  Proxy re-encryption (PRE) securely enables the re-encryption of ciphertexts
from one key to another, without relying on trusted parties, i.e., it offers
delegation of decryption rights. PRE allows a semi-trusted third party termed
as a "proxy" to securely divert encrypted files of user A (delegator) to user B
(delegatee) without revealing any information about the underlying files to the
proxy. To eliminate the necessity of having a costly certificate verification
process, Green and Ateniese introduced an identity-based PRE (IB-PRE). The
potential applicability of IB-PRE leads to intensive research from its first
instantiation. Unfortunately, till today, there is no unidirectional IB-PRE
secure in the standard model, which can withstand quantum attack. In this
paper, we provide, for the first time, a concrete construction of
unidirectional IB-PRE which is secure in standard model based on the hardness
of learning with error problem. Our technique is to use the novel trapdoor
delegation technique of Micciancio and Peikert. The way we use trapdoor
delegation technique may prove useful for functionalities other than proxy
re-encryption as well.
