> The ransomware developers could use public-key cryptography by generating a local session key to encrypt the victim's files, and then encrypting a copy of that key with the developers' public key. The paid ransomware service would then consist of decrypting that particular session key on request.
The private-key to generate the session-key has to exist somewhere -- you don't want it anywhere it can be traced back to you, so it would probably be on some automated control server - which tracks the payment addresses and forwards the decryption key on bitcoin payment receipt.
My guess best is that some of the keys got recovered in the C&C Server take-downs.
It would be possible to use bitmessage or another non-ip traceable method of communication to pass decryption keys and and keep the structure separate from the C&C servers.
The private-key to generate the session-key has to exist somewhere -- you don't want it anywhere it can be traced back to you, so it would probably be on some automated control server - which tracks the payment addresses and forwards the decryption key on bitcoin payment receipt.
My guess best is that some of the keys got recovered in the C&C Server take-downs. It would be possible to use bitmessage or another non-ip traceable method of communication to pass decryption keys and and keep the structure separate from the C&C servers.