Subject: Re: [bitcoin-dev] Analysis of Bech32 swap/insert/delete detection and next steps

From: Pieter Wuille <[email-masked]>
Date: Mon, 9 Dec 2019 22:36:20 -0800

> > So my suggestion for the next steps are:
> >
> > -   Update BIP173 to include the insertion weakness as an erratum, and
> >     the results of this analysis.
> >
>
> To clarify, this step does not modify anything about the implementation of BIP173, only adds this as an additional erratum section?

Correct - just documenting the issue.

> > -   Amend segwit addresses (either by amending BIP173, or by writing a
> >     short updated BIP to modify it) to be restricted to only length 20 or
> >     32 (as fixed-length strings are unaffected by the insertion issue, and
> >     I don't think inserting 20 characters is an interesting error class).
>
> To clarify, this refers to all SegWit address versions from 1 to 15, as this restriction exists for SegWit address v0?

Right, for v0 there is an inherent restriction to size 20 or 32
already, so this would only semantically change anything for version 1
through 16 (not 15).

> > -   Define a variant of Bech32 with the modified constant, so that
> >     non-BIP173 uses of Bech32 can choose a non-impacted version if they
> >     worry about this class of errors.
> >
>
> Okay, this probably needs to be raised in lightning-dev as well, for invoice formats, as well as planned offers feature.

It seems BOLT11 already doesn't care very much about the error
detection properties, as it's using Bech32 outside its design
parameters (max length 90 characters).

> By my understanding, best practice for readers of Bech32-based formats would be something like the below:
>
> 1.  Define two variants of checksum, the current Bech32 checksum and the modified Bech32 checksum.
> 2.  Support both variants (software tries one first, then tries the other if it fails).
> 3.  Flag or signal some deprecation warning if current Bech32 checksum was detected.
> 4.  At some undefined point in the future, drop support for the current Bech32 checksum.

I think it depends on the application and their tolerance to this sort
of errors. In some cases, these insertions may be detected already
with high probability (e.g. because of length restrictions, or the
fact that it adds random unstructured symbols at the end of the data
part).

> > -   Later, if and when we expect a need for non-32-byte witness programs
> >     in the medium term, define an updated segwit address scheme that uses
> >     the modified Bech32 variant.
>
> Okay, so we will only use the modified Bech32 if and only if we expect to need a non-32-byte witness program for a particular non-0 SegWit version.

Exactly.

-- 
Pieter