Two faces of blindness

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