Adding encryption layer in existing application architechture

I have asked to add data encryption and decryption for existing data in database.

right now i am using .dbml file for data access.

My problem is that where i will implement encryption layer so that i dont need to change much part of app as my application has many ways to read and write data.

I want to minimize the effort and make it centrally the things.

Any help and idea would be appreciated.