Decrypt Phase
• Subscriber device decrypts the ciphertext using its
private attribute keys, if it satisfies access policy.
• In case of KP-ABE, Subscriber device verifies whether
it satisfies the access policy or not. If it satisfies, then it
requests PKG to issue corresponding key set and PKG
validates the request and sends keys to the subscriber.
• For CP-ABE, subscriber who satisfies access policy
decrypts ciphertext using its private key set and information
provided in the policy and thus scheme
enables non-interactive and offline requirement of
PKG. MQTT system works without the intervention
of PKG. Once the setup and extraction phase is
completed, requirement of PKG no longer exists. Thus
this scheme is more generic and suits the requirement
of scalability for IoT, but complexity of scheme is high
w.r.t storage and computation than that of KP-ABE.