Cold Storage is just as secure as Zoolz’s regular storage; your files undergo 3 levels of encryption; 256-AES machine level encryption, transferred via secure sockets layer, and stored on encrypted 256-AES Server Side encryption. The bottom line is that your files are processed with zero knowledge.
For more information about the security in Zoolz, click here.
And for more information about the security in Amazon Glacier, please check this article.