Alice and Bob are two data scientists from different parts of the world. To track terrorist operations, they're working on sensitive government data collections. They don't want to communicate their data sets and analysis findings via existing chat programs or email services. They want to use a secure communication platform to distribute their content instead of existing options.

Task: Propose a framework for them to accomplish their goal, based on your expertise in encryption and decryption methods.

i. Explain to them how to construct a secure infrastructure using a diagram. Cryptographic algorithms and network access control techniques should be taken into consideration.

ii. Discuss the challenges that would be encountered in the key exchange process. Suggest methods to overcome challenges in such environments by employing a Certificate Authority(CA)