Elliptic curve Diffie-Hellmann is provided in `secp256k1` via the [`ecdh.h`](https://github.com/bitcoin-core/secp256k1/blob/master/include/secp256k1_ecdh.h) api. The rust wrapper provides the functionality [here](https://github.com/rust-bitcoin/rust-secp256k1/blob/master/src/ecdh.rs).
