Definition
Today's key exchanges, such as X25519 and RSA, could be broken by a large enough quantum computer. ML-KEM (Module-Lattice-Based Key-Encapsulation Mechanism) relies on lattice problems that are believed to resist both classical and quantum attacks.
How it works
Instead of both sides contributing to a Diffie-Hellman exchange, one side uses the other's public key to 'encapsulate' a random secret, which only the private key can recover. VPNs and browsers usually deploy it in hybrid mode, combining ML-KEM with X25519, so the connection stays secure if either algorithm holds.
Why it matters
The worry is 'harvest now, decrypt later': traffic recorded today could be decrypted years from now. Several VPN providers have announced hybrid post-quantum handshakes; ML-KEM is the name to look for in their documentation.