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security fundamentals for e commerce phần 4

theo cách sau. Thay vì s (ngẫu nhiên và được lựa chọn bởi người giám hộ), s0 + s1 được sử dụng. s0 được chọn bởi ví, và s1 người giám hộ. Cụ thể, nó có nghĩa là trong giao thức trên các giá trị sau được sử dụng:Nó được thực hiện theo cách như vậy mà chỉ có người giám hộ biết. | 108 Security Fundamentals for E-Commerce in the following way. Instead of s random and chosen by the guardian S0 S1 is used. S0 is chosen by the purse and S1 by the guardian. Specifically it means that in the protocol above the following values are used a gs 0 S1 mod p b m 0 S1 mod p r s 0 s 1 cx modq It is done in such a way that only the guardian knows s. This type of signature is sometimes referred as the randomized signature. An additional difficulty is that it needs to be impossible to trace the wallet on the basis of the guardian s identity. Therefore a mechanism is needed to obscure the origin of the guardian s signature key. One such mechanism was proposed in 7 . 7.2.4.2 Issuer s Signature To provide payment untraceability the signature on the coin that the purse obtains from the issuer must be blind similarly to that described in Section 7.1.1 . The purse must blind both the message and the challenge from the basic signature described earlier. The protocol steps are then as follows 7 1. Verifier Signer 2. Signer Verifier 3. Verifier Signer 4. Signer Verifier m 0 m1 mod p t random 0 t q a0 gs mod p b0m0s mod p s random 0 s q blinded challenge c0 c umodq u random 0 u q blinded response r0 s c0 x modq The unblinded signature of m is ơ m z a b r it can be computed by the verifier but not by the signer Team-Fly Digital Money Security 109 z z 10t mod p a a0gv modp b bb1 m mod p r r0 v V u mod p After Step 2 the verifier chooses u and v randomly 0 u q 0 v q so it can compute a and b but the signer cannot since it does not know u and v. Similarly after Step 3 the verifier can unblind the response r. As in the basic protocol c H m z a b which can be computed by the verifier only. On the other hand if the purse generates electronic coins and wants to obtain a blind signature on the coins from the issuer the issuer wants to be sure that the guardian has agreed to the coins. To demonstrate its agreement the guardian signs the blinded challenge C0 by using the .