I'm sorry, but I'm really baffled as I keep reading about this mysterious "double sha256" encryption. - Never heard of it in the context of asymmetric encryption.
To my knowledge, sha256 is a hashing algorithm, - not an encryption algorithm.
(For those not in-the-know, hashing algorithms are one-way computations by definition and purpose, and one cannot reverse the operation to get the original data from a hash.)
Yes, it does transform information and produce a hash value of that information. But in practice, there's no routine mechanism to reverse that computation, - only some very elaborate hacking may be able to do that.
So, yeah - it's kinda secure, if all you need is to compare it with another such hash to determine if the original data for the two hash values was the same (with a 99.9999% probability, as in very rare cases two very different pieces of data may actually produce the same hash).
But there are no keys ever involved in calculating sha256, therefore no private key would ever be able to reverse the operation (one can only use a salt value to obfuscate input data and prevent dictionary attacks on the hash).
And then, double-sha256 merely adds another round of computations where the first computed hash itself is hashed again using sha256.

I'd really like to get an explanation for the "double sha256 encryption" term, as that would constitute an incredible mathematical breakthrough.