custom private key with crypto in node.js

Question

from: https://nodejs.org/api/crypto.html#crypto_class_ecdh

const alice_key = alice.generateKeys();

will generate a random private key and the corresponding public key.

But I would like to set my own private key: e8f32e723decf...

If I use :

alice.setPrivateKey("e8f32e723decf");

the object alice_key is not affected, so later:

const bob_secret = bob.computeSecret(alice_key, 'hex', 'hex');

will be wrong. Is there a way to do something like:

const alice_key = alice.generateKeys("e8f32e723decf");

Answer 1

First of all I suppose your hex string is missing a leading 0, so it should be 0e8f32e723decf.

Then it depends on your node.js version, the implementation of ECDH.setPrivateKey() changed from 5.1 to 5.2

---

node.js 5.0

• You need to generate the keys and override them
• You need to have the public and the private key
• Working online example

const crypto = require('crypto');

// this is just to generate a private/public key pair
const warmup = crypto.createECDH('secp521r1');
warmup.generateKeys();
const warmup_private_key = warmup.getPrivateKey();
const warmup_public_key = warmup.getPublicKey();

// convert it to hex string to match the example
// you would store these strings somewhere I guess
private_key = warmup_private_key.toString('hex');
public_key = warmup_public_key.toString('hex');

// now let's create the ciphers
const alice = crypto.createECDH('secp521r1');
const bob = crypto.createECDH('secp521r1');


----------


// Bob gets created keys
bob.generateKeys();

// Generate Alice's keys - that's really annoying since you will override it
alice.generateKeys();
// now set the keys:
alice.setPrivateKey(private_key, "hex");
alice.setPublicKey(public_key, "hex");

// Exchange and generate the secret...
const alice_secret = alice.computeSecret(bob.getPublicKey());
const bob_secret = bob.computeSecret(alice.getPublicKey());

console.log("alice's shared secret: " + alice_secret.toString('hex') + "\n");
console.log("bob's shared secret:   " + bob_secret.toString('hex') + "\n");

console.log('shared secrets match: ' +  alice_secret.equals(bob_secret));

---

node.js >= 5.2

const crypto = require('crypto');
const alice = crypto.createECDH('secp256k1');
const bob = crypto.createECDH('secp256k1');
bob.generateKeys();

alice.setPrivateKey('0e8f32e723decf', 'hex');

const alice_secret = alice.computeSecret(bob.getPublicKey());
const bob_secret = bob.computeSecret(alice.getPublicKey());

console.log(alice_secret.equals(bob_secret));