Typechecking indexes

As a consequence, it will not allow accidentally double-hardened indexes.

It also won't allow strings or forgotten parameters.

src/hdnode.js

@@ -206,6 +206,8 @@ HDNode.prototype.toBase58 = function (__isPrivate) {
 
 // https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki#child-key-derivation-ckd-functions
 HDNode.prototype.derive = function (index) {
+  typeforce(types.UInt32, index)
+
   var isHardened = index >= HDNode.HIGHEST_BIT
   var data = new Buffer(37)
 
@@ -277,6 +279,8 @@ HDNode.prototype.derive = function (index) {
 }
 
 HDNode.prototype.deriveHardened = function (index) {
+  typeforce(types.UInt31, index)
+
   // Only derives hardened private keys by default
   return this.derive(index + HDNode.HIGHEST_BIT)
 }

src/types.js

@@ -12,9 +12,13 @@ function Hash256bit (value) { return nBuffer(value, 32) }
 function Buffer256bit (value) { return nBuffer(value, 32) }
 
 var UINT53_MAX = Math.pow(2, 53) - 1
+var UINT31_MAX = Math.pow(2, 31) - 1
 function UInt2 (value) { return (value & 3) === value }
 function UInt8 (value) { return (value & 0xff) === value }
 function UInt32 (value) { return (value >>> 0) === value }
+function UInt31 (value) {
+  return UInt32(value) && value <= UINT31_MAX
+}
 function UInt53 (value) {
   return typeforce.Number(value) &&
     value >= 0 &&
@@ -51,6 +55,7 @@ var types = {
   Network: Network,
   UInt2: UInt2,
   UInt8: UInt8,
+  UInt31: UInt31,
   UInt32: UInt32,
   UInt53: UInt53
 }