Mihir Bellare Sarah Shoup Two-Tier Signatures, Strongly Unforgeable Signatures, and Fiat-Shamir Without Random Oracles. 201-216 2007 conf/pkc/2007 Public Key Cryptography http://dx.doi.org/10.1007/978-3-540-71677-8_14 db/conf/pkc/pkc2007.html#BellareS07