Re: [bitcoindev] Human meaningful witness versioning

["'Ava Chow' via Bitcoin Development Mailing List" <bitcoindev@googlegroups.com>]

Hi Ethan,

I do not think that this is a good idea, and it undermines one of the 
ways that witness programs provide us flexibility when deploying soft 
forks in the form of varying witness program lengths.

Both witness version 0 and version 1 define consensus rules for the pair 
of witness version and size of the witness program. A P2WPKH address and 
a P2WSH address have very different meanings, yet share the same witness 
version and therefore the same bc1q prefix. Their different rules are 
applied depending on the size of the witness program.

While BIP 141 explicitly disallows the disallows the usage of other 
sizes when it defined version 0, BIP 341 made no such restrictions on 
version 1. Instead, the rules defined in BIP 341 only apply to witness 
programs of 32 bytes, thus allowing new rules to be applied to witness 
programs of other sizes that are also version 1.

We have already seen such a proposal be deployed - Pay to Anchor outputs 
use witness version 1 with a 2 byte witness program of a particular value.

If the witness version were chosen based on a mnemonic, then we largely 
lose the ability to define new rules for different sized witness 
programs of already in use witness versions. Such usage would cause 
mnemonics to lose their meaning, unless we expect users to also be 
checking the length of addresses, and I don't think that's a reasonable 
expectation.

Ava

On 07/18/2025 02:58 PM, Ethan Heilman wrote:
> I want to propose a new criteria for allocating Witness versions based 
> on human meaningfulness and see if there is support for this approach or 
> if the community is highly allergic to this idea.
> 
> Bech32 (BIP-0173 <https://github.com/bitcoin/bips/blob/master/ 
> bip-0173.mediawiki>) was designed such that the Witness version is the 
> first character in an address after the “bc1” address prefix
> 
> Witness Version 0: bc1q…
> Witness Version 1: bc1p…
> 
> Witness version 2: bc1z…
> 
> Witness version 3: bc1r…
> 
> Witness version 4: bc1y…
> Witness version 5: bc19…
> 
> Witness version 6: bc1x…
> 
> Witness version 7: bc18…
> 
> Witness version 8: bc1g…
> 
> …
> 
> 
> So far we have been allocating Witness Versions in incrementing numeric 
> order (0,1,...). I want to suggest we allocate Witness Versions mnemonic 
> to make it easier to look at an address and determine the output type.
> 
> 
> This originally came up over the question of if BIP-360 should use 
> Witness Version 3 to get bc1r… for P2QRH (r for resistant) or the next 
> numerically available 2, but I want to see how the community feels about 
> it as a general pattern for future softforks (z for compressed/zipped 
> output, y for yield outputs, etc…).
> 
> 
> Making it easier for users to understand the output type associated is 
> likely to grow in importance over time as we retire output types, add 
> policy restricting the relay of certain output types or output types 
> become insecure due to cryptanalytic breaks. While wallet software 
> should flag dangerous output types, some wallets may not invest in such 
> functionality or the user may be using a paper wallet. This is the same 
> argument as prefixing addresses with “bc” for mainnet and “tc” for testnet.
> 
> 
> Note: the Witness version is sometimes called the SegWit version.
> 
> Thanks,
> Ethan
> 
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