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.Comment: 32 page