Ransomware

The first known case, the 1989 AIDS Trojan distributed on floppy disks, hid and encrypted file names with a symmetric scheme whose key was recoverable from the code, so it could be reversed without paying. Young and Yung's 1996 cryptovirology paper (IEEE S&P) showed the fix from the attacker's side: embed only a public key, so the victim's machine never holds anything that decrypts. CryptoLocker (2013) industrialised that design with RSA-2048 and Bitcoin payment, and almost every modern family uses the same hybrid scheme: each file (or block) is encrypted with a fresh symmetric key, typically AES or ChaCha20, and that key is wrapped with an asymmetric public key (RSA or Curve25519) and stored in the file footer or a key blob. Only the operator's private key can unwrap it, which is why recovery without the key is only possible when the implementation is flawed, as with the reused keys or weak random number generators behind many free decryptors published through the No More Ransom initiative.

Speed and reach are engineering goals. Many families use intermittent encryption - only every n-th block, or the first megabytes of each file - which destroys usability while encrypting terabytes in minutes and evades detectors that look for bulk high-entropy writes. Hypervisor targeting is now standard: encrypting VMDK files on VMware ESXi takes down every guest at once. Before detonation, operators run the recovery-sabotage steps mapped in MITRE ATT&CK as T1490 Inhibit System Recovery - deleting Volume Shadow Copies (vssadmin or WMI), deleting Windows backup catalogues, disabling recovery mode, and targeting backup servers and their credentials - followed by T1486 Data Encrypted for Impact.

The 2017 incidents mark two edge cases. WannaCry spread as a worm through the SMBv1 EternalBlue exploit patched in MS17-010; NotPetya, delivered through a compromised update of the Ukrainian M.E.Doc accounting software and devastating for Maersk among others, looked like ransomware but was effectively a wiper, since its ransom mechanism could not restore data. Since Maze in 2019, data theft before encryption has become standard, and leak sites turn the incident into a confidentiality breach as well; some groups skip encryption entirely and extort on stolen data alone.

For recovery, the practical defences follow the kill chain: immutable or offline backup copies with separate credentials, restore tests timed against the recovery objective, EDR tuned for shadow-copy deletion and mass renames, and segmentation of management planes. Paying is legally sensitive: the US Treasury's OFAC has warned since 2020 that payments to sanctioned actors can violate sanctions law, and Danish authorities, including CFCS (now under Styrelsen for Samfundssikkerhed), advise against paying. Reporting runs in parallel: NIS2 Art. 23 (24-hour early warning, 72-hour notification, final report within one month) and, where personal data is affected, GDPR Art. 33(1) to Datatilsynet within 72 hours, because loss of availability of personal data is itself a personal data breach.