Quote from: SopaXT on January 05, 2018, 03:09:27 PM

If yes, then the problem lies in simply brute-forcing the 3 words, which is about 8 billion tries.

2048^3 conforms 8.589.934.592 different combinations. On average you need to search half of the space [1] (= 4.294.967.296).
The last word is a checksum. This makes it much easier to test whether the seed is valid or not. This makes it pretty easy for a halfway modern pc to bruteforce the seed.

Quote from: pcmbtc on January 05, 2018, 03:00:53 PM

I went through some of my older private keys and in one of them i've seem to have lost 3 out of the 24 words of my passphrase. I'll spare you the details of my idiocy Undecided

. There wasn't really that much money in there but I was wondering if bruteforcing these 3 words is possible since I have the other 21. I don't really know how this stuff works but if it is possible, how would I go about doing this?

I would recommend to take a look at btcrecover (https://github.com/gurnec/btcrecover/blob/master/docs/Seedrecover_Quick_Start_Guide.md).
This tool gives you the opportunity to bruteforce the missing words of your seed. You'll need python to run it. There are quite a few tutorials on the internet on how to install python.
If you'll need some help doing so, feel free to ask further questions.

Quote from: LoyceV on January 05, 2018, 04:04:37 PM

~snip~
No, please don't spare us, entertain me, I'm curious how you managed to lose 3 but keep the other 21.
~snip~

He may stored those 24 words seperated in 24 different places? In shoes, jackets, .. Over time he may have lost / can't remember where he stored the missing 3. Just my guess Roll Eyes

[1] For further information read more about the birthday paradox here https://en.wikipedia.org/wiki/Birthday_problem and here https://en.wikipedia.org/wiki/Birthday_attack.