Nicolas Del Valle - 2 years ago 258

Node.js Question

This is the nodejs documentation example:

`const crypto = require('crypto');`

const alice = crypto.createECDH('secp256k1');

const bob = crypto.createECDH('secp256k1');

// Note: This is a shortcut way to specify one of Alice's previous private

// keys. It would be unwise to use such a predictable private key in a real

// application.

alice.setPrivateKey(

crypto.createHash('sha256').update('alice', 'utf8').digest()

);

// Bob uses a newly generated cryptographically strong

// pseudorandom key pair bob.generateKeys();

const alice_secret = alice.computeSecret(bob.getPublicKey(), null, 'hex');

const bob_secret = bob.computeSecret(alice.getPublicKey(), null, 'hex');

// alice_secret and bob_secret should be the same shared secret value

console.log(alice_secret === bob_secret);

I don't understand where the secret comes in. Suppose I want to decrypt a message

`foo-bar`

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Answer Source

The steps above constitute the ECDH key agreement protocol to establish a shared secret (a symmetric key) between Alice and Bob which they can subsequently use to communicate securely.

The secret key **alice_secret** is computed using Alice's private key and Bob's public key at Alice's end.

The key **bob_secret** is computed using Bob's private key and Alice's public key at Bob's end.

Both keys will be equal. Now Alice and Bob has a shared secret (alice_secret=bob_secret) which they can use to ecnrypt/decrypt messages.

Note that only public keys are exchanged here and a Man In the Middle cannot get hold of either Alice's or Bob's private key.

The shared secret can be converted to a proper symmetric key suitable for algorithms like AES or used as it is. Refer KDF

Pseudo-code

Bob encrypts using bob_secret and AES:

```
var crypto = require('crypto'),
algo = 'aes-256-ctr',
var cipher = crypto.createCipher(algo,bob_secret)
var encrypted = cipher.update("foo-bar",'utf8','hex')
encrypted += cipher.final('hex');
```

Alice decrypts:

```
var decipher = crypto.createDecipher(algo,alice_secret)
var decrypted = decipher.update(encrypted,'hex','utf8')
decrypted += decipher.final('utf8');
```

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