Add DER decoder

der.js

@@ -0,0 +1,56 @@
+var BigInteger = require('cifre/rsa').BigInteger;
+
+exports.decode = decode;
+function decode(data) {
+  var offset = 0;
+
+  // Decode length headers
+  function parseLength() {
+    var value = data[offset++];
+    if (value < 0x80) { return value; }
+    return parseInteger(value & 0x7f);
+  }
+
+  function parseInteger(length) {
+    var value = 0;
+    for (var i = 0; i < length; i++) {
+      value = (value << 8) | data[offset++];
+    }
+    return value;
+  }
+
+  function parseBigInteger(length) {
+    var arr = new Array(length);
+    for (var i =0; i < length; i++) {
+      arr[i] = data[offset++];
+    };
+    return new BigInteger(arr, 256);
+  }
+
+  function parseSequence(length) {
+    var arr = [];
+    var end = offset + length;
+    while (offset < end) {
+      arr.push(parse());
+    }
+    return arr;
+  }
+
+  // Decode type headers
+  function parse() {
+    var type = data[offset++];
+    var length = parseLength();
+    var value;
+    switch (type) {
+      case 0x02:
+        if (length > 6) return parseBigInteger(length);
+        return parseInteger(length);
+      case 0x30:
+        return parseSequence(length);
+      default:
+        throw new Error("TODO: Implement data type 0x" + type.toString(16));
+    }
+  }
+
+  return parse();
+}