Add Key.encrypt(passphrase) to exported API

[NSkelsey]

In the API documentation we find key.prototype.decrypt. However, there is no accompanying key.prototype.encrypt for Key objects.

Either this was a specific design choice or it is genuinely missing – please advise.

The current code I use to preform key.encrypt(passphrase) looks like:

// lockKeyRing encrypts the private key material of the key using the passed 
// password. The primary key and all subkeys are encrypted with the password.
// { string -> bool }
function lockKeyRing(password) {
  var w = true;
  w = w && keyRing.privateKey.primaryKey.encrypt(password);
  keyRing.privateKey.subKeys.forEach(function(sk) {
      w = w && sk.subKey.encrypt(password);
   });
  return w;
}

Note that the keyRing here is a custom data structure.

P.S. Thanks for the library it is fantastic!

[evilaliv3]

Thanks @NSkelsey for the suggestion; I've written a pull request that follows the same pattern of the decrypt API and that is already used by various projects like whiteout-io: https://github.com/whiteout-io/mail/blob/master/src/js/crypto/pgp.js#L204

@tanx / @toberndo what do you think?

[tanx]

See #446