[bitcoin-dev] Progress on bech32 for future Segwit Versions (BIP-173)

David A. Harding dave at dtrt.org
Thu Oct 8 14:59:38 UTC 2020

On Thu, Oct 08, 2020 at 10:51:10AM +1030, Rusty Russell via bitcoin-dev wrote:
> Hi all,
>         I propose an alternative to length restrictions suggested by
> Russell in https://github.com/bitcoin/bips/pull/945 : use the
> https://gist.github.com/sipa/a9845b37c1b298a7301c33a04090b2eb variant,
> unless the first byte is 0.
> Here's a summary of each proposal:
> Length restrictions (future segwits must be 10, 13, 16, 20, 23, 26, 29,
> 32, 36, or 40 bytes)
>   1. Backwards compatible for v1 etc; old code it still works.
>   2. Restricts future segwit versions, may require new encoding if we
>      want a diff length (or waste chainspace if we need to have a padded
>      version for compat).
> Checksum change based on first byte:
>   1. Backwards incompatible for v1 etc; only succeeds 1 in a billion.
>   2. Weakens guarantees against typos in first two data-part letters to
>      1 in a billion.[1]

Excellent summary!

> I prefer the second because it forces upgrades, since it breaks so
> clearly.  And unfortunately we do need to upgrade, because the length
> extension bug means it's unwise to accept non-v0 addresses.

I don't think the second option forces upgrades.  It just creates
another opt-in address format that means we'll spend another several
years with every wallet having two address buttons, one for a "segwit
address" (v0) and one for a "taproot address" (v1).  Or maybe three
buttons, with the third being a "taproot-in-a-segwit-address" (v1
witness program using the original bech32 encoding).

It took a lot of community effort to get widespread support for bech32
addresses.  Rather than go through that again, I'd prefer we use the
backwards compatible proposal from BIPs PR#945 and, if we want to
maximize safety, consensus restrict v1 witness program size, e.g. reject
transactions with scriptPubKeys paying v1 witness programs that aren't
exactly 32 bytes.

Hopefully by the time we want to use segwit v2, most software will have
implemented length limits and so we won't need any additional consensus
restrictions from then on forward.

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