Adds id() method to Secp256k1PrivateKey

Feature parity with ed25519 and rsa

src/index.js

@@ -73,6 +73,25 @@ module.exports = (keysProtobuf, randomBytes, crypto) => {
       ensure(callback)
       multihashing(this.bytes, 'sha2-256', callback)
     }
+
+    /**
+     * Gets the ID of the key.
+     *
+     * The key id is the base58 encoding of the SHA-256 multihash of its public key.
+     * The public key is a protobuf encoding containing a type and the DER encoding
+     * of the PKCS SubjectPublicKeyInfo.
+     *
+     * @param {function(Error, id)} callback
+     * @returns {undefined}
+     */
+    id (callback) {
+      this.public.hash((err, hash) => {
+        if (err) {
+          return callback(err)
+        }
+        callback(null, bs58.encode(hash))
+      })
+    }
   }
 
   function unmarshalSecp256k1PrivateKey (bytes, callback) {