ASVS 5.0 V11.6.2 (Level 3) requires approved algorithms for key exchange, such as Diffie-Hellman, configured with secure parameters, so that an attacker cannot weaken the exchange or sit in the middle.
Why it matters
Small or reused Diffie-Hellman groups, and exchanges without authentication, let an attacker downgrade or break the shared secret. The handshake is where a session's whole protection starts.
How to implement it
Prefer ephemeral elliptic curve exchange (ECDHE) with X25519 or P-256 or larger. If finite field DH is unavoidable, use a standard named group of at least 3072 bits instead of generating your own. Authenticate the exchange with certificates or signatures. In nginx:
ssl_ecdh_curve X25519:secp384r1:prime256v1;
How to test it
Run a TLS scanner against your endpoints and list negotiated groups. Flag any export grade or 1024 bit group, and any suite without forward secrecy.
Common mistakes
- Using an old static dhparam file of 1024 bits
- Unauthenticated exchange between internal services
- Custom exchange protocols instead of TLS
Quick check
Why prefer ephemeral exchange?
It gives forward secrecy: recorded traffic stays safe if a long term key leaks later.
