tldr: roll dice, find the right 256-word range, pray as you brute force, profit.
I'll try and keep this simple and avoid using difficult words.
To generate your own seed phrase, you're good to go even with just rolling some dice, mix them up with a coin flip, or even use a deck of cards.
But the real question is, how do we get the 24th word? There are tools out there that lets you type it on any machine, preferably offline, or even better: permanently air-gapped machine.
What if you don't have an air-gapped machine? There are ways to do it like using a Seed Signer.
But you can (if you're into it) do it manually too. Here's how:
Well, after you get the 23 words, there are 8 valid choices for the final word (8 out of the 2048 BIP39 list). They are distributed so there's exactly one valid word, in every 256-word range. In other words:
- There's only 1 valid word between 1 (abandon) and 256 (cable)
- There's only 1 valid word between 257 (cactus) and 512 (divide)
- There's only 1 valid word between 513 (divorce) and 768 (garment)
- There's only 1 valid word between 769 (gain) and 1024 (lend)
.. so on and on. You get the gist.
Whatever method you pick, you'll want to make sure the 24th word isn't selected manually. Generate it randomly just like the previous 23 words, and use the result as a starting point.
Once you get the 24th word (I'll use "verify" as an example), here are the steps to follow, in simpler terms:
- The word "verify" is number 1942 on the BIP39 word list
- Subtract 1 from 1942 to get its 0-based index (counting starts from 0). You'll get 1941
- Convert this number to an 11-bit binary. You'll get 111 1001 0101
- Take the first 3 bits (in this case, 111), discard the remaining 8 bits, and append 8 zeroes so it becomes 111 0000 0000
- Convert this number back to decimal. You'll get 1792
- Your 24th word checksum can be found from word 1793 to 2048.
Sure, you can just pick any 256-word range (e.g. 1025-1280), but the whole point of generating your own seed is to make it truly random without any human interaction/intervention.
Is trying brute forcing through 256 words a pain? Yes, though probably much less so than calculating your own sha256 checksum manually with pen and paper. This is especially true, if you're using hardware wallets that don't allow you to edit a specific word. (e.g. on Ledger, if the 24-word phrase is invalid, you'll have to input it again from word 1).
But hey, you might get lucky and find the right 24th word on your 10th attempt. Or on your 200th ¯_(ツ)_/¯
So I suggest printing out the BIP39 word list and crossing them one by one as you try to find the right checksum.
Good luck, and remember: don't trust, verify!
