@inproceedings{DRR-2006-BentleyM,
	author        = "Jon Louis Bentley and Colin L. Mallows",
	booktitle     = "{Proceedings of the 13th Conference on Document Recognition and Retrieval}",
	doi           = "10.1117/12.650644",
	isbn          = "978-0-8194-6107-0",
	publisher     = "{SPIE}",
	title         = "{CAPTCHA challenge strings: problems and improvements}",
	year          = 2006,
}