Paper 2005/261

The Cramer-Shoup Encryption Scheme is Plaintext Aware in the Standard Model

Alexander W. Dent

Abstract

In this paper we examine the security criteria for a KEM and a DEM that are su±cient for the overall hybrid encryption scheme to be plaintext-aware in the standard model. We apply this theory to the Cramer-Shoup hybrid scheme acting on ¯xed length messages and deduce that the Cramer-Shoup scheme is plaintext-aware in the standard model. This answers a previously open conjecture of Bellare and Palacio on the existence of plaintext-aware encryption schemes.

Note: The original version of this paper contained an subtle, but substantial error in the proof of the theorem that PA1 + Simulability => PA2. This theorem has been withdrawn. The main result, that Cramer-Shoup is PA2, is now proven using a slight variation of the original technique. My apologies to anyone inconvenienced by the error.