Does using a 12 word BIP39 mnemonic phrase in combination with another 12 word BIP39 mnemonic phrase used as an additional passphrase provide the same security as a 24 word mnemonic phrase without passphrase?

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Short answer: Yes it’s the same since there are 2048^24 mnemonic + passphrase combinations in both cases.

Slightly longer answer: Technically the second case is a bit better because the 12 words in the mnemonic have a checksum so they only contain 128 bits of entropy vs 132 bits in the 12 words in the passphrase. Any 12 words is a valid passphrase but any 12 words are not a valid mnemonic.

  • Yes you can put anything in the passphrase but my understanding of OPs question is he wants to use the same wordlist to pick 12 words for a passphrase
    – Mike D
    Sep 13, 2022 at 14:29
  • @RedGrittyBrick I explicitly asked for "another 12 word seed used as an additional passphrase". The question was about a comparison of the security of a 24 word seed without passphrase vs. a 12 word seed with another 12 word seed used as passphrase. Sep 13, 2022 at 21:46
  • @wieland.gmeiner Thanks for the clarification. My comment was motivated to help authors clarify their contributions, I misunderstood your question so I'll edit it for you to make it clearer for anyone in future who has the same question and is searching for answers. Sep 14, 2022 at 9:27

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