CS 647B Advanced Cryptography

Problem set #3

due on __Friday March 24, 2000__

- Let p be a trapdoor one-way permutation. Show how Alice and Bob can use p to evaluate secretly any two-party computation on private input A and B respectively. Don’t repeat everything we have done but
**use**the things we have done and fill in the gaps. - Consider the standard basis S made of and . We know that any pure state of a qubit can be expressed as a+b where a and b are complex numbers such that ||a||
^{2}+||b||^{2}= 1. Thus any element is specified by a pair (a,b) so that corresponds to (1,0) and to (1,0). We have also seen that the diagonal basis D made of is conjugate to the standard basis which means that if we measure or according to basis D the probabilities of each outcome are exactly 1/2. - Find yet a third basis C that is conjugate to both bases S and D.
- Find a basis B=[(u,v),(x,y)] such that measuring according to B maximizes the prediction probability for elements of bases S,D,C. More precisely, find a basis and a strategy that will have the highest success probability of issuing the correct answer when a random element of either S,D or C is given and that the basis is revealed
**after**the measurement has been done. - For each of the following situations, show how the honest Alice and Bob (who have access to an authenticated public channel) can take advantage of the insecure communication channel to exchange a secret key unknown to the eavesdropper Eve:

EXTRA POINTS: when solving this question, make sure one party has all his data statistically concealed (whereas the other one is only computationally concealed).

a)

b) Here, the probabilities b and e of transmitting a bit are arbitrary (but fixed and known to everybody) and the OTs are independent.