Ingrid Biehl Sachar Paulus Tsuyoshi Takagi Efficient Undeniable Signature Schemes Based on Ideal Arithmetic in Quadratic Orders. 99-123 2004 31 Des. Codes Cryptography 2 http://dx.doi.org/10.1023/B:DESI.0000012439.20075.16 db/journals/dcc/dcc31.html#BiehlPT04