What is an encryption key?

An encryption key is the secret value an algorithm uses to transform readable data (plaintext) into unreadable data (ciphertext) and back. Ransomware encrypts a victim’s files with a key the attacker controls, then offers to sell the corresponding decryption key as the ransom.

Modern ransomware typically combines symmetric encryption for speed with asymmetric encryption to protect the keys, making decryption without the attacker’s key effectively impossible. This is why a clean, recoverable backup — verified by deep file inspection — is the dependable alternative to paying.