# HDKeys
Create and derive extended public and private keys according to the BIP32 standard for Hierarchical Deterministic (HD) keys.

## HDPrivateKey
An instance of a [PrivateKey](privatekey.md) that also contains information required to derive child keys.

Sample usage:

```javascript
var astracore = require('astracore');
var HDPrivateKey = astracore.HDPrivateKey;

var hdPrivateKey = new HDPrivateKey();
var retrieved = new HDPrivateKey('xpriv...');
var derived = hdPrivateKey.derive("m/0'"); // see deprecation warning for derive
var derivedByNumber = hdPrivateKey.derive(1).derive(2, true);
var derivedByArgument = hdPrivateKey.derive("m/1/2'");
assert(derivedByNumber.xprivkey === derivedByArgument.xprivkey);

var address = derived.privateKey.toAddress();

// obtain HDPublicKey
var hdPublicKey = hdPrivateKey.hdPublicKey;
```

## HDPublicKey
An instance of a PublicKey that can be derived to build extended public keys. Note that hardened paths are not available when deriving an HDPublicKey.

```javascript
var hdPrivateKey = new HDPrivateKey();
var hdPublicKey = hdPrivateKey.hdPublicKey;
try {
  new HDPublicKey();
} catch(e) {
  console.log("Can't generate a public key without a private key");
}

var address = new Address(hdPublicKey.publicKey, Networks.livenet);
var derivedAddress = new Address(hdPublicKey.derive(100).publicKey, Networks.testnet); // see deprecation warning for derive
```

## Deprecation Warning for `HDPublicKey.derive()` and `HDPrivateKey.derive()`


The bug only affected hardened derivations using an extended private key, and did not affect public key derivation. It also did not affect every derivation and would happen 1 in 256 times where where the private key for the extended private key had a leading zero *(e.g. any private key less than or equal to '0fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff')*. The leading zero was not included in serialization before hashing to derive a child key, as it should have been.

As a result, `HDPublicKey.derive()` and `HDPrivateKey.derive()` are now deprecated. These methods will throw an error in the next major release.
`HDPublicKey.deriveChild()`, `HDPrivateKey.deriveChild()`, and `HDPrivateKey.deriveNonCompliantChild()` have been implemented as alternatives. Note that these new methods will not be officially supported until v1.0.0.  `deriveNonCompliantChild` will derive using the non-BIP32 derivation and is equivalent to the buggy version, `derive`. The `deriveNonCompliantChild` method should not be used unless you're upgrading and need to maintain compatibility with the old derivation.