Suppose you encrypt very sensitive and valuable data with the strongest available crypto. Would it be safe to store the encrypted data in public, for example in a blog post or a public github repo?
It depends. Here are the questions that came to my mind in under 30 seconds:<p>* For how long must it be secure?<p>* Against whom must it be secured?<p>* What are your potential losses if it's broken?<p>* How can you be sure you've used a secure implementation?<p>* What do you think the "strongest available crypto" currently is?<p>* Can you be sure that the key will never be compromised?<p>* Where will you store the key?<p>* Do you need to send the key to someone else?<p>* Where will the encryption happen?<p>... and there's more.
My understanding is that NSA considers classified data that has been encrypted using a suitable cipher/key to be unclassified and suitable for transmission on unsecure systems/networks.