[Derander]

One of the interesting things about classic ciphers is that (as far as I know) they're obviously insecure by modern standards.

Most of the time what people want out of modern crypto is something called "semantic security" -- in other words, an attacker should not be able to learn even a single bit of the plaintext for any message, even if they are allowed to see the encryption of polynomially many chosen plaintexts (ignoring length, of course.)

What this means is that deterministic encryption schemes are all broken. Consider a game:

The attacker asks to see the encryption of messages a and b, receiving a' and b' as ciphertexts of a and b.

Now, the attacker is "challenged" with a new ciphertext of a or b, c. If the attacker can (with reasonable probability) distinguish which plaintext c is the encryption of, then the encryption scheme is not "semantically secure."

Notably, since our encryption scheme is deterministic, c = a' or b'. Therefore, with probability = 1.0 the attacker can distinguish which plaintext encrypted to c.

So, by modern security standards all deterministic (and therefore all classic, I think) ciphers are broken.

This is the sort of thing which is covered in the coursera class. Modern crypto is (usually provably) substantially more secure than what used to be used, and the kinds of techniques used to do modern crypto don't have much overlap with things like Caesar ciphers.