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My understanding is that 12th word is a checksum… so why are following mnemonics both valid?

gather video album produce report surge grant buzz buzz jaguar modify occur

gather video album produce report surge grant buzz buzz jaguar modify surge

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12-word BIP39 phases only have a 4-bit checksum. In other words, 8 bits of the last word are still "data".

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  • Thanks for helping out… can you please expand on this? Why are occur and surge considered valid checksums for preceding 11 words?
    – sunknudsen
    Aug 27, 2022 at 13:31
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    Your premise is wrong. It's not the whole word that is a checksum. The word is an encoding of 12 bits of data. 8 of those bits are data. The other 4 bits are checksum.
    – Pieter Wuille
    Aug 27, 2022 at 13:32
  • Can you please include example of how checksum is derived in answer? Having a hard time understanding how two different words can yield same checksum even if only part of work is checksum (perhaps a naive understanding of your answer).
    – sunknudsen
    Aug 27, 2022 at 13:37
  • @sunknudsen if you add a 4 bit checksum to some data there’s still a 1/16 chance that random data will validate the checksum
    – Mike D
    Aug 27, 2022 at 13:39
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    They may or may not be the same checksum - I haven't checked. The point is that part of the last word is part of the phrase the checksum is computed over. There shouldn't be anything surprising over the fact that two phrases can differ.
    – Pieter Wuille
    Aug 27, 2022 at 13:39

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