dart signature example (#320)

examples/signature.dart

@@ -0,0 +1,32 @@
+import 'dart:convert';
+import 'package:convert/convert.dart';
+import 'package:crypto/crypto.dart';
+
+void main() {
+	var key = hex.decode("943b421c9eb07c830af81030552c86009268de4e532ba2ee2eab8247c6da0881"); 
+	var salt = hex.decode("520f986b998545b4785e0defbc4f3c1203f22de2374a3d53cb7a7fe9fea309c5"); 
+	var url = "http://img.example.com/pretty/image.jpg"; 
+	var resizing_type = 'fill';
+	var width = 300;
+	var height = 300;
+	var gravity = 'no';
+	var enlarge = 1;
+	var extension = 'png';
+  
+	var url_encoded = urlSafeBase64(utf8.encode(url)); 
+
+	var path = "/$resizing_type/$width/$height/$gravity/$enlarge/$url_encoded.$extension"; 
+
+	var signature = sign(salt, utf8.encode(path), key);
+	print("/$signature/$path"); 
+}
+
+String urlSafeBase64(buffer) {
+	return base64.encode(buffer).replaceAll("=", "").replaceAll("+", "-").replaceAll("/", "_"); 
+}
+
+String sign(salt, path, key) {
+	var hmac = Hmac(sha256, key); 
+	var digest = hmac.convert(salt + path); 
+	return urlSafeBase64(digest.bytes); 
+}