Catálogo de publicaciones - libros

Undeniable signatures, introduced by Chaum and van Antwerpen, is a useful cryptography primitive to limit the publicly verifiable property of ordinary digital signatures. In an undeniable signature scheme, the validity or invalidity of the signature can only be verified via the confirmation/disavowal protocol with the help of the signer. An extended concept, convertible undeniable signatures, was introduced by Boyar, Chaum, Damgård and Pedersen. In the new concept, the signer can publish some selective proofs to convert one or more undeniable signatures into publicly verifiable ones, or issue a universal proof to make all his undeniable signatures publicly verifiable. In this paper, we first present a security model for convertible undeniable signature schemes, and then propose a new construction from bilinear pairings. Compared with the other schemes in the literature, the new construction has three advantages: Our scheme is both selectively and universally convertible; the signature length of our scheme is as short as BLS signature; meanwhile, all the security properties are formally proven under some conventional assumptions in the random oracle model.

The beauty of identity-based encryption (IBE) lies in the convenience of public key handling, in the sense that any identification such as an email address, a name, or an IP number can serve as a public key of a party. However, such convenience is not inherited by a system where a party possesses many identities (e.g., many email addresses) and has to use them as his public keys. When this system is handled with a standard IBE, the user must manage all the private keys that are associated with all the public keys (identities). However, keeping these private keys is inconvenient to the user. In this paper, we solve this problem by proposing a novel identity-based encryption where we set a private key that maps multiple public keys (identities); namely, we can use a private key to decrypt multiple ciphertexts; each was encrypted with a different public key (identity).