Jan Camenisch Victor Shoup Practical Verifiable Encryption and Decryption of Discrete Logarithms. 126-144 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2729&spage=126 2003 conf/crypto/2003 CRYPTO db/conf/crypto/crypto2003.html#CamenischS03