ecdh

ecdh is a key-agreement method where two sides derive a shared secret from elliptic curve math - in pgp it is what actually scrambles messages made for a curve25519 key.

ecdh stands for elliptic curve Diffie-Hellman, a key-agreement protocol: two parties exchange public values computed on an elliptic curve, and each side can then compute the same shared secret that outsiders watching the exchange cannot. the shared secret then drives a fast symmetric cipher that does the bulk encryption.

openpgp's flavor adds an ephemeral twist: every encrypted message generates a fresh throwaway curve point, so each message is scrambled under its own one-time secret even though it goes to your long-lived public key. compromise of one message's randomness does not cascade into the others.

when someone asks why a message to a modern curve25519 key cannot be decrypted with an old rsa-only implementation, the answer lives here - ecc encryption in openpgp is defined as ecdh, and the pieces have to understand each other.

for daily use none of this needs configuring; choose a curve25519 key at generation time and ecdh handles encryption under the hood. it earns a glossary entry because "ecdh" shows up in error messages, key listings and security discussions often enough that knowing it means "elliptic curve key agreement" demystifies most of them.