSec 2013

11. PISB-Generic Scheme (Intuition) Do we have to aggregate protection signature? Power of Simplicity: Server just computes a standard signature  on the aggregate signature , and define the final signature as a pair ( ,  ). Seems communication inefficient as it is not “fully aggregate”. However: ECDSA + (BLS or C-RSA ) combination is much more communication and computation efficient than Skroot and GQ schemes in [3,4]. Flexible: Allows cross data owners queries, protection signature can be any signature such as offline/online signature [10], token-ECDSA [11]. However, PISB-CSA-RSA outperforms PISB-Generic for various performance metrics. 11 DBSec 2013

12. PISB-Generic Scheme (Detailed) 12 DBSec 2013

13. Performance Analysis 13 DBSec 2013 Estimated execution times (l = 10 query elements, in ms) are measured on a computer with an Intel(R) Core(TM) i7 Q720 at 1.60GHz CPU and 2GB RAM running Ubuntu 10.10. We used MIRACL library. PISB Generic is implemented with ECDSA + BLS with pre-computed parameters End-to-end delay: Sign + Verify + transmission (remote client –server) ~40 times more efficient Small signature Overall the most versatile choice Non-cross signer Best for server Cross signer Not ideal for verifier Offline/online ECDSA+C-RSA

14. Security Analysis Immutable Existential Unforgeability under Chosen Message Attack (I-EU-CMA) for PISB: 14 DBSec 2013 I-EU-CMA is an extension of EU-CMA such that adversary wins if the forgery is a combination or subset of queried messages (i.e., signature mutations). A vector of messages Winning condition

15. Security Analysis (Cont’) 15 DBSec 2013 Any forgery on  also requires forging protection signature s’. Generating mutable signature on  requires forging s’. Simulation is indistinguishable. Theorem 1. PISB-Generic is (t, qs,  )-I-EU-CMA secure, if ASig is (t’, qs,  )-EU- CMA secure and Sig is (t’, qs,  )-EU-CMA secure, where t’= O(t) + qs(Op + Op’) and (Op,Op’) are the cost of signing for ASig and Sig, respectively. Theorem 2. PISB-CSA-RSA is (t, qs,  )-I-EU-CMA secure, if RSA is (t’, (2l)  qs,  )-EU-CMA secure, where t’= O(t) + (2l)  qs  Exp, where l and Exp denote the modular exponentiation and number of messages in a single query, respectively. Forging sequential aggregate RSA signature  is as difficult as forging RSA.  is on, producing subset/combination requires forging RSA, individual forgery of data items require forging  thereby forging RSA. Given two RSA signature oracles (O1,O2), simulator generates PISB-CSA-RSA signatures by computing a C-RSA signature via O1 and a SA-RSA signature via O2. Simulation is indistinguishable.

16. Conclusion PISB schemes are efficient immutable signatures for outsourced databases PISB-CSA-RSA Very low client computational overhead Compact constant size signature, no interaction Suitable choice for resource-limited clients PISB-Generic Very simple, various options Cross signer aggregation is possible More efficient than previous alternatives: Simplicity Provable security guarantee 16 DBSec 2013

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