So I'm reading a chapter in my networking book, and it talks about substitution and transposition ciphers. I know that most network security uses public key or symmetric encryption, but I wonder whether a simple transposition is also used. I ask because I'm not sure how they would quickly decode the message on the other side. It would seem they would need to use something like the DH Key Exchange, and even then, they could just use the traditional symmetric encryption at that point anyway.
If “transposition” and “substitution” do have applications, what are they and how secure are they?