## Cryptology ePrint Archive: Report 2015/841

An Efficient CP-ABE with Constant Size Secret Keys using ECC for Lightweight Devices

Vanga Odelu and Ashok Kumar Das and Adrijit Goswami

Abstract: The energy cost of asymmetric cryptography is a vital component of modern secure communications, which inhibits its wide spread adoption within the ultra-low energy regimes such as Implantable Medical Devices (IMDs) and Radio Frequency Identification (RFID) tags. The ciphertext-policy attribute-based encryption (CP-ABE) is a promising cryptographic tool, where an encryptor can decide the access policy that who can decrypt the data. Thus, the data will be protected from the unauthorized users. However, most of the existing CP-ABE schemes require huge storage and computational overheads. Moreover, CP-ABE schemes based on bilinear map loose the high efficiency over the elliptic curve cryptography because of the requirement of the security parameters of larger size. These drawbacks prevent the use of ultra-low energy devices in practice. In this paper, we aim to propose a novel expressive AND-gate access structured CP-ABE scheme with constant-size secret keys (CSSK) with the cost efficient solutions for the encryption and decryption using ECC, called the CP-ABE-CSSK scheme. In the proposed CP-ABE-CSSK, the size of secret key is as small as 320 bits. In addition, ECC is efficient and more suitable for the lightweight devices as compared to the bilinear pairing based cryptosystem. Thus, the proposed CP-ABE-CSSK scheme provides the low computation and storage overheads with an expressive AND-gate access structure as compared to the related existing schemes in the literature. As a result, our scheme is very suitable for CP-ABE key storage and computation cost in the ultra-low energy devices.

Category / Keywords: public-key cryptography / Attribute-based encryption, ciphertext-policy, constant-size secret key, elliptic curve cryptography

Note: In this paper, we have proposed a novel ECC-based CP-ABE-CSSK scheme with the constant size secret keys with an expressive AND gate access structure without using bilinear maps. To the best of our knowledge, it is the first ECC-based CP-ABE scheme. In addition, the proposed CP-ABE-CSSK offers the constant size secret keys, which is as small as 320-bits for the $80$-bit security. The CP-ABE-CSSK also significantly reduces the encryption and decryption costs as compared to the related existing schemes in the literature.