Obviously, SHA256 can't magically generate more entropy out of nothing, it just stretches whatever is put in. If your seed was only 32 bits then hashing wouldn't save you: every possible private key could easily be calculated in advance.


On Thu, Mar 27, 2014 at 2:12 PM, Thomas Kerin <thomas.kerin@gmail.com> wrote:
Isn't the length of the seed arbitrary anyway? Once decoded using whatever mnemonic implementation (electrums, or BIP0039) the bytestream is immediately passed to HMAC-SHA256 to generate the master key. No matter what your initial entropy is, it would be hashed anyway.


On Thu, Mar 27, 2014 at 12:49 PM, Mike Hearn <mike@plan99.net> wrote:
Ah, BIP32 allows for a range of entropy sizes and it so happens that they picked 256 bits instead of 128 bits.

I'd have thought that there is a right answer for this. 2^128 should not be brute forceable, and longer sizes have a cost in terms of making the seeds harder to write down on paper. So should this be a degree of freedom?


On Thu, Mar 27, 2014 at 1:28 PM, Mike Hearn <mike@plan99.net> wrote:
By the way, I just noticed that greenaddress.it is creating seeds that have 24 words instead of 12. Does anyone know what's up with that? They claim to be using BIP32 wallets so I wanted to see if they were using the default structure and if so, whether bitcoinj was compatible with it (before I switch to the one discussed here). But it seems we fall at the first hurdle ...


On Thu, Mar 27, 2014 at 1:06 PM, Thomas Voegtlin <thomasv1@gmx.de> wrote:


Le 27/03/2014 12:30, Marek Palatinus a écrit :
> Ah, I forget to two things, which should be into the BIP as well:
>
> a) Gap factor for addresses; as Thomas mentioned, although some software
> can watch almost unlimited amount of unused addresses, this is serious
> concern for lightweight or server-based wallets like Electrum or
> myTREZOR. myTREZOR currently uses gap factor 10, which is (from my
> experience so far) quite sane for most of users.


Yes, I was planning to increase the number of available unused addresses
to 10 or 20 in the bip32 version of Electrum.

Related to this, here is another idea I would like to submit:

Instead of using a "gap limit" (maximal number of consecutive unused
addresses), I think we should get rid of the topology, and simply count
the number of unused addresses since the beginning of the sequence.
Indeed, the topology of the sequence of addresses is of no interest to
the user. Users often misinterpret "gap limit" as the "number of unused
addresses available", so I think we should just give them what they want
:) This is easier to understand, and it makes things more predictable,
because the wallet will always display the same number of unused
addresses (except when it is waiting for confirmations).


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