  Ciphertext policy attribute based encryption (CP-ABE) is a technique in which
user with secret key containing attributes, only able to decrypt the message if
the attributes in the policy match with the attributes in secret key. The
existing methods that use reasonably computable decryption policies produce the
ciphertext of size at least linearly varying with the number of attributes with
additional pairing operations during encryption and decryption. In this paper,
we propose a scheme in which ciphertext remains constant in length,
irrespective of the number of attributes. Our scheme works for a threshold
case: the number of attributes in a policy must be a subset of attributes in a
secret key. The security of propose scheme is based on Decisional Bilinear
Diffie-Hellman (DBDH) problem.
