Subject: Re: [bitcoindev] Penlock, a paper-computer for secret-splitting BIP39 seed phrases

From: Andrew Poelstra <[email-masked]>
Date: Mon, 13 May 2024 13:40:46 +0000

On Sun, May 12, 2024 at 06:04:09PM +0000, 'Rama Gan' via Bitcoin Development Mailing List wrote:
> I am excited to introduce Penlock, a printable paper-computer that guides users
> through secret-splitting their BIP39 seed phrase without an electronic device. A
> beta release is now available for peer-reviewing and early testing:
> https://beta.penlock.io.
> 
> <snip>
>

Hi Rama,

Very interesting project. I have a few unordered thoughts about this:

* You have instructions for generating BIP39 seed words, but if the goal
  is to be compatible with existing setups, this really isn't necessary
  (or even desireable). If somebody is willing to generate a whole new
  seed and is willing to sweep their coins, they might as well just use
  codex32. (Perhaps they have an urgent need to do so, and cannot wait
  for codex32 support to arrive in mainstream wallets. Ok. But it's a
  pretty niche user who is panickedly updating their coins while having
  the patience to hand-compute things!)

* Furthermore, the "just grind checksum words til the string works"
  approach, while ergonomic for 12 words (16 iterations max), is
  unrealistic for 16 words (64 iterations) and basically impossible for
  24 words (256 iterations). Probably worth mentioning this.

* The math underlying this all seems sound -- you map BIP39 characters
  directly into the field of integers mod 29, then compute lines in this
  field. However, the resulting checksum is then as long as your
  original set of words. Again, probably ok for 12 words but
  unreasonable for 24. (BTW, we have an unofficial BIP39 compatibility
  layer for codex32 which has the same issue -- everything is horrible
  for the 24-word case. But it is possible to do, and I've done it.)

* However, the use of a characteristic-greater-than-2 field means that
  addition and subtraction are different operations and suddenly you
  need to be careful about the exact order in which your read things
  off the volvelles. It also makes recovering your share more
  complicated. I see that you currently have a table for the 2-of-3
  case where you read the volvelle in different ways depending on which
  shares you have. Clever, but this will not extend to 2-of-n and I
  suspect you'll basically need to implement the full "recovery wheel"
  from codex32 (or the "recovery tables" which are faster to use for the
  2-of-n case, though easier to use wrong).

  Recovery is not really that important because you only do it when
  you're going to put your seed into a computer, and in that case you
  might as well make the computer do the recovery for you, but it is
  unfortunate. Especially in this case where a stated goal is that the
  computer -won't- do anything for you because it doesn't know about the
  scheme.

* Furthermore, this encoding into GF29 is nonstandard. I think, for the
  checksum construction this doesn't matter -- if the encoding becomes
  lost then you can just forget about the checksum, and if it doesn't,
  then you have a pretty great checksum (which can recover any number of
  errors as long as they don't hit both the data and the checksum in the
  same place). My feeling is that it's probably a good idea for people
  to use your checksum scheme on top of their existing BIP39 words, but
  the splitting stuff I'm less comfortable with.

  Possibly you would rather just combine your checksum scheme with
  seedxor? Though seedxor has the unfortunate need to convert your data
  to binary before xoring, which is time-consuming and error-prone and
  not compatible with the checksum so you don't have any good way to
  catch or fix mistakes. (The "unofficial compatibility layer for
  codex32" I mentioned works this way as well and it's horrible. But as
  you say, for users who really don't want to sweep their coins, maybe
  they are willing to make ugly tradeoffs..) Though I believe that
  seedxor only works for 2-of-3 and cannot be generalized without making
  the scheme unrecognizable.

  Alternately, if you switched to a binary field, and chose a checksum
  whose target residue was 0 (normally *not* recommended because it
  allows some classes of errors, in particular prefixes of zeroes)
  (though it does not allow any more substitution or erasure errors,
  which is what we care about for short fixed-length like this) then
  you could use an addition volvelle in the same way, the computation
  would secretly be identical to the seedxor computation, and your
  checksum would be preserved by it. So this is another way in which you
  could try to make a "seedxor-compatible checksum". But by adding a
  multiplicaion wheel that can do Lagrange multipliers you could
  generalize it to 2-of-n in a "natural" way which would break seedxor
  compatibility only for people who wanted more than 3 shares, and
  possibly even only when actually using shares beyond the third..

  As a final note about seedxor, they have as a design goal that the
  shares look identical to full seeds; they preserve the broken bip39
  checksum, have no extra characters, etc. Personally I think this goal
  is terrible. If you are going to use obscure hand-computation tricks
  you are far more likely to lose your data (or forget how to manipulate
  it) than you are to be robbed by a thief who understands your scheme.

* More generally, you need to write up a specification and description
  of the math and maybe even a PDF :). I learned the scheme by
  reverse-engineering your Javascript, which is well-written and
  dependency-free, but still pretty abstract and indirect and anyway JS
  is not my language (nor is it likely to be the language for the
  typical hand-computer user). Sadly your volvelles also don't render
  properly in my browser (qutebrowser) which is chromium-based but maybe
  I have some settings wrong.

Best
Andrew

-- 
Andrew Poelstra
Director, Blockstream Research
Email: apoelstra at wpsoftware.net
Web:   https://www.wpsoftware.net/andrew

The sun is always shining in space
    -Justin Lewis-Webster

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