http://www.ruanyifeng.com/blog/2011/08/what_is_a_digital_signature.html
http://www.ruanyifeng.com/blog/2006/12/notes_on_cryptography.html
https://zh.wikipedia.org/wiki/%E5%85%AC%E5%BC%80%E5%AF%86%E9%92%A5%E5%8A%A0%E5%AF%86