Use of a random point to initiate scalar multiplication

All of the above

Avoiding the use of low-order points in the curve group

Limiting the number of points in the curve group to avoid small subgroup attacks

A side-channel attack revealing the private key during encryption or decryption

All of the above

The use of quantum algorithms for factoring integers

An adversary exploiting weaknesses in the key exchange protocol.

Encrypting the entire communication using the exchanged key

Changing the key generation algorithm

Reducing the size of the prime number used in the Diffie-Hellman exchange

Using digital signatures to authenticate public keys before exchange