const crypto = require('crypto');

//Keys generation
const alice = crypto.createECDH('secp256k1');
alice.generateKeys();
const bob = crypto.createECDH('secp256k1');
bob.generateKeys();

const alicePublicKeyBase64 = alice.getPublicKey().toString('base64');
const bobPublicKeyBase64 = bob.getPublicKey().toString('base64');

const aliceSharedKey = alice.computeSecret(bobPublicKeyBase64, 'base64', 'hex');
const bobSharedKey = bob.computeSecret(alicePublicKeyBase64, 'base64', 'hex');

//Alice encrypt the message
const MESSAGE = "This is a text to encrypt";
console.log("Message en clair : " + MESSAGE);

const IV = crypto.randomBytes(16);
const cipher = crypto.createCipheriv('aes-256-gcm', Buffer.from(aliceSharedKey, 'hex'), IV);

let encrypted = cipher.update(MESSAGE, 'utf8', 'hex');
encrypted += cipher.final('hex');

const auth_tag = cipher.getAuthTag().toString('hex');

console.table({
    IV: IV.toString('hex'),
    encrypted: encrypted,
    auth_tag: auth_tag
});

const payload = IV.toString('hex') + encrypted + auth_tag;
console.log("Chaine cryptée HEX : " + payload);

const payload64 = Buffer.from(payload, 'hex').toString('base64');
console.log("Chaine cryptée envoyée : " + payload64);

//Send the payload64 to bob

//Bob decrypt the message
console.log("Chaine cryptée reçue : " + payload64);
const bob_payload = Buffer.from(payload64, 'base64').toString('hex');
console.log("Chaine cryptée HEX : " + bob_payload);

const bob_iv = bob_payload.substr(0, 32);
const bob_encrypted = bob_payload.substr(32, bob_payload.length -32 -32);
const bob_auth_tag = bob_payload.substr(bob_payload.length -32, 32);

console.table({ bob_iv, bob_encrypted, bob_auth_tag });

try{
    const decipher = crypto.createDecipheriv(
        'aes-256-gcm', 
        Buffer.from(bobSharedKey, 'hex'), 
        Buffer.from(bob_iv, 'hex')
    );

    decipher.setAuthTag(Buffer.from(bob_auth_tag, 'hex'));

    let decrypted = decipher.update(bob_encrypted, 'hex', 'utf8');
    decrypted += decipher.final('utf8');

    console.log("Message en clair : " + decrypted);

}catch(error){
    console.log(error)
}