Specifically what I'm trying to do is print out a list of the 2048 bip39 words and randomly select 12 to create my own offline generated seed.
The right way to do it is to follow BIP-39:
- 1. Generate 128 random bits.
- 2. Compute the SHA-256 hash of the 128 bits.
- 3. Append the first 4 bits of the hash to the 128 bits, giving you 132 bits.
- 4. Split the 132 bits into 12 11-bit values.
- 5. Generate the phrase by using the 11-bit values as indexes into the list of 2048 words.
However, there is another way similar to what you want to do, but the result may be less secure depending on how random your input is:
- 1. Select 11 words from the word list. Duplicates are acceptable.
- 2. Concatenate the indexes into a 121 bit string.
- 3. Add another 7 bits, random, 0, or whatever.
- 4. Compute the SHA-256 hash of the 128 bits.
- 5. Append the first 4 bits of the hash to the 7 bits, giving you the index of the 12th word.
Finally, here is minor variation of the previous method. Again, the security depends on how the words are chosen:
- 1. Select 11 words from the word list. Duplicates are acceptable.
- 2. Determine the 128 words that would be valid as the 12th word.
- 3. Choose one.
