Given the issue is in the address format, not the consensus/standardness layer, it does seem somewhat strange to jump to addressing it with a consensus/standardness fix. Maybe the ship has sailed, but for the sake of considering all our options, we could also redefine bech32 to not allow such addresses.
Matt
On Nov 7, 2019, at 17:47, Greg Sanders via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:
Could the softer touch of just making them non-standard apply as a future preparation for an accepted softfork? Relaxations could easily be done later if desired.
Hello all,
A while ago it was discovered that bech32 has a mutation weakness (see
https://github.com/sipa/bech32/issues/51 for details). Specifically,
when a bech32 string ends with a "p", inserting or erasing "q"s right
before that "p" does not invalidate it. While insertion/erasure
robustness was not an explicit goal (BCH codes in general only have
guarantees about substitution errors), this is very much not by
design, and this specific issue could have been made much less
impactful with a slightly different approach. I'm sorry it wasn't
caught earlier.
This has little effect on the security of P2WPKH/P2WSH addresses, as
those are only valid (per BIP173) for specific lengths (42 and 62
characters respectively). Inserting 20 consecutive "p"s in a typo
seems highly improbable.
I'm making this post because this property may unfortunately influence
design decisions around bip-taproot, as was brought up in the review
session (https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2019-October/017427.html)
past tuesday. In the current draft, witness v1 outputs of length other
than 32 remain unencumbered, which means that for now such an
insertion or erasure would result in an output that can be spent by
anyone. If that is considered unacceptable, it could be prevented by
for example outlawing v1 witness outputs of length 31 and 33.
Thoughts?
Cheers,
--
Pieter
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