On the Security of a Multi-use CCA-Secure Proxy Re-encryption Scheme

In proxy re-encryption (PRE), a semi-trusted proxy can convert a cipher text originally intended for Alice into one which can be decrypted by Bob, while the proxy can not know the corresponding plaintext. PREcan be classified as single-use PRE and multi-use PRE according to the times the cipher text can be transformed. In multi-use PRE schemes, the cipher text can be transformed from A to B and to C and so on. In CCS´09 (post session), Wan get al. proposed a multi-use unidirectional CCA-secure proxy re-encryption scheme. Unfortunately, we show their proposal is not CCA-secure in the corresponding security models by giving concrete attacks.