Fine-Grained Access Control
(FGAC) in the Cloud
Robert Barton
Access Control Quick Review
 Fine-grained
 Why should I care?
 Why is access control necessary?
Clouds
 Shift to corporate data storage by third parties
 More cost effective
 Poses problems with data security
Issues with Cloud Storage
 Data Security
 User Revocation
 Scalability
Data Security
 It is necessary to keep the data private from the third party
 There is no clear solution to scalable FGAC but there are
many good systems to start from
Data Security:
Key Policy Attribute-Based Encryption
 Users given secret keys based on sets of attributes
 Includes one dummy attribute that every file is encrypted with
and every user has but cloud does not know about
 Files encrypted using the keys of the attributes such that a
user that has all the attributes will be able to decrypt the file
 Easy to deal with user revocation
 Easy for the cloud server to learn about users
Data Security:
Hierarchical Identity-Based Encryption
 Each user has a public key and secret key
 Secret key is made to decrypt any file encrypted using its
paired public key along with all the public keys of the user’s
ancestors
 Easy for third parties to learn about file security levels
Cloud Knowledge
 It’s safe to assume that the cloud will try to get as much
knowledge about the data it’s storing
 One proposed solution: chunks
 Each data owner has their own chunk that contains all their files
on the cloud
 Cloud doesn’t know individual file access policies
 If a user satisfies one of the access policies of the chunk he
downloads the whole chunk
Data Chunks
 Each data owner has their own chunk that contains all their files on
the cloud
 Cloud doesn’t know individual file access policies
 If a user satisfies one of the access policies of the chunk he
downloads the whole chunk
User Revocation
 Each file the user had access to needs to be re-encrypted
 Severe computational overhead on the data owner
 Two good solutions:
 Two-Layered Encryption
 Proxy Re-Encryption
 These systems have the larger resources of the cloud server
do all the work
 The only work done by the data owner is the updated key
delegation
User Revocation:
Two-Layered Encryption
 Data owner encrypts data then has the cloud encrypt a
second time
 When a user is removed the data owner has the cloud server
decrypt the second layer then re-encrypt with a different
encryption
User Revocation:
Proxy Re-Encryption
 This method has the third party re-encrypt the already
encrypted data to create a new encryption
 The third party doesn’t get to see that data decrypted so it
never learns anything
Lazy Re-Encryption
 Files are not re-encrypted until a user wants access
 Spreads out the re-encryption over time to speed up access
with the third party
Conclusion
 There is no perfect or correct solution to these problems
 It is a continuing academic and industry research area