Ransomware is software that denies you access to your files until you pay a ransom.
In the past, malicious software typically corrupted or deleted data, but now it can hold your data hostage instead. For example, the Archiveus Trojan copies the contents of the My Documents folder into a password-protected file and then deletes the original files. It leaves a message telling you that you require a 30-character password to access the folder, and that you will be sent the password if you make purchases from an online pharmacy.
In that case, as in most ransomware so far, the password or key is concealed inside the Trojan’s code and can be retrieved by malware analysts. However, in the future, hackers could use asymmetric or public-key encryption (which uses one key to encrypt data, but another to decrypt it) so that the password would not be stored on your computer.
In some cases, the threat to deny access is sufficient. For example, the Ransom-A Trojan threatens to delete a file every 30 minutes until you pay for an unlock code via Western Union. If you enter an incorrect unlock code, the Trojan warns that the computer will crash after three days. However, the threats are a bluff, as Ransom-A is not capable of doing these things.