title: Sign a string with an RSA private key
body: <p>In my app i have to sign a message with an RSA private key i already have. After this done, i want to use the signature later. 
  I've checked several libraries, but all of them do much more that i need.
  This is what i'd like to do:</p>
  
  <pre><code>NSString *message = @"This is a message";
  NSString *privateKey = ...;
  
  NSString *signature = [self signMessage:message withPrivateKey:privateKey];
  </code></pre>
  
  <p>How would</p>
  
  <pre><code>(NSString *)signMessage:(NSString *)message withPrivateKey:(NSString *)privateKey {
  }
  </code></pre>
  
  <p>look like?</p>