Problem is there is too much to know for any one person to learn it all. I've never been asked to encrypt a file in my life (programmer for 15 years), because no single company that I have worked for has cared that much about securing files being sent out. (the ones that did, just sent password protected zip files). Typically the things that need to be secure are behind firewalls, or the encryption is handled for you by other systems you are using.
Now, how do you find a good developer? Those are hard to define intangibles.
Being a quick study is one. I don't know how to do that today, but give me a day and I'll tell you all about it. I have enough of the background to learn the detail quickly and retain them. So, if the developer in question doesn't know the particular information you consider critical, ask them where they will go to figure it out.
Writing good code. That is really hard, but basically they can write code that they can easily understand and modify over time with minimal errors. That is the heart of the craft.