In this thesis we present our contribution in the field of post-quantum cryptography.
We introduce a new notion of {\em weakly Random-Self-Reducible} public-key cryptosystem and show how it can be used to implement secure Oblivious Transfer.
We also show that two recent (Post-quantum) cryptosystems can be considered as
{\em weakly Random-Self-Reducible}. We introduce a new problem called Isometric Lattice
Problem and reduce graph isomorphism and linear code
equivalence to this problem. We also show that this problem has a perfect zero-knowledge interactive proof with respect to a malicious verifier;
this is the only hard problem in lattices that is known to have this property.
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