If you use good encryption and a reasonable key size, that’s good in practice, but theoretically, that’s a lot less secure than a one time pad.

An attacker can ‘simply’ try all possible keys and use statistics to filter out those that look like natural language.

If the encrypted text is large enough, chances are you will be left with only one plausible plaintext.

Also, AFAIK, we don’t know whether good encryption using a key much shorter than the plaintext, in the sense that an attacker can’t use statistics on the encrypted text to learn something about the key, exists at all.

Your final aside, "good encryption using a key much shorter than the plaintext," is something I hadn't really thought much about and seems like a reasonable way to go directly from a one time pad to the ciphertext.

As you say, there doesn't seem to be a way to guess the key length from the ciphertext. Ignoring side channel attacks for the moment, it does seem like the one time pad could encrypt the entire message simply by using some clever way of "extending" the key.

> it does seem like the one time pad could encrypt the entire message simply by using some clever way of "extending" the key

Any way of meaningfully extending the key will be vulnerable to a kind of analysis well understood 70 years ago.

This is basically what almost the entire field of cryptography is about: Figure out how you can effectively and securely encrypt things with a key that's a lot shorter than your cleartext.

Indeed. The two simplest examples are the Caesar cipher (https://learncryptography.com/classical-encryption/caesar-ci..., key length of 1 character) and the Vigenère Cipher (https://learncryptography.com/classical-encryption/vigenere-..., key length as long as you want it to be)

Both extend the key by repeating it. That is not a good idea.