In this paper, we investigate an interesting and practical cryptographic construct - Oblivious Signature-Based Envelopes (OSBEs) - recently introduced in [15]. OSBEs allow a sender to communicate information to a receiver such that the latter's rights (or roles) are unknown to the former. At the same time, a receiver can obtain the information only if it is authorized to access it. This makes OSBEs a natural fit for anonymity-oriented and privacy-preserving applications. Previous results yielded three OSBE constructs: one based on RSA and two based on Identity-Based Encryption (IBE). Our work focuses on the ElGamal signature family: we succeed in constructing practical and secure OSBE schemes for several well-known signature schemes, including: Schnorr, Nyberg-Rueppel, ElGamal and DSA. As illustrated by experiments with a prototype implementation, our schemes are more efficient than previous techniques. Furthermore, we show that some OSBE schemes, despite offering affiliation privacy for the receiver, result in no additional cost over schemes that do not offer this feature.
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