How does CryptoDefense ransomware encrypt large data with a (relatively) small RSA key?

Question

I recently encountered some news about the recently new Ransomware called CryptoDefense: it uses RSA 2048 to encrypt your valuables and offers the private key for a price. (It's already been solved since the private key is apparently cached locally)

My question however is how does this software actually encrypt large files of an arbitrary size such as photos and word documents that can be 1MB+ in file size with an RSA 2048 key? Is it actually RSA + AES or is it only using RSA 2048 by splitting files into small blocks somehow?

I've tried to find a detailed analysis of the software on Google by failed to find anything useful since almost all articles are fixated on it's insecure RSA mechanisms or bitcoin profits.

Answer 1

Generally speaking, this is called hybrid encryption. When we need to encrypt a lot of data with RSA, well, we don't; not only would this be highly inefficient, but we also don't really know how a big chunk of data should be split into small messages to be individually encrypted with RSA: from a cryptographer's point of view, this is a non-trivial issue, which is already hard enough for block ciphers (for block ciphers, this problem results in modes of operation which are already a complex subject; with RSA things are just worse).

Instead, we generate a symmetric key K, encrypt K with RSA, and encrypt the bulk of the data with K, using an algorithm which is good at that, and that means symmetric encryption (e.g. AES in a suitable mode).

If the ransomware author knows his trade (which is to be hoped, because nothing is more irksome than being attacked by an incompetent), then he used hybrid encryption.