What I have in mind is in my latest reply (difficult to have some kind of fluent discussions on this list given the moderation and delayed posts)

I would just add that the derivation method (indeed something like what you are sketching below) should estimate that there is enough entropy from the secret, if not just throw

On Wed, Jan 2, 2019 at 3:40 AM Alan Evans via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:

I think any method that doesn't use real entropy, but some fake source of randomness, such as a book is asking to be hacked and so is not a reasonable idea.

If an algorithm for book text to BIP39 sentence ever became well used, common books will be systematically searched for accounts. People will also choose their favourite passages, so I would expect to see collisions.

I tend to have this conversation a lot ;) I'm not sure what Aymeric has in mind, but my suggestions are for use by the small few who properly understand how these things work. I am not suggesting blockchain.info require every user to choose a book passage to use as their backup phrase!

There are so many small things that could be done to make a text input unique. Choose the X number of words from the start of the Nth sentence. Replace all punctuation with exclamation points. Combine two sentences from different pages. It would be nigh impossible to brute force any of these, and would require hints/instructions from the owner to recover.

But I admit if this is not intended for standardization, discussing it on this mailing list is probably unwarranted.

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