## Cryptology ePrint Archive: Report 2007/140

Hidden Identity-Based Signatures

Aggelos Kiayias and Hong-Sheng Zhou

Abstract: This paper introduces Hidden Identity-based Signatures (Hidden-IBS), a type of digital signatures that provide mediated signer-anonymity on top of Shamir's Identity-based signatures. The motivation of our new signature primitive is to resolve an important issue with the kind of anonymity offered by group signatures'' where it is required that either the group membership list is {\em public} or that the opening authority is {\em dependent} on the group manager for its operation. Contrary to this, Hidden-IBS do not require the maintenance of a group membership list and they enable an opening authority that is totally independent of the group manager. As we argue this makes Hidden-IBS much more attractive than group signatures for a number of applications. In this paper, we provide a formal model of Hidden-IBS as well as two efficient constructions that realize the new primitive. Our elliptic curve construction that is based on the SDH/DLDH assumptions produces signatures that are merely half a Kbyte long and can be implemented very efficiently.

To demonstrate the power of the new primitive, we apply it to solve a problem of current onion-routing systems focusing on the Tor system in particular. Posting through Tor is currently blocked by sites such as Wikipedia due to the real concern that anonymous channels can be used to vandalize online content. By injecting a Hidden-IBS inside the header of an HTTP POST request and requiring the exit-policy of Tor to forward only properly signed POST requests, we demonstrate how sites like Wikipedia may allow anonymous posting while being ensured that the recovery of (say) the IP address of a vandal would be still possible through a dispute resolution system. Using our new Hidden-IBS primitive in this scenario allows to keep the listing of identities (e.g., IP addresses) of Tor users computationally hidden while maintaining an independent Opening Authority which would not have been possible with previous approaches.

Category / Keywords: cryptographic protocols /

Publication Info: Extended version for a Financial Cryptography 2007 paper

Date: received 19 Apr 2007, last revised 24 Apr 2007

Contact author: hszhou at cse uconn edu

Available format(s): PDF | BibTeX Citation

Note: Presented at FC'07 by Aggelos Kiayias; Slides are available at http://www.cse.uconn.edu/~akiayias/talks/

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