In security terms, what is the difference between 15- and 24-word seed?
Therefore, which option is recommended (from a security point of view)?
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Following the BIP39 standard, 15 word seeds are proving about 160 bits (20 bytes) of entropy, where 24 word seeds are providing around 254 bits (32 bytes) of entropy.
As long as you do everything to protect your word seed, 160 bits of entropy is plenty. In the past, people used 24 words to split it into two parts, but today they are better ways to split your seed, such as Shamir backup.
Little bit off topic, but I urge people to use hardware wallets, they are not that expensive and provide a huge increase in security over plain wallets - that is my security recommendation.
BIP39 wordlist contains 2048 words. If you use a 24-word phrase, you'll get 2048²⁴ possible combinations, or 2^264. This is an insultingly huge number.
For hackers, trying to brute force to determine even your 15-word seed phrase, 2^165 possible combinations is also a huge number. Have you ever seen at least 2^160 ? This is a number of possible bitcoin addresses.
I'm sure a 15-word recovery phrase is enough for today. The 24-word seed phrase is invented with the future in mind (don't forget about quantum equipment).
Read my Medium story for details.
For Shelley-era Daedalus wallet you must use 24-word seed, for Yoroi – 15-word seed.
For Byron-era Daedalus legacy wallet you must use 12-word seed, for Yoroi – 15-word seed.
For Cardano-cli Shelley-era wallet you can use 15, 18, 21 or 24-word seed.
The latest version of Yoroi has an ability to recover 24-word-based Daedalus wallet.