What a computer is good at
Trying things. Very fast, without getting bored, and without ever losing its place.
That is enough to end every cipher a person can do with a pencil. A shift cipher has twenty five keys. A scrambled alphabet has too many keys to try, but counting letters gets through it anyway, and counting is exactly what a machine is best at.
Enigma had more settings than there are seconds in the age of the universe, and a modern laptop breaks it in minutes, because it only has to search the settings that make sense rather than all of them.
And the two it cannot touch
A one-time pad is not hard to break. It is impossible. With truly random numbers used once, the cipher text could decode to any message of the same length, so there is nothing to search for and no way to tell a right answer from a wrong one. A faster computer does not help, because speed is not the problem.
AES can be broken in principle, by trying every key. Nobody will. The pile of keys is so large that a machine trying a billion a second would still be going long after the stars have gone out. It is not cleverness holding the door shut. It is arithmetic.
So what about artificial intelligence?
Counting letters and spotting patterns is, in a real sense, what modern AI does: it finds regularities in enormous amounts of text. Frequency analysis is a tiny, hand-sized version of the same instinct, and al-Kindi was doing it in the ninth century.
So AI is good at exactly the ciphers on this site, and it helped solve the Zodiac killer's cipher and other real puzzles that had held for decades.
But it is no use at all against AES, because there is no pattern to find. Being cleverer does not shorten a search through a number that large. Every serious cipher since the 1970s has been built precisely so that no amount of thinking replaces the work.
Which leaves the same conclusion as every other page here: the mathematics holds. The way in is the people, the habits and the assumptions.
One thing worth remembering
A code-breaking machine can also see patterns that are not there, exactly as a person can, and with far more confidence. It will happily find a word in random letters at some spacing, or a face in a cloud.
The rule from the cryptanalysis pages applies to machines too: fix what counts before you look, and be suspicious of any pattern found by something allowed to keep changing the question.
Codes that were never there Keyspace The one-time pad