1. 多媒體網路安全實驗室 Practical Searching Over Encrypted Data By Private Information Retrieval Date：2011.05.19 Reporter: Chien-Wen Huang 出處: GLOBECOM 2010, 2010 IEEE Global Telecommunications Conference

2. 多媒體網路安全實驗室 Outline INTRODUCTION 1 PREPARATION 2 PRIVATE INFORMATION RETRIEVAL 33 OUR PROPOSAL AND PERFORMANCE ANALYSIS 44 COMPARISON 35 CONCLUSION 46 2

3. 多媒體網路安全實驗室 1.INTRODUCTION  there are Sender and User (Receiver) who want to communicate mainly via the “honest- but-curious” database.  Sender: only permitted to send a couple of keywords, but not the whole data which is commonly a relatively large file(videos or photos)  User: could efficiently search and retrieve the information those Sender submitted 3

4. 多媒體網路安全實驗室 2.PREPARATION  Boneh et al.proposed the scheme:  PIR technique aims to retrieve the target data  Several techniques have been employed  Bloom filter: used only as the intermediate storage of the information on addresses of data  color survival game  modified encrypted data 4

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6. 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  n hash functions that map the data into n bits in the bloom filter 6

7. 多媒體網路安全實驗室 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 2 3 4 5 6 7 8 9 10 11 12 13 14 15 1111111 Check for 22: H1(22) = 6, h2(22) = 10 (not in filter) Check for 51: H1(51) = 3, h2(51) = 11 (false positive) 7

8. 多媒體網路安全實驗室 3.PRIVATE INFORMATION RETRIEVAL  A. IPIR  Then he sends to DB a query of whole dataset,so that DB replies all of dataset.  B. Block PIR  uses multiple databases  Like Chor et.al.(by 1995)  BlockPIR scheme is secure, if DBs do not collude together. 8

9. 多媒體網路安全實驗室  C. Computational PIR  Based on Paillier cryptosystem  the computation cost instead of communication cost here is the bottleneck of the operation time  The homomorphic encryption is assumed as follows: 1)Compute 2)Select random 3) 4) 9

10. 多媒體網路安全實驗室 4.OUR PROPOSAL AND PERFORMANCE ANALYSIS  A. Proposed Scheme  DB: has a size of N bits and can store n messages in maximum.  Buffer: has a size of M bits(we assume M is a square)  R(receiver): has the key pair and a length of cipher text is k  S(sender): uses the keyword of w words  (k,m)-Bloom Filter: has k hash functions and outputs value with a length of m bits. 10

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12. 多媒體網路安全實驗室  Assume there are two buffers, Buffer1 and Buffer2 1.S associates keyword W to the message M and send E(M) to DB. 2.DB stores E(M) in main database, returns the corresponding address ρ. 3.S inputs W to Bloom filter to get the k outputs as addresses of Buffer (1,2) 4.S then encrypts the r copies of ρ as and writes them into r addresses of Buffer 1 and Buffer 2 5.S modifies the encrypted data 12

13. 多媒體網路安全實驗室  R intends to search the keyword W associated with the message from DB. 1)Input W to Bloom filter and get the k addresses H(W) of Buffer. 2)Execute BlockPIR to the addresses k times, and get k outputs of. a)R generates random vector b)Repeat k times to recover 3) R decrypts and gets 4)R executes CPIR to the ρ of DB and gets the M associated with W. 13

14. 多媒體網路安全實驗室  B. Performance of Previous Scheme  the time required for CPIR is shown as follows:  C. Implementation with IPIR - For Comparison  the time required for IPIR is shown as follows: 14

15. 多媒體網路安全實驗室  D.Performance of Our Proposal  it is obvious to see that by using BlockPIR the computation cost is reduced a lot.  the communication cost is also acceptable considering the current networking technology. 15

16. 多媒體網路安全實驗室 5. COMPARISON 16

17. 多媒體網路安全實驗室 6.CONCLUSION  We have proposed a practical keyword search scheme which performs better than the previous work which is only theoretically interesting but less of practice  A simple but effective modification to overcome this problem, which greatly enhances the performance and furthermore enables the privacy-preserving outsourcing techniques 17

18. 多媒體網路安全實驗室