Decryption turns ciphertext back into readable data. It reverses encryption using the correct key. Without the key, decryption is computationally infeasible for modern algorithms. With the key, it is fast. The process is invisible in most systems. Your browser decrypts HTTPS traffic automatically. Your phone decrypts stored messages when you unlock it. Disk encryption decrypts files on the fly as you open them.

Decryption is where encryption systems often fail. The algorithm is usually strong. The key management is not. Keys get stored in plaintext, shared over insecure channels, or reused across systems. Ransomware uses encryption to lock files and demands payment for the decryption key. Some victims pay and never receive the key. Others pay and get a key that does not work. Law enforcement sometimes breaks encryption through legal compulsion, exploiting implementation flaws, or brute force against weak passwords. The FBI versus Apple case in 2016 ended without a court ruling because a third party unlocked the phone. Decryption is a technical process, but the politics around it are messy.

Decryption in practice

• Symmetric — same key encrypts and decrypts, fast
• Asymmetric — private key decrypts what the public key encrypted
• Key management — the hardest part of any encryption system
• Brute force — trying every key, impractical for strong algorithms

If you lose the decryption key, the data is gone. Encryption without a key backup is a data loss plan.