Abstract
A Proxy re-encryption (PRE) scheme is a relatively new cryptosystem. It allows a third party proxy to transform a cipher text under Alice's private key into another cipher text which Bob can decrypt. This cryptosystem has many useful applications in modern information technology. Shao[3] proposed a unidirectional single-use PRE and claimed its CCA security in the random oracle model. But there many argue on this scheme. We launch a chosen-cipher text attacks (CCA) on the first level cipher text to show that the scheme fails to CCA security, but is CPA security on the first level cipher text. Then we improved Shao's scheme by adding some salt to the scheme to satisfy CCA security in the random oracle model.
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