Secure API, in-transit Data, data not in transit.

I have basically 3 requirement for implementing security in project which includes (iOS & Android, Api developed in Laravel PHP ) 1. Secure API: My requirement to implement security in the the API which I have for my App with some random and unique key in each call which will be validated at server end. App will pass unique key in each call with some predefined logic and same logic will be implemented at server end to validate the key. The logic should be unique and should be tempered proof. (we can further discuss on the logic if required). 2. Encrypt/Decrypt data which is in-transit: The data which is in transit should be encrypted at source and decrypted destination. The source can be app (android and iOS) and destination can be server(php). It can be vice-versa as well where source is server(php) and destination can be app (android and iOS). The app and server are already existing, just need to encrypt the data which is passed. The data will be string or image. 3. Encrypt/Decrypt Data not in transit: Need to secure data which is not in-transit. Here the requirement is to encrypt the data at the time of storing. Here the data will be stored by one user who will use its private key to encrypt the data and then store it db, the data will be accessed by the white listed users only who will have the public key to decrypt the data. Here solution should provide option to generate unique private and public key cater (iOS, Android, PHP) We can discuss on the algorithm to be used. The solution should be algorithm agnostic and meet industry standard. The solution should be design to cater (Android, iOS & Laravel PHP). The requirement would be to implement the solution without making much change in the existing code. which mean I should be able to call function to encrypt/decrypt data(String, image), generate public/private key, generate/validate random key. Please submit the your understanding in summary while bidding for this project.