问题 问答题 简答题

随机预言机模型与标准模型

答案

参考答案:

许多密码协议的设计都需要用到哈希函数,因此在进行安全证明时,如何对待哈希函数成为密码学中的一个重要课题。在随机预言机模型里,哈希函数被视为一个随机函数,它的输出被认为是随机的,攻击者必须询问它才能知道输出,但对相同的询问,它的输出是一致的。绝大多数的密码学方案是在随机预言机模型下证明它们的安全性,其中最经典的结果包括文献[16]等等对基于RSA的签名、加密以及基于离散对数的签名的安全性证明。而在标准模型里,哈希函数被视为一个满足单向性和抗碰撞性的确定性函数。当然,在标准模型下的密码学方案还可能根本就不需要哈希函数。已有少量的实用密码学方案在标准模型下证明了安全性,其中最经典包括文献[17]等等基于标准的DDH假设对克拉梅-舒普加密方案和基于标准的CDH假设对沃特斯签名[17]的安全性证明。

阅读理解

                                                        The Cost of Higher Education

     Individuals(个人) should pay for their higher education.

     A university education is of huge and direct benefit to the individual. Graduates eam more than

non-graduates.Meanwhile,social mobility is ever more dependent on having a degree. However,only

some people have it. So the imli-

vidual,not the taxpayers,should pay for it. There are pressing calls on the resources(资源) of the

government.  Using taxpayers' money to help a small number of people to earn high incomes in the future

is not one of them.

     Full govemment funding(资助) is not very good for universities. Adam Smith worked in a Scottish

university whose teachers lived off student fees.  He knew and looked down upon 18th-century Oxford,

where the academics lived comfortably off the income received from the govemment.  Guaranteed salaries,Smith argued,were the enemy of hard work;and when the academics were lazy and incompetent,the

students were similarly lazy.

      If students have to pay for their education,they not only work harder,but also demand more from their

teachers. And their teachers have to keep them satisfied.  If that means taking teachjng seriously,and giving less time to their own research interests,that is surely something to celebrate.

         Many people believe that higher education should be free because it is good for the economy (经济) .  Many graduates clearly do contribute to national wealth,but so do all the businesses that invest (投资) and create jobs.  If you believe that the government should pay for higher education because

graduates are economically productive,you should also believe that the government should pay part of

business costs. Anyone promising to create jobs should receive a gift of capital from the govemment to

invest. Therefore,it is the individual,not the govemment,who should pay for their university education.

1. The underlined word "them" in Paragraph 2 refers to _____.

A. taxpayers                                  

B. pressing calls

C. college graduates                          

D. govemment resources

2. The author thinks that with full govemment funding _____.

A. teachers are less satisfied                  

B. students are more demanding

C. students will become more competent  

D. teachers will spend less time on teaching

3. The author mentions businesses in Paragraph 5 in order to____.

A. argue against free university education    

B. call on them to finance students' studies

C. encourage graduates to go into business        

D. show their contribution to higher education

判断题