Daniel V. Bailey Dan Boneh Eu-Jin Goh Ari Juels Covert channels in privacy-preserving identification systems. 297-306 2007 ACM Conference on Computer and Communications Security http://doi.acm.org/10.1145/1315245.1315283 conf/ccs/2007 db/conf/ccs/ccs2007.html#BaileyBGJ07