Integrating CP-ABE with IBRS to Enhance Security and Privacy in WBANs 427
Attribute-Based Encryption. Attribute-based encryption (ABE) was first
introduced by Sahai and Waters [18] with the aim to provide an error-tolerant IBE
that uses biometric identities. ABE can be viewed as an extension of the notion of
IBE in which user identity is generalized to a set of descriptive attributes instead
of a single string specifying the user identity. Compared with IBE, ABE has sig-
nificant advantage as it achieves flexible one-to-many encryption instead of one-
to-one, it is envisioned as a promising tool for addressing the problem of secure
and fine-grained data sharing and decentralized access control [10].
According to access policy is associated with the ciphertext or private key, ABE
can be divided into two categories: key-policy ABE (KP-ABE) and ciphertext-
policy ABE (CP-ABE).
In a KP-ABE system, ciphertexts are labeled by the sender with a set of
descriptive attributes, while users’ private key are issued by the trusted attribute
authority captures an policy that specifies which type of ciphertexts the key can
decrypt. The first KP-ABE construction was provided by Goyal et al. [10], which
was very expressive in that it allowed the access policies to be expressed by
any monotonic formula over encrypted data. The system was proved selectively
secure under the BDH assumption. Later, Ostrovsky et al. [16] proposed a KP-
ABE scheme where private keys can represent any access formula over attributes,
including non-monotone ones.
In a CP-ABE system, when a sender encrypts a message, they specify a spe-
cific access policy in terms of access policy over attributes in the ciphertext,
stating what kind of receivers will be able to decrypt the ciphertext. Users pos-
sess sets of attributes and obtain corresponding attribute private keys from the
attribute authority. Such a user can decrypt a ciphertext if his attributes satisfy
the access policy associated with the ciphertext. The first CP-ABE scheme was
proposed by Bethencourt et al. [3], but its security was proved in the generic
group model. Waters [24] proposed a more expressive and efficient CP-ABE
scheme, the size of a ciphertext depending linearly on the number of attributes
involved in the specific policy for that ciphertext.
Signcryption. Encryption and signature are two basic cryptographic primitives
to achieve confidentiality and authenticity. Zheng [27] first proposed the concept
of signcryption, which can perform digital signature and public key encryption
simultaneously in a single logical step with the cost in terms of both commu-
nication and computation significantly lower than sign-then-encrypt approach.
Beak et al. [1] first gave the formal security notions for signcryption scheme
via semantic security against adaptive chosen ciphertext attack and existential
unforgeability against adaptive chosen message attack.
Malone-Lee [15] extended the concept of signcryption to the identity-based set-
tings. Malone-Lee’s work spurred a great deal of research on identity-based sign-
cryption (IBSC), many IBSC schemesand IBSC schemeswith additional properties
have been proposed. In a conventional IBSC scheme, the message is hidden and thus
the validity of the signcrypted ciphertext can be verified only after the unsigncryp-
tion process. Thus, a third party will not be able to verify whether the signcrypted
ciphertext is valid or not. Selvi et al. [19] first proposed an IBSC scheme with public