Two faces of blindness

Abstract Blind signatures are a decades-old privacy enhancing technology. It is not always clearly understood that blind actually possess two separate properties: the intuitive understanding message to be signed hidden from signer, and fact resulting signature unlinkable (meaning signer cannot later tell in which session it created particular signature). The question is: how exactly should these properties defined, can they defined natural way such mutually independent yet together imply blindness? In this paper we study question, present formal definitions for indistinguishability unlinkability (and few more related ones), their relationships. We show indeed independent. Unfortunately union equivalent blindness what appear only pathological cases.