franklin.reiter0research.att.com Abstract. We introduce Verifiable Signature Sharing (VCS), a cryp-tographic primitive for protecting digital signatures. VCS enables the holder of a digitally signed document, who may or may not be the origi-nal signer, to share the signature among a set of proxies so that the honest proxies can later reconstruct it. We present efficient VCS schemes for ex-ponentiation based signatures (e.g., RSA, Rabin) and discrete log based signatures (e.g., ElGamal, Schnorr, DSA) that can tolerate the malicious (Byzantine) failure of the sharer and a constant fraction of the proxies. We also describe our implementation of these schemes and evaluate their performance. Among the applications of VXS is the incorporation of dig-ital cash into multiparty protocols, e.g., to enable cash escrow and secure distributed auctions.
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