Encrypting ransomware reappeared in September 2013 with a Trojan known as CryptoLocker and used to encrypt files using a whitelist of specific file extensions .
CryptoLocker generated a 2048 - bit RSA key pair and uploaded in turn to a command - and - control server .
Encrypting ransomware reappeared in September 2013 with a Trojan known as CryptoLocker , which generated a 2048 - bit RSA key pair and uploaded in turn to a command - and - control server , and used to encrypt files using a whitelist of specific file extensions .