I have the need in my app to ask a user for a username/password that will be used for an external app (my app will be using these credentials to connect to the external service and perform operations on behalf of the user that provided them). Assume they are giving me their reddit username/pass

I will be storing these in the database and was wondering what is the right way encrypt->save to db then retrieve>decrypt.

My question is specifically what algo should i be using to encrypt/key etc.