Create signature-cloudfront-functions.js

Modified signature.js for use in AWS CloudFront Functions.

About JS runtime and restrictions - 
https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/functions-javascript-runtime-features.html

examples/signature-cloudfront-functions.js

@@ -0,0 +1,19 @@
+var crypto = require('crypto');
+
+var KEY = '943b421c9eb07c830af81030552c86009268de4e532ba2ee2eab8247c6da0881'
+var SALT = '520f986b998545b4785e0defbc4f3c1203f22de2374a3d53cb7a7fe9fea309c5'
+
+var hexDecode = (hex) => String.bytesFrom(hex, 'hex')
+
+var sign = (salt, target, secret) => {
+    var hmac = crypto.createHmac('sha256', hexDecode(secret))
+    hmac.update(hexDecode(salt))
+    hmac.update(target)
+    
+    return hmac.digest('base64url')
+}
+
+var path = "/rs:fit:300:300/plain/http://img.example.com/pretty/image.jpg"
+var signature = sign(SALT,path,KEY)
+var result = `/${signature}${path}`
+console.log(result)