BIP-373: denote different public key types/purposes consistently

theStack commented at 0:43 am on November 29, 2024: contributor

While reviewing https://github.com/bitcoin/bitcoin/pull/31247 a while ago I noticed that the mentioned public keys in the PSBT fields of this BIP are quite confusing. This is mostly due to a non-consistent mention of types (plain vs. x-only) and a purpose that is sometimes missing. This PR aims to improve this in the following ways:

be specific about the purpose of pubkey types in PSBT fields (“plain pubkey” alone doesn’t say a lot, especially if it’s used repeatedly within a field; explicitly call it “aggregate pubkey” or “participant pubkey” if applicable)
replace all uses of “compressed pubkey” by “plain pubkey” (the only category that should matter is whether the pubkey type is “x-only” or “plain”; in the latter case, it’s obvious from the size of 33 bytes that the key is compressed)
use consistent word order, e.g. prefer “plain aggregate public key” over “aggregate plain public key”

Another possibility would be to even get rid of the “plain” terminology completely. From the fact that “x-only” is not explicitly mentioned and the size of 33 bytes it should be obvious that this is a plain public key.

BIP-373: denote different public key types consistently

Improve the clarity of the BIP w.r.t. pubkeys in the following ways:
- be specific about the purpose of pubkey types in PSBT fields
  ("plain pubkey" alone doesn't say a lot, especially if it's used
   repeatedly within a field)
- replace all uses of "compressed pubkey" by "plain pubkey"
  (the only category that should matter is whether the pubkey type
   is "x-only" or "plain"; in the latter case, it's obvious from
   the size of 33 bytes that the key is compressed)
- use consistent word order, e.g. prefer
  "plain aggregate public key" over "aggregate plain public key"

953e807ca2

theStack force-pushed on Nov 29, 2024

Sjors commented at 9:08 am on November 29, 2024: member

replace all uses of “compressed pubkey” by “plain pubkey” (the only category that should matter is whether the pubkey type is “x-only” or “plain”; in the latter case, it’s obvious from the size of 33 bytes that the key is compressed)

I find the term “compressed” more clear than “plain”, so I don’t think that part of the change is an improvement.

Similarly I find “compressed” vs “x-only” easier to understand than “plain 33 bytes” vs “x-only”.

I would just change: “Why the plain aggregate public key instead of x-only?” to say “compressed”.

in bip-0373.mediawiki:49 in 953e807ca2

45@@ -46,53 +46,53 @@ The new per-input types are defined as follows:
46 | rowspan="2"|MuSig2 Participant Public Keys
47 | rowspan="2"|<tt>PSBT_IN_MUSIG2_PARTICIPANT_PUBKEYS = 0x1a</tt>
48 | <tt><33 byte plain aggregate pubkey></tt>
49-| <tt><33 byte compressed pubkey>*</tt>
50+| <tt><33 byte plain participant pubkey>*</tt>

Sjors commented at 9:10 am on November 29, 2024:

Could also say “33 byte participant pubkey (compressed)”

in bip-0373.mediawiki:54 in 953e807ca2

51 | rowspan="2"|
52 | rowspan="2"|
53 | rowspan="2"| 0, 2
54 |-
55-| The MuSig2 aggregate plain public key<ref>'''Why the plain aggregate public key instead of x-only?'''
56+| The MuSig2 plain aggregate public key<ref>'''Why the plain aggregate public key instead of x-only?'''

Sjors commented at 9:11 am on November 29, 2024:

Maybe “The MuSig2 aggregate public key (compressed)”

in bip-0373.mediawiki:58 in 953e807ca2

56+| The MuSig2 plain aggregate public key<ref>'''Why the plain aggregate public key instead of x-only?'''
57 BIP 32 requires public keys to include their evenness byte. Aggregate public keys are expected to be
58 derived from, following [[bip-0328.mediawiki|BIP 328]], and therefore will
59 need to include the evenness. Furthermore, PSBT_IN_TAP_BIP32_DERIVATION fields include fingerprints
60-to identify master keys, and these fingerprints require full compressed public keys. By including
61+to identify master keys, and these fingerprints require full plain public keys. By including

Sjors commented at 9:13 am on November 29, 2024:

require the y-coordinate of the public key, so x-only serialisation can’t be used.

theStack commented at 11:50 am on November 29, 2024: contributor

replace all uses of “compressed pubkey” by “plain pubkey” (the only category that should matter is whether the pubkey type is “x-only” or “plain”; in the latter case, it’s obvious from the size of 33 bytes that the key is compressed)

I find the term “compressed” more clear than “plain”, so I don’t think that part of the change is an improvement.

I think more important than the concrete term is to stick to one and use it consistently, so I’m also fine with changing all occurences to “compressed” if there are no objections. My impression was that “compressed” is significantly less used now than it was in the past (when there was only 65 vs. 33 bytes), especially since x-only pubkeys could then be seen as “even more compressed” 😅

// EDIT: slightly off-topic for the BIP repo here, but if we change everything to “compressed”, the RPC help for decodepsbt (https://github.com/bitcoin/bitcoin/pull/31247/commits/abb8228944a213443c7d64e47c60ac96a21cd422) should probably updated to use that as well for consistency

Sjors commented at 12:45 pm on November 29, 2024: member

My impression was that “compressed” is significantly less used now than it was in the past (when there was only 65 vs. 33 bytes), especially since x-only pubkeys could then be seen as “even more compressed” 😅

That’s true, but there’s no other word for x-inclusive key.

BIP-373: denote different public key types/purposes consistently #1705