Subject: [bitcoindev] BIP39 Extension for Manual Seed Phrase Creation

From: Eric Kvam <[email-masked]>
Date: Fri, 23 May 2025 04:25:32 -0700 (PDT)

*Motivation*
Make it easy for users to manually create their seed phrase so that they 
don't have to trust a "black box" and allow for encoding derivation path in 
seed phrase to simplify recovery

*How*
Use every eighth word from the wordlist to generate 16 word phrases with 
128 bits of entropy (no checksum).  The most significant eight bits of each 
word are used as entropy.  The least significant three bits of each word 
specify the derivation path.

   - *000* Derivation Path Not Specified
   - *001* m/44'/0'/0'
   - *010* m/49'/0'/0'
   - *011* m/84'/0'/0'
   - *100* m/48'/0'/0'/2'
   - *101* m/86'/0'/0'

Up to seven derivation paths can be specified if all words have the same 
least significant bits.  If the least significant bits of each word vary, 
there are 48 bits that can be used to encode meta-data.  As long as 
meta-data is limited to certain allowable values, this provides a mechanism 
for error detection, similar to a checksum.

*Benefits of Suggested Implementation*

   - The word length determines how the seed phrase should be interpreted.  
   User only needs to know how many words they have and how many words the 
   wallet supports to check for compatibility with this extension
   - Uses same wordlist to represent the same entropy as a 12 word phrase 
   (could be a revision to BIP39 instead of a new BIP)
   - Manual procedure is very simple, each derivation path can use a 
   shortened 256 word list which enjoys improved alphabetical separation of 
   words
   - May prevent naive word selections which aren't limited to every eighth 
   word (similar to what checksum does)
   - Can be extended further.  For example, a 32 word phrase with the same 
   entropy as a 24 word phrase could also be added.  We can keep adding 
   formats with unique word length and keep adding uses for the meta data as 
   needed.

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