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1. Access Control in Decentralized
Online Social Networks: Applying a
Policy-Hiding Cryptographic
Scheme and Evaluating Its
Performance
Oleksandr Bodriagov, Gunnar Kreitz, and Sonja Buchegger
KTH Royal Institute of Technology, School of Computer Science and Communication
Stockholm, Sweden

2. Abstract
Privacy concerns in online social networking services have prompted a number of
proposals for decentralized online social networks (DOSN) that remove the central
provider and aim at giving the users control over their data and who can access it.
This is usually done by cryptographic means. Existing DOSNs use cryptographic
primitives that hide the data but reveal the access policies. At the same time, there
are privacy-preserving variants of these cryptographic primitives that do not reveal
access policies. They are, however, not suitable for usage in the DOSN context
because of performance or storage constraints.
A DOSN needs to achieve both privacy and performance to be useful.
We analyze predicate encryption (PE) and adapt it to the DOSN context. We
propose a univariate polynomial construction for access policies in PE that
drastically increases performance of the scheme but leaks some part of the access
policy to users with access rights. We utilize Bloom filters as a means of decreasing
decryption time and indicate objects that can be decrypted by a particular user.
We evaluate the performance of the adapted scheme in the concrete
scenario of a news feed. Our PE scheme is best suited for encrypting for groups or

3. Existing System
Existing DOSNs use cryptographic primitives that hide the data but reveal the access
policies. At the same time, there are privacy-preserving variants of these
cryptographic primitives that do not reveal access policies. They are, however, not
suitable for usage in the DOSN context because of performance or storage
constraints.

4. Disadvantages of Existing System
 A drawback of the existing system is that the standard implementations reveal the
access policy specified together with each ciphertext (as the access policy is
needed for decryption).
 Efficiency is lost because of the quadratic growth of the ciphertext size in the
number of attributes.
 Standard implementations reveal the access policy specified together with each
ciphertext (as the access policy is needed for decryption).
 Computational efficiency and storage efficiency are crucial for DOSNs which are
characterized by a large number of users and objects, and by the absence of a
centralized storage.

5. Proposed System
Proposed system is a Predicate Encryption (PE) is a cryptographic primitive that
provides access control of encrypted data using attribute based policies. When
creating a ciphertext, the encrypt or specifies an access policy and only those users
whose keys satisfy the policy can decrypt. The decryption keys are generated by the
encrypt or using a master secret.

6. Advantages of Proposed System
 Provides access control of encrypted data using attribute based policies.
 The proposed system employs a univariate polynomial construction for access
policies that has a short vector and thus drastically increases performance of the
scheme.

7. Architecture

8. Modules
 System Initialization
 User Registration
 KDC setup
 Attribute generation
 Sign
 Verify
 Bloom filters

9. Modules Description
System Initialization
Select a prime q, and groups G1 and G2, which are of order q. We define the mapping
ˆe : G1 ×G1 → G2. Let g1, g2 be generators of G1 and hj be generators of G2, for j ∈
[tmax], for arbitrary tmax. Let H be a hash function. Let A0 = ha0 0 , where a0 ∈ Z∗ q
is chosen at random. (TSig,TV er) mean TSig is the private key with which a message
is signed and TV er is the public key used for verification. The secret key for the
trustee is TSK = (a0, TSig) and public key is TPK = (G1,G2,H, g1,A0, h0, h1, . . . ,
htmax, g2, TV er).

10.  User Registration
For a user with identity Uu the KDC draws at random Kbase ∈ G.Let K0 = K1/a0 base
. The following token γ is output γ = (u,Kbase,K0, ρ), where ρ is signature on
u||Kbase using the signing key TSig.

11.  KDC setup
We emphasize that clouds should take a decentralized approach while distributing
secret keys and attributes to users. It is also quite natural for clouds to have many
KDCs in different locations in the world. The architecture is decentralized, meaning
that there can be several KDCs for key management.

12.  Attribute generation
The token verification algorithm verifies the signature contained in γ using the
signature verification key TV er in TPK. This algorithm extracts Kbase from γ using (a,
b) from ASK[i] and computes Kx = K1/(a+bx) base , x ∈ J[i, u]. The key Kx can be
checked for consistency using algorithm ABS.KeyCheck(TPK,APK[i], γ,Kx), which
checks ˆe(Kx,AijBx ij) = ˆe(Kbase, hj), for all x ∈ J[i, u] and j ∈ [tmax].

13.  Sign
The access policy decides who can access the data stored in the cloud. The creator
decides on a claim policy Y, to prove her authenticity and signs the message under
this claim. The ciphertext C with signature is c, and is sent to the cloud. The cloud
verifies the signature and stores the ciphertext C. When a reader wants to read, the
cloud sends C. If the user has attributes matching with access policy, it can decrypt
and get back original message.

14.  Verify
The verification process to the cloud, it relieves the individual users from time
consuming verifications. When a reader wants to read some data stored in the cloud,
it tries to decrypt it using the secret keys it receives from the KDCs.

15.  Bloom filters
A profile in the DOSN contains multiple objects encrypted for different users. It is
impossible for a user to determine if an object is encrypted for him without trying to
decrypt it since the ciphertexts do not reveal access policies. The user could use a
trial-and-error approach (sequentially trying to decrypt objects) for rendering the
profile, but this becomes prohibitively expensive with the large number of objects.
Therefore, we utilize Bloom filters to speed up rendering and to show users in a
privacy-preserving manner whether they can decrypt objects.

16. Conclusion
We have proposed to apply a privacy preserving scheme to the DOSN context: inner-product predicate
encryption (PE). It is too expensive to use out of the box. Therefore for PE we proposed a construction
for access policies that drastically increases performance, but introduces some trade-offs: it allows
encrypting for a bounded set of groups/users; this bound is a trade-off between efficiency and
functionality of the scheme; the number of groups in the system is unlimited; a user has 2g different
decryption keys, where g is the number of groups a user is a member of; having multiple keys leaks
some information about access policies. PE is most suitable for encrypting for groups or small sets of
separate identities. We designed an experiment that showed that for newsfeed assembly from all
friends our scheme shows good performance and thus user experience. For schemes that do not
reveal access policies and have relatively slow decryption, we proposed to use Bloom filters to indicate
to users which files they can decrypt. Bloom filters are both performant and space-efficient, and thus
are suitable for DOSNs.
In this paper, we focused the evaluation on performance to see if PE is even feasible under the
constraints of decentralized online social networks, starting from the security and privacy properties of
the original scheme. The next steps are to focus on security and privacy, as well as semantics of
access policies of our modifications.

17. Minimum Hardware Configuration of the
proposed system
 Processor : Intel/AMD
 Speed : 1.1 GHz
 RAM : 256 MB
 Hard Disk : 20 GB
 Keyboard : Standard Keyboard
 Mouse : Standard Mouse
 Monitor : SVGA/LCD

18. Software Configuration of the proposed
system
 Operating System : Windows
 Java Version : JDK 1.7/1.8
 Application Server : Tomcat 6/7
 Front End : HTML, Java, JSP, CSS
 Scripts : JavaScript
 Database : MySQL 5.5
 Database Connectivity : JDBC

19. References
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