CryptoLocker is a trojan which targets computers running Microsoft Windows. It may come in several forms, often disguised as a legitimate email attachment. Or, it may be uploaded to a computer already recruited to a botnet during a previous trojan infection.
When activated, CryptoLocker encrypts files (e.g., photos and those with Microsoft Office, OpenDocument, and AutoCAD file extensions) using RSA public key cryptography and with the private key stored only CryptoLocker’s control servers. This renders the files inaccessible to the user.