The message:
Twenty-five tries
A shift cipher has exactly twenty-five wrong keys and one right one. So do not think about it. Shift the message back by one and read it. Then by two. Keep going.
A computer does this in less than a thousandth of a second. A child with a pencil does it in about four minutes. Either way the cipher is finished, and no cleverness was required.
This is why the shift cipher is the first one on this site and never the last: it is the only one where the answer is simply to try everything.
When trying everything stops working
Add one more letter to the key and the number of keys does not go up by one, it multiplies. A key word of five letters gives about twelve million. A scrambled alphabet gives four hundred million million million million.
At that point a computer trying a billion keys a second would still be going when the sun burns out. Brute force is over, and you have to find a pattern instead — which is what frequency analysis is for.
Every serious cipher since has been built on that single fact: make the pile of keys too big to walk through. That is what keyspace means, and it is the whole reason modern encryption works.
Frequency analysis How big is too big A history of cryptographyRead more elsewhere
Longer, harder and written for grown-ups. Worth a look when this page runs out.