Add swift signing example

HMAC is slightly complex as it was not yet ported to pure Swift

Can be complied and run as

  echo '#import <CommonCrypto/CommonCrypto.h>' > bridge.h
  swiftc examples/signature.swift -import-objc-header bridge.h
  ./signature

examples/signature.swift

@@ -0,0 +1,65 @@
+//
+// You also need a Bridging-Header.h to import CommonCrypto:
+//   #import <CommonCrypto/CommonCrypto.h>
+//
+
+import Foundation
+
+// https://stackoverflow.com/a/41965688/326257
+extension Data {
+    func hmac256(key: Data) -> String {
+        let cKey = (key as NSData).bytes
+        let cData = (self as NSData).bytes
+        var result = [CUnsignedChar](repeating: 0, count: Int(CC_SHA256_DIGEST_LENGTH))
+        CCHmac(CCHmacAlgorithm(kCCHmacAlgSHA256), cKey, key.count, cData, self.count, &result)
+        let hmacData:NSData = NSData(bytes: result, length: (Int(CC_SHA256_DIGEST_LENGTH)))
+        return customBase64(input: hmacData as Data)
+    }
+}
+
+// https://stackoverflow.com/a/26502285/326257
+extension String {
+    func hexadecimal() -> Data? {
+        var data = Data(capacity: characters.count / 2)
+
+        let regex = try! NSRegularExpression(pattern: "[0-9a-f]{1,2}", options: .caseInsensitive)
+        regex.enumerateMatches(in: self, range: NSMakeRange(0, utf16.count)) { match, flags, stop in
+            let byteString = (self as NSString).substring(with: match!.range)
+            var num = UInt8(byteString, radix: 16)!
+            data.append(&num, count: 1)
+        }
+
+        guard data.count > 0 else { return nil }
+
+        return data
+    }
+
+}
+
+func customBase64(input: Data) -> String {
+    return input.base64EncodedString()
+        .replacingOccurrences(of: "+", with: "-")
+        .replacingOccurrences(of: "/", with: "_")
+        .replacingOccurrences(of: "=+$", with: "", options: .regularExpression)
+}
+
+let key = "943b421c9eb07c830af81030552c86009268de4e532ba2ee2eab8247c6da0881".hexadecimal()!;
+let salt = "520f986b998545b4785e0defbc4f3c1203f22de2374a3d53cb7a7fe9fea309c5".hexadecimal()!;
+
+let resizing = "fill";
+let width = 300;
+let height = 300;
+let gravity = "no";
+let enlarge = 1;
+let originalUrl = "http://img.example.com/pretty/image.jpg";
+
+let encodedUrl = customBase64(input: Data(originalUrl.utf8))
+let format = "png";
+
+let partialPath = "/\(resizing)/\(width)/\(height)/\(gravity)/\(enlarge)/\(encodedUrl).\(format)"
+let toSign = salt + partialPath.utf8
+
+let signature = toSign.hmac256(key: key)
+
+let path = "/\(signature)\(partialPath)"
+print(path)