Contract Signing Protocols
简答：CSP的目的和作用
双方签订协议；合同对于双方已知，不进行协商（对“拍卖”不适用）；攻击者可以是第三方、或签订合同的对方。
公平交易：双方都想签订协议，但是都不想先签名；协议要保证同时成功、或同时失败。
名词解释：TTP
TTP=Trusted Third Party
It is impossible to solve strong fair exchange without a trusted third party. The proof is by relating strong fair exchange to the problem of consensus and adapting the impossibility result of Fischer, Lynch and Paterson. via
Asokan-Shoup-Waidner乐观协议
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Garay-Jakobsson-MacKenzie Abuse-Free协议
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