Codes #3

Although the codes I had mentioned earlier
were effective, they can't be sent directly
over the Internet as letters as they have to be
converted into binary and also because Credit
Card transactions have to be sent through
the Internet, so it becomes unsafe to use it.
So, a new system was created, called RSA.

To explain this would need some Heavy-
Weight maths, so I'll give it to you the way I
had been made to understand it.

The post system is unsafe, but 
you need to send a high-importance 
message without it getting into 
the wrong hands. So you and your 
correspondent set up a system: 

Call the message A

Any padlocks or safeguards 
will be denoted by brackets []

You send [A]
He receives it and again adds safety:
[[A]]
You receive it and remove your padlock:
[A]
He receives it and removes his padlock:
A

So, now he receives the message without any need 
to send a key across, lest it be lost in the post system.

But you can't do this on numbers, or can you?
Because factorisation of numbers is difficult, 
even with computers, your credit card number 
or even blog post cannot be intercepted until 
it reaches its desired destination, like this blog.