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Hình 9,4 cho thấy hoạt động hiện tại và định hướng tương lai trong khu vực dịch vụ web không dây, cùng với các hoạt động trong lĩnh vực dịch vụ web. Phân chia các hoạt động trong các dịch vụ web không dây vào hai thế hệ: kỷ nguyên Dấu chân nhỏ, và kỷ nguyên các Ubiquitous. | CRYPTOGRAPHIC ALGORITHMS AND PROTOCOLS FOR XG 305 Suppose that the jth person wants to construct a ring signature on the message M. In this case he knows all the public keys n1 e1 n2 e2 . nr er but he only knows his own private key pj qj dj . The signing process works as follows. He first picks values si at random for 1 i r i j. For each such si he sets mi s ee mod ni. Next he computes T m1 . mj_1 mj 1 . mr and he sets mj T M. He next uses his signing exponent dj to sign mj by computing Sj mdj mod n. The ring signature on M consists of S1 . sr .To check the validity of the signature the verifier checks that M mi . mr where mi sei mod n for 1 i r. The verifier cannot determine which signing key the signer used and so his identity is hidden. However one can show that only someone with knowledge of one of signing exponents di could have signed assuming that the RSA signature scheme is secure . Such a proof is beyond our scope. Ring signatures have two noteworthy properties 1. The verifier must know the public verification keys of each ring member. 2. Once the signature is issued it is impossible for anyone no matter how powerful to determine the original signer that is there is no anonymity escrow capability. Another related property is that a ring signature requires the signer to specify the ring members and hence the number of bits he transmits may be linear in the ring size. One can imagine that in certain settings these properties may not always be desirable. Group signatures which predate ring signatures are a related cryptographic construct that address these issues. Naturally we stress that there are situations in which a ring signature is preferable to a group signature. Group signature schemes allow members of a given group to digitally sign a document as a member of - or on behalf of - the entire collective. Signature verification can be done with respect to a single group public key. Furthermore given a message together with its signature only a designated
