BIP-352: limit number of recipients to 2^32-1 #2055
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cc: @RubenSomsen, @josibake |
murchandamus
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| For silent payments v0 a transaction MUST be scanned if and only if all of the following are true: | ||
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| * The transaction contains at least one BIP341 taproot output (note: spent transactions optionally can be skipped by only considering transactions with at least one unspent taproot output) | ||
| * The transaction does not contain more than 4294967295 (2<sup>32</sup>-1) taproot outputs<ref name="maximum_recipients">'''Why specify a limit of recipients, if the maximum number of possible transaction outputs is practically constrained by Bitcoin's consensus rules (block weight limit) anyway?''' Specifying an explicit limit improves clarity and ensures that implementations can safely choose compatible data types without the risk of becoming non-compliant. The limit matches the maximum possible value for ''k'' in the course of the shared secret scalar calculation ''t<sub>k</sub>'', where ''k'' is serialized as a 4-bytes sequence.</ref> |
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nit: I think it would be clearer and harder to misinterpret to state directly that both k and the number of recipients are unsigned 32-bit integers.
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+1. The block size limits transactions to fewer than 23,300 P2TR outputs, so stating such a high limit seems more confusing than explaining what type of number are used.
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Makes sense. Rephrased without stating the limit explicitly, but rather using the phrasing "fits within an unsigned 32-bit integer". Suggestions welcome.
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| For silent payments v0 a transaction MUST be scanned if and only if all of the following are true: | ||
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| * The transaction contains at least one BIP341 taproot output (note: spent transactions optionally can be skipped by only considering transactions with at least one unspent taproot output) | ||
| * The number of taproot outputs in the transaction fits within an unsigned 32-bit integer<ref name="maximum_recipients">'''Why specify a limit of recipients, if the maximum number of possible transaction outputs is practically constrained by Bitcoin's consensus rules (block weight limit) anyway?''' Specifying an explicit limit improves clarity and ensures that implementations can safely choose compatible data types without the risk of becoming non-compliant. The limit is inferred from the data type of ''k'', which is serialized as a 4-bytes sequence in the course of the shared secret scalar calculation ''t<sub>k</sub>''.</ref> |
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If we ignore the block size limits, and consider this restriction, a transaction with 4294967295 + N1 taproot outputs with only a single silent payment output wouldn't be scanned.
I recommend to keep the reference for the recipient case but remove this constraint, because even if the case was possible, the limit would be something arbitrary.
Footnotes
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N > 0 ↩
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I assume this is intended as humorous, or do you actually propose a change in the phrasing?
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I wasn't trying to. Maybe I'm going too far with the implications of the sentence, I just want to avoid someone thinking there is a magic number besides which you can ignore the transaction and don't have to scan more outputs, and later suggest we can avoid the worst performing scanning case by lowering this number again.
I'm saying The number of taproot outputs in the transaction fits within an unsigned 32-bit integer is an arbitrary limit that can be dropped without harm, because is not there where the problem lies.
From following the referenced comments above, I understood the original idea of this change was to drop checks on the recipients side (while creating sp outputs), not on the receiving side (while scanning sp outputs).
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I see. From my cursory following along here, it seemed like you were joking about the large numbers involved. I’ll leave it up to the author to reply then.
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This seems ready to go, but I would prefer to get sign-off from one of the BIP authors, @RubenSomsen or @josibake.
| For silent payments v0 a transaction MUST be scanned if and only if all of the following are true: | ||
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| * The transaction contains at least one BIP341 taproot output (note: spent transactions optionally can be skipped by only considering transactions with at least one unspent taproot output) | ||
| * The number of taproot outputs in the transaction fits within an unsigned 32-bit integer<ref name="maximum_recipients">'''Why specify a limit of recipients, if the maximum number of possible transaction outputs is practically constrained by Bitcoin's consensus rules (block weight limit) anyway?''' Specifying an explicit limit improves clarity and ensures that implementations can safely choose compatible data types without the risk of becoming non-compliant. The limit is inferred from the data type of ''k'', which is serialized as a 4-bytes sequence in the course of the shared secret scalar calculation ''t<sub>k</sub>''.</ref> |
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I assume this is intended as humorous, or do you actually propose a change in the phrasing?
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Explicitly specifying a limit on the number of SP recipients / taproot outputs to scan for was suggested in the course of reviewing the BIP-352 module implementation in libsecp256k1, see bitcoin-core/secp256k1#1765 (comment) (related earlier discussion on take 3: bitcoin-core/secp256k1#1698 (comment) ff.).