Towards Black-Box Accountable Authority IBE with Short Ciphertexts and
Private Keys

B. Libert and D. Vergnaud

At Crypto'07, Goyal introduced the concept of Accountable Authority
Identity-Based Encryption as a convenient tool to reduce the amount of trust
in authorities in Identity-Based Encryption. In this model, if the Private
Key Generator (PKG) maliciously re-distributes users' decryption keys, it
runs the risk of being caught and prosecuted. Goyal proposed two
constructions : the first one is efficient but can only trace well-formed
decryption keys to their source ; the second one allows tracing obfuscated
decryption boxes in a model (called weak black-box model) where cheating
authorities have no decryption oracle. The latter scheme is unfortunately far
less efficient in terms of decryption cost and ciphertext size. In this work,
we propose a new construction that combines the efficiency of Goyal's first
proposal with a very simple weak black-box tracing mechanism. Our scheme is
described in the selective-ID model but readily extends to meet all security
properties in the adaptive-ID sense, which is not known to be true for prior
black-box schemes.