crypto: error in ECDH example

[jasnell]

@nodejs/crypto ... In crypto.markdown, the example for ECDH.setPublicKey(public_key[, encoding]) gives an error. Documenting this as a todo to fix (note, the examples use the updated syntax being worked on in #4282)

    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);

crypto.js:526
  var key = this._handle.getPublicKey(f);
                         ^

Error: You should generate ECDH keys first
    at Error (native)
    at ECDH.getPublicKey (crypto.js:526:26)
    at Object.<anonymous> (/Users/james/tmp/test.js:17:48)
    at Module._compile (module.js:435:26)
    at Object.Module._extensions..js (module.js:442:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:311:12)
    at Function.Module.runMain (module.js:467:10)
    at startup (node.js:134:18)
    at node.js:961:3

Adding alice.generateKeys() allows the example to work.

    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 and Alice use a newly generated cryptographically strong pseudorandom key pair
    bob.generateKeys();
    alice.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);

[shigeki]

@jasnell Are you running the example on Node-v4? #3511 included ECDH.genrateKeys() in ECDH.computeSecret() so that the example running on Node-v5 would have no error.

[jasnell]

Aha! That's right. Forgot about that change!
On Dec 15, 2015 6:42 PM, "Shigeki Ohtsu" notifications@github.com wrote:

@jasnell https://github.com/jasnell Are you running the example on
Node-v4? #3511 #3511 included
ECDH.genrateKeys() in ECDH.computeSecret() so that the example running on
Node-v5 would have no error.

—
Reply to this email directly or view it on GitHub
#4296 (comment).

[jasnell]

hmm.. wouldn't that also mean we could drop bob.generateKeys(); from the example also? sigh.. nevermind... I need to read the example better. sigh lol

[shigeki]

It can be dropped. The example just shows that ECDH.generateKeys() can also be effective before invoking ECDH.comuputeSecret() and it's not harmful