Securing Cloud Data - Cloud Computing with e-Science Applications - page 48

Sometimes, the list of users is unknown. In RBAC (introduced in [11]), users
are classified based on their individual roles. Data can be accessed by users
who have matching roles, which are defined by the system. For example, in
the case of medical records, the personal information regarding insurance
and address might be available only to the hospital staff but not to the doc-
tors and nurses. ABAC is wider in scope; users are given attributes, and the
data have an attached access policy. Only users with a valid set of attributes,
satisfying the access policy, can access the data. For instance, in the example,
medical records are accessed by only the neurologist or psychiatrist in only
one hospital but no others. Some advantages and disadvantages of RBAC
and ABAC have been discussed [22]. Most of the work in ABAC makes use of
a cryptographic primitive known as the ABE.
ABAC in clouds has been studied by several researchers [e.g., 24, 31, 32,
39, 41, 42]. Some of these focused on storage of health records [e.g., 24, 41].
Using ABE, the records are encrypted under some access policy and stored
in the cloud. Users are given sets of attributes and corresponding keys by a
key distribution center (KDC). The keys are computed using key generation
algorithms in ABE. Only when the users have a matching set of attributes
can they decrypt the information stored in the cloud.
Online social networking is yet another domain where users (members)
store their personal information, pictures, music, and videos and share
them with selected groups of users (friends/acquaintances) or commu-
nities to which they belong. All such information is stored in clouds and
given to users who satisfy matching criteria. Access control in online social
networking has been studied [18]. Most of these schemes use simple cipher-
text policy attribute-based encryption (CP-ABE) to achieve access control,
assuming that there is only one trusted KDC.
Before we discuss how access control is achieved in clouds, we briefly talk
about ABE.
3.3.1 Attribute-Based Encryption
Attribute-based encryption was proposed by Sahai and Waters [34]. In ABE,
a user is given a set of attributes by an attribute authority (AA) along with
a unique identity. Identity-based encryption (IBE), proposed by Shamir [37],
is a public key encryption technique that eliminates the need for certification
authorities and has been extensively studied. Each user in an IBE protocol
has a unique identity, and the public key is the unique information about
the user. IBE is a special case of ABE. There are two main variants of ABE.
Key-policy ABE (KP-ABE, proposed by Goyal et al. [15]), is ABE in which
the sender has attributes and encrypts data with the attributes that it has.
The receiver has access policies and receives secret keys from the AA, which
are constructed using the access policy. On receiving an encrypted message,
the receiver can decrypt if it has matching attributes. Ciphertext-policy ABE
(CP-ABE; proposed by Bethencourt et al. [4]) is the reverse of KP-ABE; the