What is Harvest Now Decrypt Later?

Harvest Now, Decrypt Later (HNDL) is a strategy where an attacker collects encrypted data today and stores it, expecting future capabilities (e.g. a large quantum computer) to decrypt it later. It is a time-based risk, not a current break of encryption. The concern applies mainly to public-key cryptography (key exchange, signatures), which protects today's TLS, VPNs, and many stored secrets.