Why 24 words?

[BlackHatCoiner]

...

I understood how it works. Thank you. On 12 words I guess that you have 4 bits with 16 different combinations. If we take 1 in every 16 words, we end up with... Oh.. 128 possibilities. This was what pooya87 trying to say. Brute forcing with 24 words is 128/16=8 times faster than with 12 words.

Even if all these are true, I still have my doubts that I'll ever use this in the future. Once you write down your mnemonic, you only have one job. Do it right.

[Coin-Keeper]

One scenario I imagine is if you split your seed into three to store as 2 of 3 factors of authentication, in case of 24 words, attacker would have to crack more words if they get their hands on one copy versus if you were using 12 words.

You really, really don't want to do that!!  Have a good read about creating/using Shamir backup if you want to spread your seed words around to add security!  Its very easy to generate Shamir Backup SEED using my Trezors.

[pooya87]

Keep in mind that the number of valid checksums with different combinations are an estimate because we are talking about collision and it can technically be between 0 collisions and maximum possible ones meaning 2048 with 1 missing word.
Probabilistically we say with 1 missing in 24 words there can be 8 valid combinations on average.

Example:
Only 5 are valid here:

Code:

goddess crane order member daring title renew autumn drive * sword slim lesson home focus flee silent dash shock adapt success laundry physical base

While there are 9 valid combinations here:

Code:

goddess crane order member daring title renew autumn drive rally sword slim lesson home focus flee silent dash shock adapt success laundry * base

And 13 here:

Code:

* crane order member daring title renew autumn drive rally sword slim lesson home focus flee silent dash shock adapt success laundry physical base

With 12 words:
135:

Code:

lava reason future wonder image hard hunt step basket frost * poet

130:

Code:

lava reason future * image hard hunt step basket frost invite poet

128

Code:

* reason future wonder image hard hunt step basket frost invite poet

[igor72]

For example, trezor chooses to use 24 words.

The Trezor Model T generates 12-word seed phrase.

[HCP]

The Trezor Model T generates 12-word seed phrase.

It generates 12 words by default... but as I understand it, you can override that and select 24 words if you want, but it requires using the "trezorctl" commandline tools

In fact both devices are able to use 12, 18 or 24 word seeds.


source: https://wiki.trezor.io/Recovery_seed#Recovery_seed_and_Trezor

[Pmalek]

Snip

It's explained nicely in this post:
https://www.reddit.com/r/TREZOR/comments/9tfl38/trezor_t_24_word_seed/

The entire command is

trezorctl reset-device -t256 -p

-t256 generates a 24-word recovery phrase.

Someone else pointed out that you can generate your seed on a Ledger hardware wallet or another hardware wallet that supports 24-words seeds, and than import that seed into your Trezor.