Date: Sun, 29 Aug 2021 14:42:07 +0000
To: ts <ts@cronosurf.com>,
 Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
From: Pieter Wuille <bitcoin-dev@wuille.net>
Reply-To: Pieter Wuille <bitcoin-dev@wuille.net>
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Subject: Re: [bitcoin-dev] Human readable checksum (verification code) to
	avoid errors on BTC public addresses
Precedence: list

On Thursday, August 19th, 2021 at 1:02 PM, ts via bitcoin-dev <bitcoin-dev@=
lists.linuxfoundation.org> wrote:

> > In any case --- the last 5 characters of a bech32 string are already a =
human-readable 5-digit code, with fairly good properties, why is it not usa=
ble for this case?

Side note: it's actually the last six characters.

>
> Well, because
>
> a) most people don't know that
>
> b) it is specific to bech32
>
> c) it is not easily readable being the last digits of a long address (alt=
hough this could be

I think this is a misconception. For the purpose of verifying that you have=
 the *right* address (rather than just a valid one), the checksum, or even =
the knowledge that a checksum is present, is completely irrelevant.

In honestly-generated addresses, every character except the prefix (the ~2 =
first characters for P2PKH and P2SH, and the ~4 first characters for BIP173=
/BIP350 native segwit addresses) has exactly the same amount of entropy. In=
stead of adding say a 4 character code, just tell people to compare any 4 c=
haracters of their choosing. Or more - I would hope people are already comp=
aring (much) more than 4 characters already.

It doesn't matter if the characters being compared are checksum characters =
or data characters. In honestly-generated addresses, both are equally rando=
m.

Adding a special 4 character "external" checksum IMO would instead encourag=
e people to perhaps just compare those 4 characters instead of the rest (or=
 at least, focus mostly on those). That could easily worsen how well compar=
isons are done in practice...

Cheers,

--
Pieter