Subject: Re: [bitcoin-dev] Bech32m BIP: new checksum, and usage for segwit address

From: Pieter Wuille <[email-masked]>
Date: Mon, 18 Jan 2021 04:15:49 +0000

Hi all,

A few updates, in response to comments here and in a few other places:

- Updated several reference implementations (C, C++, Python, Javascript) to support Bech32m: https://github.com/sipa/bech32/tree/bech32m (but contributions to update other languages are welcome!)

- Updated website, including error-locating JS decoder, and demo: http://bitcoin.sipa.be/bech32/demo/demo.html

- Opened a Bitcoin Core PR: https://github.com/bitcoin/bitcoin/pull/20861

- Updates to the BIP draft (https://github.com/sipa/bips/blob/bip-bech32m/bip-bech32m.mediawiki):
  * Made the title clearer (so it doesn't imply Bech32m is used for v0)
  * Added rationale for not permitting both Bech32 and Bech32m for v0
  * Added a section on error location
  * Added links for more reference implementations

On Friday, January 15, 2021 12:01 AM, nakagat <[email-masked]> wrote:

> I read the BIP draft of Bech32m and implemented it in Go.

Cool! Do feel like contributing it to https://github.com/sipa/bech32/tree/bech32m?

> Let me ask you one question.
> Does Checksum have to be fixed?
> The 'bech32_verify_checksum' function has hrp and data as parameters,
> so how about committing Checksum with these two values?
>
> For example, calculate Checksum from hrp and data using hash, chacha20, etc.

I'm not entirely sure what you mean. Do you mean:

1) Can we use a hash function to compute the checksum instead of Bech32's algorithm?

If you compute the checksum using the HRP and the data using a hash function, you just 2^-30 failure probability for any error. The idea behind Bech32 was doing better than that for common errors: any error that consists of up to 4 substitutions are a failure probability of 0 - far better than a hash can do.

2) Can we keep using Bech32's algorithm, but compute the final xorred-in constant from the HRP and the data using a hash function?

That would be functionally equivalent to (1).

3) Can we keep using Bech32's algorithm, but compute the final xorred-in constant from the HRP (but not the data) using a hash function?

It would mean that some (very) small set of potential HRPs would exhibit much worse behavior than others - including the 'q'-before-'p' that the original Bech32 has.

Does that clarify things?

Cheers,

--
Pieter