implement diffie hellman key exchange with elliptic curve, plus generate key pair and get certificate

Hi, help needed for implementing Diffie-Hellmen key exchange algorithm into existing client/chat system in the same java environment using multithread and slots. I already implemented basic encryption/decryption capabilities and integrated that into the communication/chat system. However I did not have any way to establish secret keys or how to authenticate users. I needed someone's help to implement the key exchange between clients, and key handling needs to be in Elliptic Curve Diffie-Hellman key agreement as a method of key exchange as part of the chat system. Basically Diffie-Hellman class is needed for the following purpose, each client will send it's private key to diffie helman to get its public key to be calculated. and each client will share the public keys each other thru the socket(we need to convert public key into string in order to pass thru the socket). then each client will combine the other side's public key with its private key and send it to the diffie-helman and get back a secret key. this secret key will be in use for encryption/decryption. Basically Diffie-Hellman class is needed for the following purpose, each client will send it's private key to diffie helman to get its public key to be calculated. and each client will share the public keys each other thru the socket(we need to convert public key into string in order to pass thru the socket). then each client will combine the other side's public key with its private key and send it to the diffie-helman and get back a secret key. this secret key will be in use for encryption/decryption. PLUS, need to generate keypair and get a certificate.