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

From: Pieter Wuille <[email-masked]>
Date: Fri, 18 Dec 2020 02:02:04 +0000

On Tuesday, December 8, 2020 9:39 AM, Ryan Grant <[email-masked]> wrote:

> It looks like a good strategy for a bech32 library that is external to
> Bitcoin Core would be:
>
> -   Default to the new M, under the same bech32 brand.
> -   Provide an interface to explicitly use both M=1 and M=0x2bc830a3.
> -   If decoding fails, throw an error; but in constructing that error
>     inform whether the other M would have succeeded.
>
> -   Provide an interface for a BIP173 implementation to peek at the
>     witness version byte of the data part, which may also involve
>     sanity-checking that byte for errors using a BIP173-specific
>     understanding of the appropriate checksum.

I think there are two possible interfaces that make sense:

- Have the caller explicitly specify whether they want bech32 or bech32m (which someone - I think Rusty? - started using in reference to this new code and I'm going to adopt now).

- Have the bech32 decoding function return a tristate (failed, valid as bech32, valid as bech32m). No string is ever valid as both, so there is no loss of information here.

The former is a bit cleaner, and also the only real choice if error location hinting is desired. The second is more efficient if decoding twice is a performance concern.

Cheers,

--
Pieter