This is a defense against potential attacks from quantum computing powerful enough to break certain cryptographic algorithms. In general, symmetric ciphers such as AES-GCM and Chacha20-Poly1305 that are used for data-channel encryption, are still safe to use. No change is necessary in this area. The cryptographic algorithms that are affected are the asymmetric algorithms. These are commonly used to establish the symmetric encryption keys for data encryption.

OpenVPN has two channels for communication - the control channel and the data channel. The control channel is where authentication and key agreement occurs, whereas the data channel transports the encrypted payload. The tls-crypt and tls-crypt-v2 options work by using pre-shared symmetric keys to encrypt the whole control channel and the TLS handshake that occurs there. In essence symmetric keys protect the asymmetric part of the process. Often this kind of protection is called "poor man's post-quantum cryptography".

provides "poor-man's" post-quantum security, against attackers who will never know the pre-shared key (i.e. no forward secrecy).

The key agreement is what TLS server and client use to agree on a common secret that will be used to derive all encryption keys for the connection. Typically this should be done in a way that ensures the common secret is not recoverable later, even if the secrets of the client and/or server are at some point compromised. This principle is called [Perfect Forward Secrecy](https://de.wikipedia.org/wiki/Perfect_Forward_Secrecy) and is typically achieved by using finite field Diffie-Hellman (see also `--dh`) or [Elliptic-curve Diffie–Hellman](https://en.wikipedia.org/wiki/Elliptic-curve_Diffie–Hellman). TLS 1.3 typically uses ECDH with the X25519 curve for the key agreement.

To achieve perfect forward secrecy with post-quantum a [hybrid ECDHE-MLKEM Key Agreement](https://datatracker.ietf.org/doc/draft-ietf-tls-ecdhe-mlkem/) is used. Examples are X25519MLKEM768, SecP256r1MLKEM768, and SecP384r1MLKEM1024. Typically X25519MLKEM768 is used.

will only allow connections that use this post-quantum safe key-agreement. A connection that cannot fulfill this requirement will fail similar to this: