1. 2011 IEEE TrustCom-11 Sushmita Ruj Amiya Nayak and Ivan Stojmenovic 2013.04.29 Regular Seminar Tae Hoon Kim

2. /21 Contents 1. Introduction 2. Related work 3. Background A. Model and assumptions B. Formats of access policies C. Mathmatical background D. Lewko-Waters ABE Scheme 4. Proposed scheme: DACC 5. Security of DACC 6. Performance 7. Conclusion and Future work 2

3. /21 Introduction  Cloud is emerging technology  User only require a terminal, a smart phone, table connected internet  User can rent the storage, computing resources of a server provided by a company  Application services  e.g) Google Apps, Microsoft online  Infrastructural support  e.g)Amazon’s EC2, Eucalyptus, Nimbus  Platform  e.g)Amazon’s S3, Windows Azure 3

4. /21 Introduction  Two important to preserve  security of data  In cloud, Users trying to access data  privacy of users  Only services are authorized user  Using public key cryptographic techniques  User should ensure that  The cloud is not tampering with user’s data and computational results 4

5. /21 Introduction  Method of not tampering user data & computational results  Hide the data from the clouds  Use of homomorphic encryption techniques[9][10]  Code obfuscation[11]  Important problem : Anonymously search data in Cloud(range, multidimensional, complex queries)  Is achieved searchable encryption[12][13]  Takes keywords in encrypted form and return results Without knowing the keyword or the retrieved records 5

6. /21 Introduction  Consider following scenarios  Patients store their medical records in cloud  Different users can access different data field  The same data fields might be accessed by a selective group of people(authorized set)  Example)the patient’s medical history and drug administration can be accessed by doctors and nurses, but not by hospital management staff  Access Control : Granting access rights to certain users and forbidding other users to access data 6

7. /21 Introduction  Method of granting access  To attach list of all valid users to data  problem : 1. Each time the list has to be checked to see if user is valid -> huge computation and storage cost  To encrypt data is by using public keys of valid users  Problem : 1. The same data then must be encrypted several times -> may result in huge storage cost  ABE(Attribute Based Encryption) : owners encrypted data with attributes that they possess and store the information in the clouds 7

8. /21 Introduction  KDC(Key Distributed Center)  Users are given attributes and secret keys by KDC  Set of attributes are able to decrypt the information  E.g)(A, doctor, Key), (B, doctor, Key), (A, staff, Key) (A, patient, Key) (C, doctor, Key)  Earlier work[17] : owners encrypt data with attributes(Not requried KDC)  Problem : increase the total number of secret keys given to users- >increases storage and communication overhead  In this paper, proposes DACC is  Cloud has cipher text(unable to decrypt cipher text)  Owner decide on attributes that users should have and users receive decryption keys  KDC distributes secret keys to user 8

9. /21 Related work  Sahai and Waters[19] ABE  A user has a set of attributes in addition to its unique ID  Shamir[20] IBE(Identity-based-encryption)  Each user in IBE scheme has a unique identity  Bethencourt et al,. [21] Ciphertext-policy  Receiver has the access policy in the form of a tree, with attributes as leaves and monotonic access structure with AND, OR and other threshold gates. 9

10. /21 Related work  KDC(attribute authority) is assumed to be honest  May not hold; a distributed system, authorities can fail or be corrupt  Chase[23] multi-authority ABE  Distribute attribute, and secret keys to users  Wang et al[26],. Hierarchical access control mechanism  Relies on Bethencoure et al[21], and Hierarchical IBE[27] 10

11. /21 Background A. Model and assumptions  KDCs which may be even servers scattered in different countries, that generate secret keys for the users  Assume that the cloud is honest but curious 11

12. /21 Background : B. Format of access policies 12

13. /21 Background : C. D.  C. Mathematical background  Use bilinear pairings on elliptic curves  D. Lewko-Waters ABE scheme  Consist of four steps  1)System Initialization  2)Key and attribute distribution to users by KDCs  3)Encryption of message by sender  4)Decryption by receiver 13

14. /21 Proposed Scheme:DACC  A. Sketch of DACC  1. Initially the parameters of the scheme and the size of group are decided(2 32 +1)  2. KDC A j selects the set of attributes L j  3. An owner U u who wants to store information in cloud, chooses a set of attributes I u which are specific to the data it wants to encrypt’  4. convert the access tree to a Matrix R 14 A j = j 번째 KDC, L j = KDC A j 가 소유한 attribute 의 잡합 I [j,u] U u 가 준 attribute 의 집합

15. /21 Proposed Scheme:DACC  An Example; professional n/w  J1:Engineering, J2(CS Research), J3(Faculty positions)  P1:Canada P2:US 15

16. /21 Security of DACC, Performance  Security of DACC  Show that only authorized users can decrypt the data in clouds  Theorem : Our access control scheme is secure, collusion resistant and allows access only to authorized users  Proof  Performance  Calculate the computation and communication overhead of DACC scheme and DACC with revocation 16

17. /21 Conclusion  In DACC, the cloud is assumed to be honest  If not possible to satisfy, care should be taken  The authenticity of the data must be verified by the user  hide the identity of the users and owners at the same time provide their authentication  Future work  Hide the access structure from the cloud, by scrambling the matrix in some way 17

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19. /21 Appendix : Method  Method 1 : One way to hide the data from clouds  Method 2 : Use of homomorphic encryption techniques(4 generation) data Sends msg : Homomorphicly encrypted data Return result : Return encrypted data http://www2.readersnews.com/sub_read.html?uid=34467&section=sc1 19

20. /21 Appendix : Method  Method 3 : Code obfuscation  Method 4 : Searchable encryption : Obfuscation Code : Return data Provide result encrypted data 20

21. /21 Appendix : DACC KDC 2 posses KDC of list (HospitalA, doctor, SK 2,1), (HospitalB, doctor, SK 2,2), (HospitalA, Staff, SK2,3) A2A2 A 2 of attribute set L j Owner UuUu Access tree User n1 User n2 IuIu User n1 User n2 Matrix R SSH:Secure Shell protocol : Using SSH 1 1 2 2 3 3 Cyper text Matrix R C1C2 21