BIP-352: limit number of recipients to 2^32-1 #2055
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cc: @RubenSomsen, @josibake |
murchandamus
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Proposed BIP modification
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furszy
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Dec 12, 2025
bip-0352.mediawiki
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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.
Eunovo
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Dec 28, 2025
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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.).