when you say "For 18 words, 1500 years", you mean to just generate ALL possible combinations, right??
Based on the benchmark provided by btcrecover, that would be to exhaust 50% of the search space, which is the average amount of the space you would need to search to reach the desired combination.
and my script generate that exact sequence as the 4th result, is it not the same ??
have i found it in some seconds?
Yes. But it is equally likely that you find it in the 4th result or that you find it in the 4th last result after searching 99.9999....% of combinations.
and what about the "last" word, you call CHECKSUM, in my case "cat17".
it is not just a word "cat17" ??
That word encodes 11 bits of data. Of those bits of data, some of them represent a checksum. For a 12 word seed phrase, 4 bits are a checksum. For a 24 word seed phrase, it is 8 bits.
On average, to crack a key, you should assume that you need to exhaust the search space. If you average out every cracking attempt, then you will find that you should only find them when you're nearing the end of the search space and it should follow an exponential distribution.
That's not right, On average you need to exhaust half the search space. There is a 50% chance you find it in the first half, and a 50% chance you find it in the second half.