I have said that I want to take at least 4 courses on coursera during 2015. Now I am following cryptography 1 by Stanford University and cryptography 2 will follow after 1 ends. Last year, I have taken algorithm 1&2 and machine learning by Stanford University. I feel like Stanford takes online courses very seriously, especially algorithm course. Tim Roughgarden speaks very quickly and doesn't lower difficulty of this object. I also approve University of Washington and National Taiwan University. You will not feel fooled by these universities. Now I want to mention John Hopkins University. I have enrolled twice in data science specialization but it doesn't make me feel to continue so I un-enrolled just after course begins.
That's end of nonsense. Now I want to talk a little bit of cryptography course.
To guarantee security of online communication, we have to develop methods to encrypt message before delivery and decrypt it after delivery. Usually sender and receiver will share one common secret key and use it to encrypt and also decrypt. Before we go on, we need to know cryptography is a great way and basic for many security mechanisms but it cannot solve every security problem. If you know some cryptography, you have to use it properly or else you will not get protected. The most important is that it is not something you should try to invent yourself! So many examples have shown that self-invent models are very easily to break up.
In the following, I will talk about cryptography methods in the course. But before that, what is a secure cipher? One criterion is any two plain messages with same length always have same probability to be encrypted to one deterministic. Then attackers will not get any information to differentiate messages with same length.