But after the 2020 clues (another 13 letters), it became clear that it wasn't any single ACA cipher type, and it was probably something very difficult (because of K4's very low index of coincidence, i.e. 0.036 just below "random" 26-letter text at 1/26, plus the huge number of revealed plaintext letters "in place" i.e. letter-for-lettter correspondence).
That plausibly left a combination of two or more well-known cipher types, but if they were somewhat complex ciphers, the chance of solution would be rather remote.
Hence I always thought a "good" end to the puzzle would be like the book "Masquerade" by Kit Williams where the only guy in cahoots with the creator (Bamber Gascoigne) thought the initial puzzle was an unrealistic challenge, but Williams released clues which enabled two schoolteachers to solve it. So that part was satisfactory, even if hardly anybody remembers the solvers' names!
In contrast, the cribs for K4 haven't helped at all.
Edit: Unless the one time pad is a well known relative document, such as the Declaration of Independence.
Wikipedia has a good example: https://en.wikipedia.org/wiki/One-time_pad
In their example, "HELLO" is the plain text, "XMCKL" is the key, and the ciphertext is "EQNVZ". However, with a one time pad, an equally plausible plain text is "later" with the key "TQURI". Thus, without anymore data, it is simply impossible to know what the original message is.
Reasonable puzzles can be worked out (albeit maybe with a lot of work) with information provided by the puzzle or available somewhere in the environment.
Unreasonable puzzles (like some old Sierra games cough) are impossible without secret inside knowledge by the puzzle maker and/or brute force. And sometimes not even with brute force.
The hash/video example might just be an Easter egg hunt requiring looking across a wide set of videos (somewhat reasonable but boring), or completely unreasonable depending on circumstances.
Starting with the n-char plaintext, make it a loop. Now move the second letter two places to its right, the third three places, and so on ... until arriving at the original nth letter (painted red?) Or, starting with the digits of pi, move the second letter 3 to the right, the third 1, the fourth 4, und so weiter.
Doing a frequency on 97 weird letters wouldn't help much.
Good puzzles, even hard ones, should have some idea which way to approach them and should offer a method of attack other than brute force.
https://www.rrauction.com/jim-sanborn-kryptos-k4-solution-au...
The Wired piece has Sanborn saying the reserve should be "around $300,000."
It sounds like Sanborn really doesn't think it'll be solved before the auction date of 20 November (his 80th is on 14 November). If it does get solved due to this publicity bump, that's huge earnings foregone.
Perhaps he knows it is still an "unreasonable" challenge even with the 24 known letters.
You would think that one of the lessons of that is that someone could jump in right at the end and solve it after several clues were released. That hasn't worked with K4, which is increasing people's skepticism.