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Guest talk: New Privacy-Preserving Proxy-Type Signature Schemes

New Privacy-Preserving Proxy-Type Signature Schemes, Christian Hanser, Graz University of Technology, 8010 Graz, Austria

In this talk, we present two new proxy type signature schemes, which, in contrast to conventional proxy signature schemes, hide the delegated message space from outsiders. In short, proxy signature schemes allow an originator to delegate signing rights for messages out of a predefined message space to a semi-trusted party, the so-called proxy. The proxy is allowed sign messages on behalf of the originator and any third party is then able to verify, whether the proxy has produced a valid signature for a message out of the intended message space. So far, however, the originator was required to publish and sign an abstract description of the delegated message space, which may not always be desireable. We give two new types of proxy signature schemes, Blank Digital Signatures and Warrant-Hiding Proxy Signatures, which address this issue by hiding messages not signed by proxies from outsiders. While the message space in case of Warrant-Hiding Proxy Signatures is an enumeration of all possible messages, the message space in case of Blank Digital Signatures is more complex. It can be seen as a form, which consists of elements with fixed content and elements that allow to select one out of several choices in order to derive a valid message.