Proxy Signatures: Delegation of the Power to Sign Messages

In this paper a new type of digital proxy signature is proposed. The proxy signature allows a designated per-son, called a proxy signer, to sign on behalf of an original signer.Classification of the proxy signatures is shown from the point of view of the degree of delegation, and the necessary conditions of a proxy signature are clarified. The proposed proxy signature scheme is based on either the discrete logarithm problem or the problem of taking the square root modulo of a composite number. Compared to the consecutive execution of the ordinary digital signature schemes, it has a direct form, and a verifier does not need a public key of a user other than the original signer in the verification stage. Moreover, it requires less computational work than the consecutive execution of the signature schemes.Due to this efficiency together with the delegation property, an organization, e.g. a software company, can very efficiently create many signatures of its own by delegating its signing power to multiple employees. Another attractive feature is that the proxy signature based on the discrete logarithm problem is highly applicable to other ordinary signature schemes based on the same problem. For instance, designated confirmer proxy signatures can be constructed. As a stronger form of proxy signature for partial delegation, another type of proxy signature scheme is proposed in which even an original signer cannot create a proxy signature.Furthermore, using a proposed on-line proxy updating protocol,the original signer can revoke proxies of dishonest proxy signers.key words: proxy, proxy signature, proxy signer delegation, seal