Blank digital signatures

Abstract

In this paper we present a novel type of digital signatures, which we call blank digital signatures. The basic idea behind this scheme is that an originator can define and sign a message template, describing fixed parts of a message as well as multiple choices for exchangeable parts of a message. One may think of a form with blank fields, where for such fields the originator specifies all the allowed strings to choose from. Then, a proxy is given the power to sign an instantiation of the template signed by the originator by using some secret information. By an instantiation, the proxy commits to one allowed choice per blank field in the template. The resulting message signature can be publicly verified under the originator's and the proxy's signature verification keys. Thereby, no verifying party except the originator and the proxy learn anything about the "unused" choices from the message template given a message signature. Consequently, the template is hidden from verifiers.