Linkability of Two Blind Signature Schemes

Blind signature allows a user to obtain signatures from an authority on any document, in such a way that the authority learns nothing about the message that is being signed. The blindness is an important property in blind signature scheme. In this work, we analyze security of two ID-based blind signature schemes, and show that the two schemes don´t satisfy blindness, in other words, the signer is able to link a valid message-signature pair obtained by some user. It means that the two blind signature schemes are insecure